runtime: implement SUID/SGID protections

[gostls13.git] / src / runtime / proc.go

// Copyright 2014 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import (
        "internal/abi"
        "internal/cpu"
        "internal/goarch"
        "runtime/internal/atomic"
        "runtime/internal/sys"
        "unsafe"
)

// set using cmd/go/internal/modload.ModInfoProg
var modinfo string

// Goroutine scheduler
// The scheduler's job is to distribute ready-to-run goroutines over worker threads.
//
// The main concepts are:
// G - goroutine.
// M - worker thread, or machine.
// P - processor, a resource that is required to execute Go code.
//     M must have an associated P to execute Go code, however it can be
//     blocked or in a syscall w/o an associated P.
//
// Design doc at https://golang.org/s/go11sched.

// Worker thread parking/unparking.
// We need to balance between keeping enough running worker threads to utilize
// available hardware parallelism and parking excessive running worker threads
// to conserve CPU resources and power. This is not simple for two reasons:
// (1) scheduler state is intentionally distributed (in particular, per-P work
// queues), so it is not possible to compute global predicates on fast paths;
// (2) for optimal thread management we would need to know the future (don't park
// a worker thread when a new goroutine will be readied in near future).
//
// Three rejected approaches that would work badly:
// 1. Centralize all scheduler state (would inhibit scalability).
// 2. Direct goroutine handoff. That is, when we ready a new goroutine and there
//    is a spare P, unpark a thread and handoff it the thread and the goroutine.
//    This would lead to thread state thrashing, as the thread that readied the
//    goroutine can be out of work the very next moment, we will need to park it.
//    Also, it would destroy locality of computation as we want to preserve
//    dependent goroutines on the same thread; and introduce additional latency.
// 3. Unpark an additional thread whenever we ready a goroutine and there is an
//    idle P, but don't do handoff. This would lead to excessive thread parking/
//    unparking as the additional threads will instantly park without discovering
//    any work to do.
//
// The current approach:
//
// This approach applies to three primary sources of potential work: readying a
// goroutine, new/modified-earlier timers, and idle-priority GC. See below for
// additional details.
//
// We unpark an additional thread when we submit work if (this is wakep()):
// 1. There is an idle P, and
// 2. There are no "spinning" worker threads.
//
// A worker thread is considered spinning if it is out of local work and did
// not find work in the global run queue or netpoller; the spinning state is
// denoted in m.spinning and in sched.nmspinning. Threads unparked this way are
// also considered spinning; we don't do goroutine handoff so such threads are
// out of work initially. Spinning threads spin on looking for work in per-P
// run queues and timer heaps or from the GC before parking. If a spinning
// thread finds work it takes itself out of the spinning state and proceeds to
// execution. If it does not find work it takes itself out of the spinning
// state and then parks.
//
// If there is at least one spinning thread (sched.nmspinning>1), we don't
// unpark new threads when submitting work. To compensate for that, if the last
// spinning thread finds work and stops spinning, it must unpark a new spinning
// thread. This approach smooths out unjustified spikes of thread unparking,
// but at the same time guarantees eventual maximal CPU parallelism
// utilization.
//
// The main implementation complication is that we need to be very careful
// during spinning->non-spinning thread transition. This transition can race
// with submission of new work, and either one part or another needs to unpark
// another worker thread. If they both fail to do that, we can end up with
// semi-persistent CPU underutilization.
//
// The general pattern for submission is:
// 1. Submit work to the local run queue, timer heap, or GC state.
// 2. #StoreLoad-style memory barrier.
// 3. Check sched.nmspinning.
//
// The general pattern for spinning->non-spinning transition is:
// 1. Decrement nmspinning.
// 2. #StoreLoad-style memory barrier.
// 3. Check all per-P work queues and GC for new work.
//
// Note that all this complexity does not apply to global run queue as we are
// not sloppy about thread unparking when submitting to global queue. Also see
// comments for nmspinning manipulation.
//
// How these different sources of work behave varies, though it doesn't affect
// the synchronization approach:
// * Ready goroutine: this is an obvious source of work; the goroutine is
//   immediately ready and must run on some thread eventually.
// * New/modified-earlier timer: The current timer implementation (see time.go)
//   uses netpoll in a thread with no work available to wait for the soonest
//   timer. If there is no thread waiting, we want a new spinning thread to go
//   wait.
// * Idle-priority GC: The GC wakes a stopped idle thread to contribute to
//   background GC work (note: currently disabled per golang.org/issue/19112).
//   Also see golang.org/issue/44313, as this should be extended to all GC
//   workers.

var (
        m0           m
        g0           g
        mcache0      *mcache
        raceprocctx0 uintptr
        raceFiniLock mutex
)

// This slice records the initializing tasks that need to be
// done to start up the runtime. It is built by the linker.
var runtime_inittasks []*initTask

// main_init_done is a signal used by cgocallbackg that initialization
// has been completed. It is made before _cgo_notify_runtime_init_done,
// so all cgo calls can rely on it existing. When main_init is complete,
// it is closed, meaning cgocallbackg can reliably receive from it.
var main_init_done chan bool

//go:linkname main_main main.main
func main_main()

// mainStarted indicates that the main M has started.
var mainStarted bool

// runtimeInitTime is the nanotime() at which the runtime started.
var runtimeInitTime int64

// Value to use for signal mask for newly created M's.
var initSigmask sigset

// The main goroutine.
func main() {
        mp := getg().m

        // Racectx of m0->g0 is used only as the parent of the main goroutine.
        // It must not be used for anything else.
        mp.g0.racectx = 0

        // Max stack size is 1 GB on 64-bit, 250 MB on 32-bit.
        // Using decimal instead of binary GB and MB because
        // they look nicer in the stack overflow failure message.
        if goarch.PtrSize == 8 {
                maxstacksize = 1000000000
        } else {
                maxstacksize = 250000000
        }

        // An upper limit for max stack size. Used to avoid random crashes
        // after calling SetMaxStack and trying to allocate a stack that is too big,
        // since stackalloc works with 32-bit sizes.
        maxstackceiling = 2 * maxstacksize

        // Allow newproc to start new Ms.
        mainStarted = true

        if GOARCH != "wasm" { // no threads on wasm yet, so no sysmon
                systemstack(func() {
                        newm(sysmon, nil, -1)
                })
        }

        // Lock the main goroutine onto this, the main OS thread,
        // during initialization. Most programs won't care, but a few
        // do require certain calls to be made by the main thread.
        // Those can arrange for main.main to run in the main thread
        // by calling runtime.LockOSThread during initialization
        // to preserve the lock.
        lockOSThread()

        if mp != &m0 {
                throw("runtime.main not on m0")
        }

        // Record when the world started.
        // Must be before doInit for tracing init.
        runtimeInitTime = nanotime()
        if runtimeInitTime == 0 {
                throw("nanotime returning zero")
        }

        if debug.inittrace != 0 {
                inittrace.id = getg().goid
                inittrace.active = true
        }

        doInit(runtime_inittasks) // Must be before defer.

        // Defer unlock so that runtime.Goexit during init does the unlock too.
        needUnlock := true
        defer func() {
                if needUnlock {
                        unlockOSThread()
                }
        }()

        gcenable()

        main_init_done = make(chan bool)
        if iscgo {
                if _cgo_pthread_key_created == nil {
                        throw("_cgo_pthread_key_created missing")
                }

                if _cgo_thread_start == nil {
                        throw("_cgo_thread_start missing")
                }
                if GOOS != "windows" {
                        if _cgo_setenv == nil {
                                throw("_cgo_setenv missing")
                        }
                        if _cgo_unsetenv == nil {
                                throw("_cgo_unsetenv missing")
                        }
                }
                if _cgo_notify_runtime_init_done == nil {
                        throw("_cgo_notify_runtime_init_done missing")
                }

                // Set the x_crosscall2_ptr C function pointer variable point to crosscall2.
                if set_crosscall2 == nil {
                        throw("set_crosscall2 missing")
                }
                set_crosscall2()

                // Start the template thread in case we enter Go from
                // a C-created thread and need to create a new thread.
                startTemplateThread()
                cgocall(_cgo_notify_runtime_init_done, nil)
        }

        // Run the initializing tasks. Depending on build mode this
        // list can arrive a few different ways, but it will always
        // contain the init tasks computed by the linker for all the
        // packages in the program (excluding those added at runtime
        // by package plugin).
        for _, m := range activeModules() {
                doInit(m.inittasks)
        }

        // Disable init tracing after main init done to avoid overhead
        // of collecting statistics in malloc and newproc
        inittrace.active = false

        close(main_init_done)

        needUnlock = false
        unlockOSThread()

        if isarchive || islibrary {
                // A program compiled with -buildmode=c-archive or c-shared
                // has a main, but it is not executed.
                return
        }
        fn := main_main // make an indirect call, as the linker doesn't know the address of the main package when laying down the runtime
        fn()
        if raceenabled {
                runExitHooks(0) // run hooks now, since racefini does not return
                racefini()
        }

        // Make racy client program work: if panicking on
        // another goroutine at the same time as main returns,
        // let the other goroutine finish printing the panic trace.
        // Once it does, it will exit. See issues 3934 and 20018.
        if runningPanicDefers.Load() != 0 {
                // Running deferred functions should not take long.
                for c := 0; c < 1000; c++ {
                        if runningPanicDefers.Load() == 0 {
                                break
                        }
                        Gosched()
                }
        }
        if panicking.Load() != 0 {
                gopark(nil, nil, waitReasonPanicWait, traceBlockForever, 1)
        }
        runExitHooks(0)

        exit(0)
        for {
                var x *int32
                *x = 0
        }
}

// os_beforeExit is called from os.Exit(0).
//
//go:linkname os_beforeExit os.runtime_beforeExit
func os_beforeExit(exitCode int) {
        runExitHooks(exitCode)
        if exitCode == 0 && raceenabled {
                racefini()
        }
}

// start forcegc helper goroutine
func init() {
        go forcegchelper()
}

func forcegchelper() {
        forcegc.g = getg()
        lockInit(&forcegc.lock, lockRankForcegc)
        for {
                lock(&forcegc.lock)
                if forcegc.idle.Load() {
                        throw("forcegc: phase error")
                }
                forcegc.idle.Store(true)
                goparkunlock(&forcegc.lock, waitReasonForceGCIdle, traceBlockSystemGoroutine, 1)
                // this goroutine is explicitly resumed by sysmon
                if debug.gctrace > 0 {
                        println("GC forced")
                }
                // Time-triggered, fully concurrent.
                gcStart(gcTrigger{kind: gcTriggerTime, now: nanotime()})
        }
}

// Gosched yields the processor, allowing other goroutines to run. It does not
// suspend the current goroutine, so execution resumes automatically.
//
//go:nosplit
func Gosched() {
        checkTimeouts()
        mcall(gosched_m)
}

// goschedguarded yields the processor like gosched, but also checks
// for forbidden states and opts out of the yield in those cases.
//
//go:nosplit
func goschedguarded() {
        mcall(goschedguarded_m)
}

// goschedIfBusy yields the processor like gosched, but only does so if
// there are no idle Ps or if we're on the only P and there's nothing in
// the run queue. In both cases, there is freely available idle time.
//
//go:nosplit
func goschedIfBusy() {
        gp := getg()
        // Call gosched if gp.preempt is set; we may be in a tight loop that
        // doesn't otherwise yield.
        if !gp.preempt && sched.npidle.Load() > 0 {
                return
        }
        mcall(gosched_m)
}

// Puts the current goroutine into a waiting state and calls unlockf on the
// system stack.
//
// If unlockf returns false, the goroutine is resumed.
//
// unlockf must not access this G's stack, as it may be moved between
// the call to gopark and the call to unlockf.
//
// Note that because unlockf is called after putting the G into a waiting
// state, the G may have already been readied by the time unlockf is called
// unless there is external synchronization preventing the G from being
// readied. If unlockf returns false, it must guarantee that the G cannot be
// externally readied.
//
// Reason explains why the goroutine has been parked. It is displayed in stack
// traces and heap dumps. Reasons should be unique and descriptive. Do not
// re-use reasons, add new ones.
func gopark(unlockf func(*g, unsafe.Pointer) bool, lock unsafe.Pointer, reason waitReason, traceReason traceBlockReason, traceskip int) {
        if reason != waitReasonSleep {
                checkTimeouts() // timeouts may expire while two goroutines keep the scheduler busy
        }
        mp := acquirem()
        gp := mp.curg
        status := readgstatus(gp)
        if status != _Grunning && status != _Gscanrunning {
                throw("gopark: bad g status")
        }
        mp.waitlock = lock
        mp.waitunlockf = unlockf
        gp.waitreason = reason
        mp.waitTraceBlockReason = traceReason
        mp.waitTraceSkip = traceskip
        releasem(mp)
        // can't do anything that might move the G between Ms here.
        mcall(park_m)
}

// Puts the current goroutine into a waiting state and unlocks the lock.
// The goroutine can be made runnable again by calling goready(gp).
func goparkunlock(lock *mutex, reason waitReason, traceReason traceBlockReason, traceskip int) {
        gopark(parkunlock_c, unsafe.Pointer(lock), reason, traceReason, traceskip)
}

func goready(gp *g, traceskip int) {
        systemstack(func() {
                ready(gp, traceskip, true)
        })
}

//go:nosplit
func acquireSudog() *sudog {
        // Delicate dance: the semaphore implementation calls
        // acquireSudog, acquireSudog calls new(sudog),
        // new calls malloc, malloc can call the garbage collector,
        // and the garbage collector calls the semaphore implementation
        // in stopTheWorld.
        // Break the cycle by doing acquirem/releasem around new(sudog).
        // The acquirem/releasem increments m.locks during new(sudog),
        // which keeps the garbage collector from being invoked.
        mp := acquirem()
        pp := mp.p.ptr()
        if len(pp.sudogcache) == 0 {
                lock(&sched.sudoglock)
                // First, try to grab a batch from central cache.
                for len(pp.sudogcache) < cap(pp.sudogcache)/2 && sched.sudogcache != nil {
                        s := sched.sudogcache
                        sched.sudogcache = s.next
                        s.next = nil
                        pp.sudogcache = append(pp.sudogcache, s)
                }
                unlock(&sched.sudoglock)
                // If the central cache is empty, allocate a new one.
                if len(pp.sudogcache) == 0 {
                        pp.sudogcache = append(pp.sudogcache, new(sudog))
                }
        }
        n := len(pp.sudogcache)
        s := pp.sudogcache[n-1]
        pp.sudogcache[n-1] = nil
        pp.sudogcache = pp.sudogcache[:n-1]
        if s.elem != nil {
                throw("acquireSudog: found s.elem != nil in cache")
        }
        releasem(mp)
        return s
}

//go:nosplit
func releaseSudog(s *sudog) {
        if s.elem != nil {
                throw("runtime: sudog with non-nil elem")
        }
        if s.isSelect {
                throw("runtime: sudog with non-false isSelect")
        }
        if s.next != nil {
                throw("runtime: sudog with non-nil next")
        }
        if s.prev != nil {
                throw("runtime: sudog with non-nil prev")
        }
        if s.waitlink != nil {
                throw("runtime: sudog with non-nil waitlink")
        }
        if s.c != nil {
                throw("runtime: sudog with non-nil c")
        }
        gp := getg()
        if gp.param != nil {
                throw("runtime: releaseSudog with non-nil gp.param")
        }
        mp := acquirem() // avoid rescheduling to another P
        pp := mp.p.ptr()
        if len(pp.sudogcache) == cap(pp.sudogcache) {
                // Transfer half of local cache to the central cache.
                var first, last *sudog
                for len(pp.sudogcache) > cap(pp.sudogcache)/2 {
                        n := len(pp.sudogcache)
                        p := pp.sudogcache[n-1]
                        pp.sudogcache[n-1] = nil
                        pp.sudogcache = pp.sudogcache[:n-1]
                        if first == nil {
                                first = p
                        } else {
                                last.next = p
                        }
                        last = p
                }
                lock(&sched.sudoglock)
                last.next = sched.sudogcache
                sched.sudogcache = first
                unlock(&sched.sudoglock)
        }
        pp.sudogcache = append(pp.sudogcache, s)
        releasem(mp)
}

// called from assembly.
func badmcall(fn func(*g)) {
        throw("runtime: mcall called on m->g0 stack")
}

func badmcall2(fn func(*g)) {
        throw("runtime: mcall function returned")
}

func badreflectcall() {
        panic(plainError("arg size to reflect.call more than 1GB"))
}

//go:nosplit
//go:nowritebarrierrec
func badmorestackg0() {
        writeErrStr("fatal: morestack on g0\n")
}

//go:nosplit
//go:nowritebarrierrec
func badmorestackgsignal() {
        writeErrStr("fatal: morestack on gsignal\n")
}

//go:nosplit
func badctxt() {
        throw("ctxt != 0")
}

func lockedOSThread() bool {
        gp := getg()
        return gp.lockedm != 0 && gp.m.lockedg != 0
}

var (
        // allgs contains all Gs ever created (including dead Gs), and thus
        // never shrinks.
        //
        // Access via the slice is protected by allglock or stop-the-world.
        // Readers that cannot take the lock may (carefully!) use the atomic
        // variables below.
        allglock mutex
        allgs    []*g

        // allglen and allgptr are atomic variables that contain len(allgs) and
        // &allgs[0] respectively. Proper ordering depends on totally-ordered
        // loads and stores. Writes are protected by allglock.
        //
        // allgptr is updated before allglen. Readers should read allglen
        // before allgptr to ensure that allglen is always <= len(allgptr). New
        // Gs appended during the race can be missed. For a consistent view of
        // all Gs, allglock must be held.
        //
        // allgptr copies should always be stored as a concrete type or
        // unsafe.Pointer, not uintptr, to ensure that GC can still reach it
        // even if it points to a stale array.
        allglen uintptr
        allgptr **g
)

func allgadd(gp *g) {
        if readgstatus(gp) == _Gidle {
                throw("allgadd: bad status Gidle")
        }

        lock(&allglock)
        allgs = append(allgs, gp)
        if &allgs[0] != allgptr {
                atomicstorep(unsafe.Pointer(&allgptr), unsafe.Pointer(&allgs[0]))
        }
        atomic.Storeuintptr(&allglen, uintptr(len(allgs)))
        unlock(&allglock)
}

// allGsSnapshot returns a snapshot of the slice of all Gs.
//
// The world must be stopped or allglock must be held.
func allGsSnapshot() []*g {
        assertWorldStoppedOrLockHeld(&allglock)

        // Because the world is stopped or allglock is held, allgadd
        // cannot happen concurrently with this. allgs grows
        // monotonically and existing entries never change, so we can
        // simply return a copy of the slice header. For added safety,
        // we trim everything past len because that can still change.
        return allgs[:len(allgs):len(allgs)]
}

// atomicAllG returns &allgs[0] and len(allgs) for use with atomicAllGIndex.
func atomicAllG() (**g, uintptr) {
        length := atomic.Loaduintptr(&allglen)
        ptr := (**g)(atomic.Loadp(unsafe.Pointer(&allgptr)))
        return ptr, length
}

// atomicAllGIndex returns ptr[i] with the allgptr returned from atomicAllG.
func atomicAllGIndex(ptr **g, i uintptr) *g {
        return *(**g)(add(unsafe.Pointer(ptr), i*goarch.PtrSize))
}

// forEachG calls fn on every G from allgs.
//
// forEachG takes a lock to exclude concurrent addition of new Gs.
func forEachG(fn func(gp *g)) {
        lock(&allglock)
        for _, gp := range allgs {
                fn(gp)
        }
        unlock(&allglock)
}

// forEachGRace calls fn on every G from allgs.
//
// forEachGRace avoids locking, but does not exclude addition of new Gs during
// execution, which may be missed.
func forEachGRace(fn func(gp *g)) {
        ptr, length := atomicAllG()
        for i := uintptr(0); i < length; i++ {
                gp := atomicAllGIndex(ptr, i)
                fn(gp)
        }
        return
}

const (
        // Number of goroutine ids to grab from sched.goidgen to local per-P cache at once.
        // 16 seems to provide enough amortization, but other than that it's mostly arbitrary number.
        _GoidCacheBatch = 16
)

// cpuinit sets up CPU feature flags and calls internal/cpu.Initialize. env should be the complete
// value of the GODEBUG environment variable.
func cpuinit(env string) {
        switch GOOS {
        case "aix", "darwin", "ios", "dragonfly", "freebsd", "netbsd", "openbsd", "illumos", "solaris", "linux":
                cpu.DebugOptions = true
        }
        cpu.Initialize(env)

        // Support cpu feature variables are used in code generated by the compiler
        // to guard execution of instructions that can not be assumed to be always supported.
        switch GOARCH {
        case "386", "amd64":
                x86HasPOPCNT = cpu.X86.HasPOPCNT
                x86HasSSE41 = cpu.X86.HasSSE41
                x86HasFMA = cpu.X86.HasFMA

        case "arm":
                armHasVFPv4 = cpu.ARM.HasVFPv4

        case "arm64":
                arm64HasATOMICS = cpu.ARM64.HasATOMICS
        }
}

// getGodebugEarly extracts the environment variable GODEBUG from the environment on
// Unix-like operating systems and returns it. This function exists to extract GODEBUG
// early before much of the runtime is initialized.
func getGodebugEarly() string {
        const prefix = "GODEBUG="
        var env string
        switch GOOS {
        case "aix", "darwin", "ios", "dragonfly", "freebsd", "netbsd", "openbsd", "illumos", "solaris", "linux":
                // Similar to goenv_unix but extracts the environment value for
                // GODEBUG directly.
                // TODO(moehrmann): remove when general goenvs() can be called before cpuinit()
                n := int32(0)
                for argv_index(argv, argc+1+n) != nil {
                        n++
                }

                for i := int32(0); i < n; i++ {
                        p := argv_index(argv, argc+1+i)
                        s := unsafe.String(p, findnull(p))

                        if hasPrefix(s, prefix) {
                                env = gostring(p)[len(prefix):]
                                break
                        }
                }
        }
        return env
}

// The bootstrap sequence is:
//
//      call osinit
//      call schedinit
//      make & queue new G
//      call runtime·mstart
//
// The new G calls runtime·main.
func schedinit() {
        lockInit(&sched.lock, lockRankSched)
        lockInit(&sched.sysmonlock, lockRankSysmon)
        lockInit(&sched.deferlock, lockRankDefer)
        lockInit(&sched.sudoglock, lockRankSudog)
        lockInit(&deadlock, lockRankDeadlock)
        lockInit(&paniclk, lockRankPanic)
        lockInit(&allglock, lockRankAllg)
        lockInit(&allpLock, lockRankAllp)
        lockInit(&reflectOffs.lock, lockRankReflectOffs)
        lockInit(&finlock, lockRankFin)
        lockInit(&cpuprof.lock, lockRankCpuprof)
        traceLockInit()
        // Enforce that this lock is always a leaf lock.
        // All of this lock's critical sections should be
        // extremely short.
        lockInit(&memstats.heapStats.noPLock, lockRankLeafRank)

        // raceinit must be the first call to race detector.
        // In particular, it must be done before mallocinit below calls racemapshadow.
        gp := getg()
        if raceenabled {
                gp.racectx, raceprocctx0 = raceinit()
        }

        sched.maxmcount = 10000

        // The world starts stopped.
        worldStopped()

        moduledataverify()
        stackinit()
        mallocinit()
        godebug := getGodebugEarly()
        initPageTrace(godebug) // must run after mallocinit but before anything allocates
        cpuinit(godebug)       // must run before alginit
        alginit()              // maps, hash, fastrand must not be used before this call
        fastrandinit()         // must run before mcommoninit
        mcommoninit(gp.m, -1)
        modulesinit()   // provides activeModules
        typelinksinit() // uses maps, activeModules
        itabsinit()     // uses activeModules
        stkobjinit()    // must run before GC starts

        sigsave(&gp.m.sigmask)
        initSigmask = gp.m.sigmask

        goargs()
        goenvs()
        secure()
        parsedebugvars()
        gcinit()

        // if disableMemoryProfiling is set, update MemProfileRate to 0 to turn off memprofile.
        // Note: parsedebugvars may update MemProfileRate, but when disableMemoryProfiling is
        // set to true by the linker, it means that nothing is consuming the profile, it is
        // safe to set MemProfileRate to 0.
        if disableMemoryProfiling {
                MemProfileRate = 0
        }

        lock(&sched.lock)
        sched.lastpoll.Store(nanotime())
        procs := ncpu
        if n, ok := atoi32(gogetenv("GOMAXPROCS")); ok && n > 0 {
                procs = n
        }
        if procresize(procs) != nil {
                throw("unknown runnable goroutine during bootstrap")
        }
        unlock(&sched.lock)

        // World is effectively started now, as P's can run.
        worldStarted()

        if buildVersion == "" {
                // Condition should never trigger. This code just serves
                // to ensure runtime·buildVersion is kept in the resulting binary.
                buildVersion = "unknown"
        }
        if len(modinfo) == 1 {
                // Condition should never trigger. This code just serves
                // to ensure runtime·modinfo is kept in the resulting binary.
                modinfo = ""
        }
}

func dumpgstatus(gp *g) {
        thisg := getg()
        print("runtime:   gp: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
        print("runtime: getg:  g=", thisg, ", goid=", thisg.goid, ",  g->atomicstatus=", readgstatus(thisg), "\n")
}

// sched.lock must be held.
func checkmcount() {
        assertLockHeld(&sched.lock)

        // Exclude extra M's, which are used for cgocallback from threads
        // created in C.
        //
        // The purpose of the SetMaxThreads limit is to avoid accidental fork
        // bomb from something like millions of goroutines blocking on system
        // calls, causing the runtime to create millions of threads. By
        // definition, this isn't a problem for threads created in C, so we
        // exclude them from the limit. See https://go.dev/issue/60004.
        count := mcount() - int32(extraMInUse.Load()) - int32(extraMLength.Load())
        if count > sched.maxmcount {
                print("runtime: program exceeds ", sched.maxmcount, "-thread limit\n")
                throw("thread exhaustion")
        }
}

// mReserveID returns the next ID to use for a new m. This new m is immediately
// considered 'running' by checkdead.
//
// sched.lock must be held.
func mReserveID() int64 {
        assertLockHeld(&sched.lock)

        if sched.mnext+1 < sched.mnext {
                throw("runtime: thread ID overflow")
        }
        id := sched.mnext
        sched.mnext++
        checkmcount()
        return id
}

// Pre-allocated ID may be passed as 'id', or omitted by passing -1.
func mcommoninit(mp *m, id int64) {
        gp := getg()

        // g0 stack won't make sense for user (and is not necessary unwindable).
        if gp != gp.m.g0 {
                callers(1, mp.createstack[:])
        }

        lock(&sched.lock)

        if id >= 0 {
                mp.id = id
        } else {
                mp.id = mReserveID()
        }

        lo := uint32(int64Hash(uint64(mp.id), fastrandseed))
        hi := uint32(int64Hash(uint64(cputicks()), ^fastrandseed))
        if lo|hi == 0 {
                hi = 1
        }
        // Same behavior as for 1.17.
        // TODO: Simplify this.
        if goarch.BigEndian {
                mp.fastrand = uint64(lo)<<32 | uint64(hi)
        } else {
                mp.fastrand = uint64(hi)<<32 | uint64(lo)
        }

        mpreinit(mp)
        if mp.gsignal != nil {
                mp.gsignal.stackguard1 = mp.gsignal.stack.lo + stackGuard
        }

        // Add to allm so garbage collector doesn't free g->m
        // when it is just in a register or thread-local storage.
        mp.alllink = allm

        // NumCgoCall() iterates over allm w/o schedlock,
        // so we need to publish it safely.
        atomicstorep(unsafe.Pointer(&allm), unsafe.Pointer(mp))
        unlock(&sched.lock)

        // Allocate memory to hold a cgo traceback if the cgo call crashes.
        if iscgo || GOOS == "solaris" || GOOS == "illumos" || GOOS == "windows" {
                mp.cgoCallers = new(cgoCallers)
        }
}

func (mp *m) becomeSpinning() {
        mp.spinning = true
        sched.nmspinning.Add(1)
        sched.needspinning.Store(0)
}

func (mp *m) hasCgoOnStack() bool {
        return mp.ncgo > 0 || mp.isextra
}

var fastrandseed uintptr

func fastrandinit() {
        s := (*[unsafe.Sizeof(fastrandseed)]byte)(unsafe.Pointer(&fastrandseed))[:]
        getRandomData(s)
}

// Mark gp ready to run.
func ready(gp *g, traceskip int, next bool) {
        if traceEnabled() {
                traceGoUnpark(gp, traceskip)
        }

        status := readgstatus(gp)

        // Mark runnable.
        mp := acquirem() // disable preemption because it can be holding p in a local var
        if status&^_Gscan != _Gwaiting {
                dumpgstatus(gp)
                throw("bad g->status in ready")
        }

        // status is Gwaiting or Gscanwaiting, make Grunnable and put on runq
        casgstatus(gp, _Gwaiting, _Grunnable)
        runqput(mp.p.ptr(), gp, next)
        wakep()
        releasem(mp)
}

// freezeStopWait is a large value that freezetheworld sets
// sched.stopwait to in order to request that all Gs permanently stop.
const freezeStopWait = 0x7fffffff

// freezing is set to non-zero if the runtime is trying to freeze the
// world.
var freezing atomic.Bool

// Similar to stopTheWorld but best-effort and can be called several times.
// There is no reverse operation, used during crashing.
// This function must not lock any mutexes.
func freezetheworld() {
        freezing.Store(true)
        // stopwait and preemption requests can be lost
        // due to races with concurrently executing threads,
        // so try several times
        for i := 0; i < 5; i++ {
                // this should tell the scheduler to not start any new goroutines
                sched.stopwait = freezeStopWait
                sched.gcwaiting.Store(true)
                // this should stop running goroutines
                if !preemptall() {
                        break // no running goroutines
                }
                usleep(1000)
        }
        // to be sure
        usleep(1000)
        preemptall()
        usleep(1000)
}

// All reads and writes of g's status go through readgstatus, casgstatus
// castogscanstatus, casfrom_Gscanstatus.
//
//go:nosplit
func readgstatus(gp *g) uint32 {
        return gp.atomicstatus.Load()
}

// The Gscanstatuses are acting like locks and this releases them.
// If it proves to be a performance hit we should be able to make these
// simple atomic stores but for now we are going to throw if
// we see an inconsistent state.
func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
        success := false

        // Check that transition is valid.
        switch oldval {
        default:
                print("runtime: casfrom_Gscanstatus bad oldval gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
                dumpgstatus(gp)
                throw("casfrom_Gscanstatus:top gp->status is not in scan state")
        case _Gscanrunnable,
                _Gscanwaiting,
                _Gscanrunning,
                _Gscansyscall,
                _Gscanpreempted:
                if newval == oldval&^_Gscan {
                        success = gp.atomicstatus.CompareAndSwap(oldval, newval)
                }
        }
        if !success {
                print("runtime: casfrom_Gscanstatus failed gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
                dumpgstatus(gp)
                throw("casfrom_Gscanstatus: gp->status is not in scan state")
        }
        releaseLockRank(lockRankGscan)
}

// This will return false if the gp is not in the expected status and the cas fails.
// This acts like a lock acquire while the casfromgstatus acts like a lock release.
func castogscanstatus(gp *g, oldval, newval uint32) bool {
        switch oldval {
        case _Grunnable,
                _Grunning,
                _Gwaiting,
                _Gsyscall:
                if newval == oldval|_Gscan {
                        r := gp.atomicstatus.CompareAndSwap(oldval, newval)
                        if r {
                                acquireLockRank(lockRankGscan)
                        }
                        return r

                }
        }
        print("runtime: castogscanstatus oldval=", hex(oldval), " newval=", hex(newval), "\n")
        throw("castogscanstatus")
        panic("not reached")
}

// casgstatusAlwaysTrack is a debug flag that causes casgstatus to always track
// various latencies on every transition instead of sampling them.
var casgstatusAlwaysTrack = false

// If asked to move to or from a Gscanstatus this will throw. Use the castogscanstatus
// and casfrom_Gscanstatus instead.
// casgstatus will loop if the g->atomicstatus is in a Gscan status until the routine that
// put it in the Gscan state is finished.
//
//go:nosplit
func casgstatus(gp *g, oldval, newval uint32) {
        if (oldval&_Gscan != 0) || (newval&_Gscan != 0) || oldval == newval {
                systemstack(func() {
                        print("runtime: casgstatus: oldval=", hex(oldval), " newval=", hex(newval), "\n")
                        throw("casgstatus: bad incoming values")
                })
        }

        acquireLockRank(lockRankGscan)
        releaseLockRank(lockRankGscan)

        // See https://golang.org/cl/21503 for justification of the yield delay.
        const yieldDelay = 5 * 1000
        var nextYield int64

        // loop if gp->atomicstatus is in a scan state giving
        // GC time to finish and change the state to oldval.
        for i := 0; !gp.atomicstatus.CompareAndSwap(oldval, newval); i++ {
                if oldval == _Gwaiting && gp.atomicstatus.Load() == _Grunnable {
                        throw("casgstatus: waiting for Gwaiting but is Grunnable")
                }
                if i == 0 {
                        nextYield = nanotime() + yieldDelay
                }
                if nanotime() < nextYield {
                        for x := 0; x < 10 && gp.atomicstatus.Load() != oldval; x++ {
                                procyield(1)
                        }
                } else {
                        osyield()
                        nextYield = nanotime() + yieldDelay/2
                }
        }

        if oldval == _Grunning {
                // Track every gTrackingPeriod time a goroutine transitions out of running.
                if casgstatusAlwaysTrack || gp.trackingSeq%gTrackingPeriod == 0 {
                        gp.tracking = true
                }
                gp.trackingSeq++
        }
        if !gp.tracking {
                return
        }

        // Handle various kinds of tracking.
        //
        // Currently:
        // - Time spent in runnable.
        // - Time spent blocked on a sync.Mutex or sync.RWMutex.
        switch oldval {
        case _Grunnable:
                // We transitioned out of runnable, so measure how much
                // time we spent in this state and add it to
                // runnableTime.
                now := nanotime()
                gp.runnableTime += now - gp.trackingStamp
                gp.trackingStamp = 0
        case _Gwaiting:
                if !gp.waitreason.isMutexWait() {
                        // Not blocking on a lock.
                        break
                }
                // Blocking on a lock, measure it. Note that because we're
                // sampling, we have to multiply by our sampling period to get
                // a more representative estimate of the absolute value.
                // gTrackingPeriod also represents an accurate sampling period
                // because we can only enter this state from _Grunning.
                now := nanotime()
                sched.totalMutexWaitTime.Add((now - gp.trackingStamp) * gTrackingPeriod)
                gp.trackingStamp = 0
        }
        switch newval {
        case _Gwaiting:
                if !gp.waitreason.isMutexWait() {
                        // Not blocking on a lock.
                        break
                }
                // Blocking on a lock. Write down the timestamp.
                now := nanotime()
                gp.trackingStamp = now
        case _Grunnable:
                // We just transitioned into runnable, so record what
                // time that happened.
                now := nanotime()
                gp.trackingStamp = now
        case _Grunning:
                // We're transitioning into running, so turn off
                // tracking and record how much time we spent in
                // runnable.
                gp.tracking = false
                sched.timeToRun.record(gp.runnableTime)
                gp.runnableTime = 0
        }
}

// casGToWaiting transitions gp from old to _Gwaiting, and sets the wait reason.
//
// Use this over casgstatus when possible to ensure that a waitreason is set.
func casGToWaiting(gp *g, old uint32, reason waitReason) {
        // Set the wait reason before calling casgstatus, because casgstatus will use it.
        gp.waitreason = reason
        casgstatus(gp, old, _Gwaiting)
}

// casgstatus(gp, oldstatus, Gcopystack), assuming oldstatus is Gwaiting or Grunnable.
// Returns old status. Cannot call casgstatus directly, because we are racing with an
// async wakeup that might come in from netpoll. If we see Gwaiting from the readgstatus,
// it might have become Grunnable by the time we get to the cas. If we called casgstatus,
// it would loop waiting for the status to go back to Gwaiting, which it never will.
//
//go:nosplit
func casgcopystack(gp *g) uint32 {
        for {
                oldstatus := readgstatus(gp) &^ _Gscan
                if oldstatus != _Gwaiting && oldstatus != _Grunnable {
                        throw("copystack: bad status, not Gwaiting or Grunnable")
                }
                if gp.atomicstatus.CompareAndSwap(oldstatus, _Gcopystack) {
                        return oldstatus
                }
        }
}

// casGToPreemptScan transitions gp from _Grunning to _Gscan|_Gpreempted.
//
// TODO(austin): This is the only status operation that both changes
// the status and locks the _Gscan bit. Rethink this.
func casGToPreemptScan(gp *g, old, new uint32) {
        if old != _Grunning || new != _Gscan|_Gpreempted {
                throw("bad g transition")
        }
        acquireLockRank(lockRankGscan)
        for !gp.atomicstatus.CompareAndSwap(_Grunning, _Gscan|_Gpreempted) {
        }
}

// casGFromPreempted attempts to transition gp from _Gpreempted to
// _Gwaiting. If successful, the caller is responsible for
// re-scheduling gp.
func casGFromPreempted(gp *g, old, new uint32) bool {
        if old != _Gpreempted || new != _Gwaiting {
                throw("bad g transition")
        }
        gp.waitreason = waitReasonPreempted
        return gp.atomicstatus.CompareAndSwap(_Gpreempted, _Gwaiting)
}

// stwReason is an enumeration of reasons the world is stopping.
type stwReason uint8

// Reasons to stop-the-world.
//
// Avoid reusing reasons and add new ones instead.
const (
        stwUnknown                     stwReason = iota // "unknown"
        stwGCMarkTerm                                   // "GC mark termination"
        stwGCSweepTerm                                  // "GC sweep termination"
        stwWriteHeapDump                                // "write heap dump"
        stwGoroutineProfile                             // "goroutine profile"
        stwGoroutineProfileCleanup                      // "goroutine profile cleanup"
        stwAllGoroutinesStack                           // "all goroutines stack trace"
        stwReadMemStats                                 // "read mem stats"
        stwAllThreadsSyscall                            // "AllThreadsSyscall"
        stwGOMAXPROCS                                   // "GOMAXPROCS"
        stwStartTrace                                   // "start trace"
        stwStopTrace                                    // "stop trace"
        stwForTestCountPagesInUse                       // "CountPagesInUse (test)"
        stwForTestReadMetricsSlow                       // "ReadMetricsSlow (test)"
        stwForTestReadMemStatsSlow                      // "ReadMemStatsSlow (test)"
        stwForTestPageCachePagesLeaked                  // "PageCachePagesLeaked (test)"
        stwForTestResetDebugLog                         // "ResetDebugLog (test)"
)

func (r stwReason) String() string {
        return stwReasonStrings[r]
}

// If you add to this list, also add it to src/internal/trace/parser.go.
// If you change the values of any of the stw* constants, bump the trace
// version number and make a copy of this.
var stwReasonStrings = [...]string{
        stwUnknown:                     "unknown",
        stwGCMarkTerm:                  "GC mark termination",
        stwGCSweepTerm:                 "GC sweep termination",
        stwWriteHeapDump:               "write heap dump",
        stwGoroutineProfile:            "goroutine profile",
        stwGoroutineProfileCleanup:     "goroutine profile cleanup",
        stwAllGoroutinesStack:          "all goroutines stack trace",
        stwReadMemStats:                "read mem stats",
        stwAllThreadsSyscall:           "AllThreadsSyscall",
        stwGOMAXPROCS:                  "GOMAXPROCS",
        stwStartTrace:                  "start trace",
        stwStopTrace:                   "stop trace",
        stwForTestCountPagesInUse:      "CountPagesInUse (test)",
        stwForTestReadMetricsSlow:      "ReadMetricsSlow (test)",
        stwForTestReadMemStatsSlow:     "ReadMemStatsSlow (test)",
        stwForTestPageCachePagesLeaked: "PageCachePagesLeaked (test)",
        stwForTestResetDebugLog:        "ResetDebugLog (test)",
}

// stopTheWorld stops all P's from executing goroutines, interrupting
// all goroutines at GC safe points and records reason as the reason
// for the stop. On return, only the current goroutine's P is running.
// stopTheWorld must not be called from a system stack and the caller
// must not hold worldsema. The caller must call startTheWorld when
// other P's should resume execution.
//
// stopTheWorld is safe for multiple goroutines to call at the
// same time. Each will execute its own stop, and the stops will
// be serialized.
//
// This is also used by routines that do stack dumps. If the system is
// in panic or being exited, this may not reliably stop all
// goroutines.
func stopTheWorld(reason stwReason) {
        semacquire(&worldsema)
        gp := getg()
        gp.m.preemptoff = reason.String()
        systemstack(func() {
                // Mark the goroutine which called stopTheWorld preemptible so its
                // stack may be scanned.
                // This lets a mark worker scan us while we try to stop the world
                // since otherwise we could get in a mutual preemption deadlock.
                // We must not modify anything on the G stack because a stack shrink
                // may occur. A stack shrink is otherwise OK though because in order
                // to return from this function (and to leave the system stack) we
                // must have preempted all goroutines, including any attempting
                // to scan our stack, in which case, any stack shrinking will
                // have already completed by the time we exit.
                // Don't provide a wait reason because we're still executing.
                casGToWaiting(gp, _Grunning, waitReasonStoppingTheWorld)
                stopTheWorldWithSema(reason)
                casgstatus(gp, _Gwaiting, _Grunning)
        })
}

// startTheWorld undoes the effects of stopTheWorld.
func startTheWorld() {
        systemstack(func() { startTheWorldWithSema() })

        // worldsema must be held over startTheWorldWithSema to ensure
        // gomaxprocs cannot change while worldsema is held.
        //
        // Release worldsema with direct handoff to the next waiter, but
        // acquirem so that semrelease1 doesn't try to yield our time.
        //
        // Otherwise if e.g. ReadMemStats is being called in a loop,
        // it might stomp on other attempts to stop the world, such as
        // for starting or ending GC. The operation this blocks is
        // so heavy-weight that we should just try to be as fair as
        // possible here.
        //
        // We don't want to just allow us to get preempted between now
        // and releasing the semaphore because then we keep everyone
        // (including, for example, GCs) waiting longer.
        mp := acquirem()
        mp.preemptoff = ""
        semrelease1(&worldsema, true, 0)
        releasem(mp)
}

// stopTheWorldGC has the same effect as stopTheWorld, but blocks
// until the GC is not running. It also blocks a GC from starting
// until startTheWorldGC is called.
func stopTheWorldGC(reason stwReason) {
        semacquire(&gcsema)
        stopTheWorld(reason)
}

// startTheWorldGC undoes the effects of stopTheWorldGC.
func startTheWorldGC() {
        startTheWorld()
        semrelease(&gcsema)
}

// Holding worldsema grants an M the right to try to stop the world.
var worldsema uint32 = 1

// Holding gcsema grants the M the right to block a GC, and blocks
// until the current GC is done. In particular, it prevents gomaxprocs
// from changing concurrently.
//
// TODO(mknyszek): Once gomaxprocs and the execution tracer can handle
// being changed/enabled during a GC, remove this.
var gcsema uint32 = 1

// stopTheWorldWithSema is the core implementation of stopTheWorld.
// The caller is responsible for acquiring worldsema and disabling
// preemption first and then should stopTheWorldWithSema on the system
// stack:
//
//      semacquire(&worldsema, 0)
//      m.preemptoff = "reason"
//      systemstack(stopTheWorldWithSema)
//
// When finished, the caller must either call startTheWorld or undo
// these three operations separately:
//
//      m.preemptoff = ""
//      systemstack(startTheWorldWithSema)
//      semrelease(&worldsema)
//
// It is allowed to acquire worldsema once and then execute multiple
// startTheWorldWithSema/stopTheWorldWithSema pairs.
// Other P's are able to execute between successive calls to
// startTheWorldWithSema and stopTheWorldWithSema.
// Holding worldsema causes any other goroutines invoking
// stopTheWorld to block.
func stopTheWorldWithSema(reason stwReason) {
        if traceEnabled() {
                traceSTWStart(reason)
        }
        gp := getg()

        // If we hold a lock, then we won't be able to stop another M
        // that is blocked trying to acquire the lock.
        if gp.m.locks > 0 {
                throw("stopTheWorld: holding locks")
        }

        lock(&sched.lock)
        sched.stopwait = gomaxprocs
        sched.gcwaiting.Store(true)
        preemptall()
        // stop current P
        gp.m.p.ptr().status = _Pgcstop // Pgcstop is only diagnostic.
        sched.stopwait--
        // try to retake all P's in Psyscall status
        for _, pp := range allp {
                s := pp.status
                if s == _Psyscall && atomic.Cas(&pp.status, s, _Pgcstop) {
                        if traceEnabled() {
                                traceGoSysBlock(pp)
                                traceProcStop(pp)
                        }
                        pp.syscalltick++
                        sched.stopwait--
                }
        }
        // stop idle P's
        now := nanotime()
        for {
                pp, _ := pidleget(now)
                if pp == nil {
                        break
                }
                pp.status = _Pgcstop
                sched.stopwait--
        }
        wait := sched.stopwait > 0
        unlock(&sched.lock)

        // wait for remaining P's to stop voluntarily
        if wait {
                for {
                        // wait for 100us, then try to re-preempt in case of any races
                        if notetsleep(&sched.stopnote, 100*1000) {
                                noteclear(&sched.stopnote)
                                break
                        }
                        preemptall()
                }
        }

        // sanity checks
        bad := ""
        if sched.stopwait != 0 {
                bad = "stopTheWorld: not stopped (stopwait != 0)"
        } else {
                for _, pp := range allp {
                        if pp.status != _Pgcstop {
                                bad = "stopTheWorld: not stopped (status != _Pgcstop)"
                        }
                }
        }
        if freezing.Load() {
                // Some other thread is panicking. This can cause the
                // sanity checks above to fail if the panic happens in
                // the signal handler on a stopped thread. Either way,
                // we should halt this thread.
                lock(&deadlock)
                lock(&deadlock)
        }
        if bad != "" {
                throw(bad)
        }

        worldStopped()
}

func startTheWorldWithSema() int64 {
        assertWorldStopped()

        mp := acquirem() // disable preemption because it can be holding p in a local var
        if netpollinited() {
                list := netpoll(0) // non-blocking
                injectglist(&list)
        }
        lock(&sched.lock)

        procs := gomaxprocs
        if newprocs != 0 {
                procs = newprocs
                newprocs = 0
        }
        p1 := procresize(procs)
        sched.gcwaiting.Store(false)
        if sched.sysmonwait.Load() {
                sched.sysmonwait.Store(false)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)

        worldStarted()

        for p1 != nil {
                p := p1
                p1 = p1.link.ptr()
                if p.m != 0 {
                        mp := p.m.ptr()
                        p.m = 0
                        if mp.nextp != 0 {
                                throw("startTheWorld: inconsistent mp->nextp")
                        }
                        mp.nextp.set(p)
                        notewakeup(&mp.park)
                } else {
                        // Start M to run P.  Do not start another M below.
                        newm(nil, p, -1)
                }
        }

        // Capture start-the-world time before doing clean-up tasks.
        startTime := nanotime()
        if traceEnabled() {
                traceSTWDone()
        }

        // Wakeup an additional proc in case we have excessive runnable goroutines
        // in local queues or in the global queue. If we don't, the proc will park itself.
        // If we have lots of excessive work, resetspinning will unpark additional procs as necessary.
        wakep()

        releasem(mp)

        return startTime
}

// usesLibcall indicates whether this runtime performs system calls
// via libcall.
func usesLibcall() bool {
        switch GOOS {
        case "aix", "darwin", "illumos", "ios", "solaris", "windows":
                return true
        case "openbsd":
                return GOARCH == "386" || GOARCH == "amd64" || GOARCH == "arm" || GOARCH == "arm64"
        }
        return false
}

// mStackIsSystemAllocated indicates whether this runtime starts on a
// system-allocated stack.
func mStackIsSystemAllocated() bool {
        switch GOOS {
        case "aix", "darwin", "plan9", "illumos", "ios", "solaris", "windows":
                return true
        case "openbsd":
                switch GOARCH {
                case "386", "amd64", "arm", "arm64":
                        return true
                }
        }
        return false
}

// mstart is the entry-point for new Ms.
// It is written in assembly, uses ABI0, is marked TOPFRAME, and calls mstart0.
func mstart()

// mstart0 is the Go entry-point for new Ms.
// This must not split the stack because we may not even have stack
// bounds set up yet.
//
// May run during STW (because it doesn't have a P yet), so write
// barriers are not allowed.
//
//go:nosplit
//go:nowritebarrierrec
func mstart0() {
        gp := getg()

        osStack := gp.stack.lo == 0
        if osStack {
                // Initialize stack bounds from system stack.
                // Cgo may have left stack size in stack.hi.
                // minit may update the stack bounds.
                //
                // Note: these bounds may not be very accurate.
                // We set hi to &size, but there are things above
                // it. The 1024 is supposed to compensate this,
                // but is somewhat arbitrary.
                size := gp.stack.hi
                if size == 0 {
                        size = 8192 * sys.StackGuardMultiplier
                }
                gp.stack.hi = uintptr(noescape(unsafe.Pointer(&size)))
                gp.stack.lo = gp.stack.hi - size + 1024
        }
        // Initialize stack guard so that we can start calling regular
        // Go code.
        gp.stackguard0 = gp.stack.lo + stackGuard
        // This is the g0, so we can also call go:systemstack
        // functions, which check stackguard1.
        gp.stackguard1 = gp.stackguard0
        mstart1()

        // Exit this thread.
        if mStackIsSystemAllocated() {
                // Windows, Solaris, illumos, Darwin, AIX and Plan 9 always system-allocate
                // the stack, but put it in gp.stack before mstart,
                // so the logic above hasn't set osStack yet.
                osStack = true
        }
        mexit(osStack)
}

// The go:noinline is to guarantee the getcallerpc/getcallersp below are safe,
// so that we can set up g0.sched to return to the call of mstart1 above.
//
//go:noinline
func mstart1() {
        gp := getg()

        if gp != gp.m.g0 {
                throw("bad runtime·mstart")
        }

        // Set up m.g0.sched as a label returning to just
        // after the mstart1 call in mstart0 above, for use by goexit0 and mcall.
        // We're never coming back to mstart1 after we call schedule,
        // so other calls can reuse the current frame.
        // And goexit0 does a gogo that needs to return from mstart1
        // and let mstart0 exit the thread.
        gp.sched.g = guintptr(unsafe.Pointer(gp))
        gp.sched.pc = getcallerpc()
        gp.sched.sp = getcallersp()

        asminit()
        minit()

        // Install signal handlers; after minit so that minit can
        // prepare the thread to be able to handle the signals.
        if gp.m == &m0 {
                mstartm0()
        }

        if fn := gp.m.mstartfn; fn != nil {
                fn()
        }

        if gp.m != &m0 {
                acquirep(gp.m.nextp.ptr())
                gp.m.nextp = 0
        }
        schedule()
}

// mstartm0 implements part of mstart1 that only runs on the m0.
//
// Write barriers are allowed here because we know the GC can't be
// running yet, so they'll be no-ops.
//
//go:yeswritebarrierrec
func mstartm0() {
        // Create an extra M for callbacks on threads not created by Go.
        // An extra M is also needed on Windows for callbacks created by
        // syscall.NewCallback. See issue #6751 for details.
        if (iscgo || GOOS == "windows") && !cgoHasExtraM {
                cgoHasExtraM = true
                newextram()
        }
        initsig(false)
}

// mPark causes a thread to park itself, returning once woken.
//
//go:nosplit
func mPark() {
        gp := getg()
        notesleep(&gp.m.park)
        noteclear(&gp.m.park)
}

// mexit tears down and exits the current thread.
//
// Don't call this directly to exit the thread, since it must run at
// the top of the thread stack. Instead, use gogo(&gp.m.g0.sched) to
// unwind the stack to the point that exits the thread.
//
// It is entered with m.p != nil, so write barriers are allowed. It
// will release the P before exiting.
//
//go:yeswritebarrierrec
func mexit(osStack bool) {
        mp := getg().m

        if mp == &m0 {
                // This is the main thread. Just wedge it.
                //
                // On Linux, exiting the main thread puts the process
                // into a non-waitable zombie state. On Plan 9,
                // exiting the main thread unblocks wait even though
                // other threads are still running. On Solaris we can
                // neither exitThread nor return from mstart. Other
                // bad things probably happen on other platforms.
                //
                // We could try to clean up this M more before wedging
                // it, but that complicates signal handling.
                handoffp(releasep())
                lock(&sched.lock)
                sched.nmfreed++
                checkdead()
                unlock(&sched.lock)
                mPark()
                throw("locked m0 woke up")
        }

        sigblock(true)
        unminit()

        // Free the gsignal stack.
        if mp.gsignal != nil {
                stackfree(mp.gsignal.stack)
                // On some platforms, when calling into VDSO (e.g. nanotime)
                // we store our g on the gsignal stack, if there is one.
                // Now the stack is freed, unlink it from the m, so we
                // won't write to it when calling VDSO code.
                mp.gsignal = nil
        }

        // Remove m from allm.
        lock(&sched.lock)
        for pprev := &allm; *pprev != nil; pprev = &(*pprev).alllink {
                if *pprev == mp {
                        *pprev = mp.alllink
                        goto found
                }
        }
        throw("m not found in allm")
found:
        // Delay reaping m until it's done with the stack.
        //
        // Put mp on the free list, though it will not be reaped while freeWait
        // is freeMWait. mp is no longer reachable via allm, so even if it is
        // on an OS stack, we must keep a reference to mp alive so that the GC
        // doesn't free mp while we are still using it.
        //
        // Note that the free list must not be linked through alllink because
        // some functions walk allm without locking, so may be using alllink.
        mp.freeWait.Store(freeMWait)
        mp.freelink = sched.freem
        sched.freem = mp
        unlock(&sched.lock)

        atomic.Xadd64(&ncgocall, int64(mp.ncgocall))

        // Release the P.
        handoffp(releasep())
        // After this point we must not have write barriers.

        // Invoke the deadlock detector. This must happen after
        // handoffp because it may have started a new M to take our
        // P's work.
        lock(&sched.lock)
        sched.nmfreed++
        checkdead()
        unlock(&sched.lock)

        if GOOS == "darwin" || GOOS == "ios" {
                // Make sure pendingPreemptSignals is correct when an M exits.
                // For #41702.
                if mp.signalPending.Load() != 0 {
                        pendingPreemptSignals.Add(-1)
                }
        }

        // Destroy all allocated resources. After this is called, we may no
        // longer take any locks.
        mdestroy(mp)

        if osStack {
                // No more uses of mp, so it is safe to drop the reference.
                mp.freeWait.Store(freeMRef)

                // Return from mstart and let the system thread
                // library free the g0 stack and terminate the thread.
                return
        }

        // mstart is the thread's entry point, so there's nothing to
        // return to. Exit the thread directly. exitThread will clear
        // m.freeWait when it's done with the stack and the m can be
        // reaped.
        exitThread(&mp.freeWait)
}

// forEachP calls fn(p) for every P p when p reaches a GC safe point.
// If a P is currently executing code, this will bring the P to a GC
// safe point and execute fn on that P. If the P is not executing code
// (it is idle or in a syscall), this will call fn(p) directly while
// preventing the P from exiting its state. This does not ensure that
// fn will run on every CPU executing Go code, but it acts as a global
// memory barrier. GC uses this as a "ragged barrier."
//
// The caller must hold worldsema.
//
//go:systemstack
func forEachP(fn func(*p)) {
        mp := acquirem()
        pp := getg().m.p.ptr()

        lock(&sched.lock)
        if sched.safePointWait != 0 {
                throw("forEachP: sched.safePointWait != 0")
        }
        sched.safePointWait = gomaxprocs - 1
        sched.safePointFn = fn

        // Ask all Ps to run the safe point function.
        for _, p2 := range allp {
                if p2 != pp {
                        atomic.Store(&p2.runSafePointFn, 1)
                }
        }
        preemptall()

        // Any P entering _Pidle or _Psyscall from now on will observe
        // p.runSafePointFn == 1 and will call runSafePointFn when
        // changing its status to _Pidle/_Psyscall.

        // Run safe point function for all idle Ps. sched.pidle will
        // not change because we hold sched.lock.
        for p := sched.pidle.ptr(); p != nil; p = p.link.ptr() {
                if atomic.Cas(&p.runSafePointFn, 1, 0) {
                        fn(p)
                        sched.safePointWait--
                }
        }

        wait := sched.safePointWait > 0
        unlock(&sched.lock)

        // Run fn for the current P.
        fn(pp)

        // Force Ps currently in _Psyscall into _Pidle and hand them
        // off to induce safe point function execution.
        for _, p2 := range allp {
                s := p2.status
                if s == _Psyscall && p2.runSafePointFn == 1 && atomic.Cas(&p2.status, s, _Pidle) {
                        if traceEnabled() {
                                traceGoSysBlock(p2)
                                traceProcStop(p2)
                        }
                        p2.syscalltick++
                        handoffp(p2)
                }
        }

        // Wait for remaining Ps to run fn.
        if wait {
                for {
                        // Wait for 100us, then try to re-preempt in
                        // case of any races.
                        //
                        // Requires system stack.
                        if notetsleep(&sched.safePointNote, 100*1000) {
                                noteclear(&sched.safePointNote)
                                break
                        }
                        preemptall()
                }
        }
        if sched.safePointWait != 0 {
                throw("forEachP: not done")
        }
        for _, p2 := range allp {
                if p2.runSafePointFn != 0 {
                        throw("forEachP: P did not run fn")
                }
        }

        lock(&sched.lock)
        sched.safePointFn = nil
        unlock(&sched.lock)
        releasem(mp)
}

// runSafePointFn runs the safe point function, if any, for this P.
// This should be called like
//
//      if getg().m.p.runSafePointFn != 0 {
//          runSafePointFn()
//      }
//
// runSafePointFn must be checked on any transition in to _Pidle or
// _Psyscall to avoid a race where forEachP sees that the P is running
// just before the P goes into _Pidle/_Psyscall and neither forEachP
// nor the P run the safe-point function.
func runSafePointFn() {
        p := getg().m.p.ptr()
        // Resolve the race between forEachP running the safe-point
        // function on this P's behalf and this P running the
        // safe-point function directly.
        if !atomic.Cas(&p.runSafePointFn, 1, 0) {
                return
        }
        sched.safePointFn(p)
        lock(&sched.lock)
        sched.safePointWait--
        if sched.safePointWait == 0 {
                notewakeup(&sched.safePointNote)
        }
        unlock(&sched.lock)
}

// When running with cgo, we call _cgo_thread_start
// to start threads for us so that we can play nicely with
// foreign code.
var cgoThreadStart unsafe.Pointer

type cgothreadstart struct {
        g   guintptr
        tls *uint64
        fn  unsafe.Pointer
}

// Allocate a new m unassociated with any thread.
// Can use p for allocation context if needed.
// fn is recorded as the new m's m.mstartfn.
// id is optional pre-allocated m ID. Omit by passing -1.
//
// This function is allowed to have write barriers even if the caller
// isn't because it borrows pp.
//
//go:yeswritebarrierrec
func allocm(pp *p, fn func(), id int64) *m {
        allocmLock.rlock()

        // The caller owns pp, but we may borrow (i.e., acquirep) it. We must
        // disable preemption to ensure it is not stolen, which would make the
        // caller lose ownership.
        acquirem()

        gp := getg()
        if gp.m.p == 0 {
                acquirep(pp) // temporarily borrow p for mallocs in this function
        }

        // Release the free M list. We need to do this somewhere and
        // this may free up a stack we can use.
        if sched.freem != nil {
                lock(&sched.lock)
                var newList *m
                for freem := sched.freem; freem != nil; {
                        wait := freem.freeWait.Load()
                        if wait == freeMWait {
                                next := freem.freelink
                                freem.freelink = newList
                                newList = freem
                                freem = next
                                continue
                        }
                        // Free the stack if needed. For freeMRef, there is
                        // nothing to do except drop freem from the sched.freem
                        // list.
                        if wait == freeMStack {
                                // stackfree must be on the system stack, but allocm is
                                // reachable off the system stack transitively from
                                // startm.
                                systemstack(func() {
                                        stackfree(freem.g0.stack)
                                })
                        }
                        freem = freem.freelink
                }
                sched.freem = newList
                unlock(&sched.lock)
        }

        mp := new(m)
        mp.mstartfn = fn
        mcommoninit(mp, id)

        // In case of cgo or Solaris or illumos or Darwin, pthread_create will make us a stack.
        // Windows and Plan 9 will layout sched stack on OS stack.
        if iscgo || mStackIsSystemAllocated() {
                mp.g0 = malg(-1)
        } else {
                mp.g0 = malg(8192 * sys.StackGuardMultiplier)
        }
        mp.g0.m = mp

        if pp == gp.m.p.ptr() {
                releasep()
        }

        releasem(gp.m)
        allocmLock.runlock()
        return mp
}

// needm is called when a cgo callback happens on a
// thread without an m (a thread not created by Go).
// In this case, needm is expected to find an m to use
// and return with m, g initialized correctly.
// Since m and g are not set now (likely nil, but see below)
// needm is limited in what routines it can call. In particular
// it can only call nosplit functions (textflag 7) and cannot
// do any scheduling that requires an m.
//
// In order to avoid needing heavy lifting here, we adopt
// the following strategy: there is a stack of available m's
// that can be stolen. Using compare-and-swap
// to pop from the stack has ABA races, so we simulate
// a lock by doing an exchange (via Casuintptr) to steal the stack
// head and replace the top pointer with MLOCKED (1).
// This serves as a simple spin lock that we can use even
// without an m. The thread that locks the stack in this way
// unlocks the stack by storing a valid stack head pointer.
//
// In order to make sure that there is always an m structure
// available to be stolen, we maintain the invariant that there
// is always one more than needed. At the beginning of the
// program (if cgo is in use) the list is seeded with a single m.
// If needm finds that it has taken the last m off the list, its job
// is - once it has installed its own m so that it can do things like
// allocate memory - to create a spare m and put it on the list.
//
// Each of these extra m's also has a g0 and a curg that are
// pressed into service as the scheduling stack and current
// goroutine for the duration of the cgo callback.
//
// It calls dropm to put the m back on the list,
// 1. when the callback is done with the m in non-pthread platforms,
// 2. or when the C thread exiting on pthread platforms.
//
// The signal argument indicates whether we're called from a signal
// handler.
//
//go:nosplit
func needm(signal bool) {
        if (iscgo || GOOS == "windows") && !cgoHasExtraM {
                // Can happen if C/C++ code calls Go from a global ctor.
                // Can also happen on Windows if a global ctor uses a
                // callback created by syscall.NewCallback. See issue #6751
                // for details.
                //
                // Can not throw, because scheduler is not initialized yet.
                writeErrStr("fatal error: cgo callback before cgo call\n")
                exit(1)
        }

        // Save and block signals before getting an M.
        // The signal handler may call needm itself,
        // and we must avoid a deadlock. Also, once g is installed,
        // any incoming signals will try to execute,
        // but we won't have the sigaltstack settings and other data
        // set up appropriately until the end of minit, which will
        // unblock the signals. This is the same dance as when
        // starting a new m to run Go code via newosproc.
        var sigmask sigset
        sigsave(&sigmask)
        sigblock(false)

        // getExtraM is safe here because of the invariant above,
        // that the extra list always contains or will soon contain
        // at least one m.
        mp, last := getExtraM()

        // Set needextram when we've just emptied the list,
        // so that the eventual call into cgocallbackg will
        // allocate a new m for the extra list. We delay the
        // allocation until then so that it can be done
        // after exitsyscall makes sure it is okay to be
        // running at all (that is, there's no garbage collection
        // running right now).
        mp.needextram = last

        // Store the original signal mask for use by minit.
        mp.sigmask = sigmask

        // Install TLS on some platforms (previously setg
        // would do this if necessary).
        osSetupTLS(mp)

        // Install g (= m->g0) and set the stack bounds
        // to match the current stack. If we don't actually know
        // how big the stack is, like we don't know how big any
        // scheduling stack is, but we assume there's at least 32 kB.
        // If we can get a more accurate stack bound from pthread,
        // use that.
        setg(mp.g0)
        gp := getg()
        gp.stack.hi = getcallersp() + 1024
        gp.stack.lo = getcallersp() - 32*1024
        if !signal && _cgo_getstackbound != nil {
                // Don't adjust if called from the signal handler.
                // We are on the signal stack, not the pthread stack.
                // (We could get the stack bounds from sigaltstack, but
                // we're getting out of the signal handler very soon
                // anyway. Not worth it.)
                var bounds [2]uintptr
                asmcgocall(_cgo_getstackbound, unsafe.Pointer(&bounds))
                // getstackbound is an unsupported no-op on Windows.
                if bounds[0] != 0 {
                        gp.stack.lo = bounds[0]
                        gp.stack.hi = bounds[1]
                }
        }
        gp.stackguard0 = gp.stack.lo + stackGuard

        // Should mark we are already in Go now.
        // Otherwise, we may call needm again when we get a signal, before cgocallbackg1,
        // which means the extram list may be empty, that will cause a deadlock.
        mp.isExtraInC = false

        // Initialize this thread to use the m.
        asminit()
        minit()

        // mp.curg is now a real goroutine.
        casgstatus(mp.curg, _Gdead, _Gsyscall)
        sched.ngsys.Add(-1)
}

// Acquire an extra m and bind it to the C thread when a pthread key has been created.
//
//go:nosplit
func needAndBindM() {
        needm(false)

        if _cgo_pthread_key_created != nil && *(*uintptr)(_cgo_pthread_key_created) != 0 {
                cgoBindM()
        }
}

// newextram allocates m's and puts them on the extra list.
// It is called with a working local m, so that it can do things
// like call schedlock and allocate.
func newextram() {
        c := extraMWaiters.Swap(0)
        if c > 0 {
                for i := uint32(0); i < c; i++ {
                        oneNewExtraM()
                }
        } else if extraMLength.Load() == 0 {
                // Make sure there is at least one extra M.
                oneNewExtraM()
        }
}

// oneNewExtraM allocates an m and puts it on the extra list.
func oneNewExtraM() {
        // Create extra goroutine locked to extra m.
        // The goroutine is the context in which the cgo callback will run.
        // The sched.pc will never be returned to, but setting it to
        // goexit makes clear to the traceback routines where
        // the goroutine stack ends.
        mp := allocm(nil, nil, -1)
        gp := malg(4096)
        gp.sched.pc = abi.FuncPCABI0(goexit) + sys.PCQuantum
        gp.sched.sp = gp.stack.hi
        gp.sched.sp -= 4 * goarch.PtrSize // extra space in case of reads slightly beyond frame
        gp.sched.lr = 0
        gp.sched.g = guintptr(unsafe.Pointer(gp))
        gp.syscallpc = gp.sched.pc
        gp.syscallsp = gp.sched.sp
        gp.stktopsp = gp.sched.sp
        // malg returns status as _Gidle. Change to _Gdead before
        // adding to allg where GC can see it. We use _Gdead to hide
        // this from tracebacks and stack scans since it isn't a
        // "real" goroutine until needm grabs it.
        casgstatus(gp, _Gidle, _Gdead)
        gp.m = mp
        mp.curg = gp
        mp.isextra = true
        // mark we are in C by default.
        mp.isExtraInC = true
        mp.lockedInt++
        mp.lockedg.set(gp)
        gp.lockedm.set(mp)
        gp.goid = sched.goidgen.Add(1)
        if raceenabled {
                gp.racectx = racegostart(abi.FuncPCABIInternal(newextram) + sys.PCQuantum)
        }
        if traceEnabled() {
                traceOneNewExtraM(gp)
        }
        // put on allg for garbage collector
        allgadd(gp)

        // gp is now on the allg list, but we don't want it to be
        // counted by gcount. It would be more "proper" to increment
        // sched.ngfree, but that requires locking. Incrementing ngsys
        // has the same effect.
        sched.ngsys.Add(1)

        // Add m to the extra list.
        addExtraM(mp)
}

// dropm puts the current m back onto the extra list.
//
// 1. On systems without pthreads, like Windows
// dropm is called when a cgo callback has called needm but is now
// done with the callback and returning back into the non-Go thread.
//
// The main expense here is the call to signalstack to release the
// m's signal stack, and then the call to needm on the next callback
// from this thread. It is tempting to try to save the m for next time,
// which would eliminate both these costs, but there might not be
// a next time: the current thread (which Go does not control) might exit.
// If we saved the m for that thread, there would be an m leak each time
// such a thread exited. Instead, we acquire and release an m on each
// call. These should typically not be scheduling operations, just a few
// atomics, so the cost should be small.
//
// 2. On systems with pthreads
// dropm is called while a non-Go thread is exiting.
// We allocate a pthread per-thread variable using pthread_key_create,
// to register a thread-exit-time destructor.
// And store the g into a thread-specific value associated with the pthread key,
// when first return back to C.
// So that the destructor would invoke dropm while the non-Go thread is exiting.
// This is much faster since it avoids expensive signal-related syscalls.
//
// NOTE: this always runs without a P, so, nowritebarrierrec required.
//
//go:nowritebarrierrec
func dropm() {
        // Clear m and g, and return m to the extra list.
        // After the call to setg we can only call nosplit functions
        // with no pointer manipulation.
        mp := getg().m

        // Return mp.curg to dead state.
        casgstatus(mp.curg, _Gsyscall, _Gdead)
        mp.curg.preemptStop = false
        sched.ngsys.Add(1)

        // Block signals before unminit.
        // Unminit unregisters the signal handling stack (but needs g on some systems).
        // Setg(nil) clears g, which is the signal handler's cue not to run Go handlers.
        // It's important not to try to handle a signal between those two steps.
        sigmask := mp.sigmask
        sigblock(false)
        unminit()

        setg(nil)

        putExtraM(mp)

        msigrestore(sigmask)
}

// bindm store the g0 of the current m into a thread-specific value.
//
// We allocate a pthread per-thread variable using pthread_key_create,
// to register a thread-exit-time destructor.
// We are here setting the thread-specific value of the pthread key, to enable the destructor.
// So that the pthread_key_destructor would dropm while the C thread is exiting.
//
// And the saved g will be used in pthread_key_destructor,
// since the g stored in the TLS by Go might be cleared in some platforms,
// before the destructor invoked, so, we restore g by the stored g, before dropm.
//
// We store g0 instead of m, to make the assembly code simpler,
// since we need to restore g0 in runtime.cgocallback.
//
// On systems without pthreads, like Windows, bindm shouldn't be used.
//
// NOTE: this always runs without a P, so, nowritebarrierrec required.
//
//go:nosplit
//go:nowritebarrierrec
func cgoBindM() {
        if GOOS == "windows" || GOOS == "plan9" {
                fatal("bindm in unexpected GOOS")
        }
        g := getg()
        if g.m.g0 != g {
                fatal("the current g is not g0")
        }
        if _cgo_bindm != nil {
                asmcgocall(_cgo_bindm, unsafe.Pointer(g))
        }
}

// A helper function for EnsureDropM.
func getm() uintptr {
        return uintptr(unsafe.Pointer(getg().m))
}

var (
        // Locking linked list of extra M's, via mp.schedlink. Must be accessed
        // only via lockextra/unlockextra.
        //
        // Can't be atomic.Pointer[m] because we use an invalid pointer as a
        // "locked" sentinel value. M's on this list remain visible to the GC
        // because their mp.curg is on allgs.
        extraM atomic.Uintptr
        // Number of M's in the extraM list.
        extraMLength atomic.Uint32
        // Number of waiters in lockextra.
        extraMWaiters atomic.Uint32

        // Number of extra M's in use by threads.
        extraMInUse atomic.Uint32
)

// lockextra locks the extra list and returns the list head.
// The caller must unlock the list by storing a new list head
// to extram. If nilokay is true, then lockextra will
// return a nil list head if that's what it finds. If nilokay is false,
// lockextra will keep waiting until the list head is no longer nil.
//
//go:nosplit
func lockextra(nilokay bool) *m {
        const locked = 1

        incr := false
        for {
                old := extraM.Load()
                if old == locked {
                        osyield_no_g()
                        continue
                }
                if old == 0 && !nilokay {
                        if !incr {
                                // Add 1 to the number of threads
                                // waiting for an M.
                                // This is cleared by newextram.
                                extraMWaiters.Add(1)
                                incr = true
                        }
                        usleep_no_g(1)
                        continue
                }
                if extraM.CompareAndSwap(old, locked) {
                        return (*m)(unsafe.Pointer(old))
                }
                osyield_no_g()
                continue
        }
}

//go:nosplit
func unlockextra(mp *m, delta int32) {
        extraMLength.Add(delta)
        extraM.Store(uintptr(unsafe.Pointer(mp)))
}

// Return an M from the extra M list. Returns last == true if the list becomes
// empty because of this call.
//
// Spins waiting for an extra M, so caller must ensure that the list always
// contains or will soon contain at least one M.
//
//go:nosplit
func getExtraM() (mp *m, last bool) {
        mp = lockextra(false)
        extraMInUse.Add(1)
        unlockextra(mp.schedlink.ptr(), -1)
        return mp, mp.schedlink.ptr() == nil
}

// Returns an extra M back to the list. mp must be from getExtraM. Newly
// allocated M's should use addExtraM.
//
//go:nosplit
func putExtraM(mp *m) {
        extraMInUse.Add(-1)
        addExtraM(mp)
}

// Adds a newly allocated M to the extra M list.
//
//go:nosplit
func addExtraM(mp *m) {
        mnext := lockextra(true)
        mp.schedlink.set(mnext)
        unlockextra(mp, 1)
}

var (
        // allocmLock is locked for read when creating new Ms in allocm and their
        // addition to allm. Thus acquiring this lock for write blocks the
        // creation of new Ms.
        allocmLock rwmutex

        // execLock serializes exec and clone to avoid bugs or unspecified
        // behaviour around exec'ing while creating/destroying threads. See
        // issue #19546.
        execLock rwmutex
)

// These errors are reported (via writeErrStr) by some OS-specific
// versions of newosproc and newosproc0.
const (
        failthreadcreate  = "runtime: failed to create new OS thread\n"
        failallocatestack = "runtime: failed to allocate stack for the new OS thread\n"
)

// newmHandoff contains a list of m structures that need new OS threads.
// This is used by newm in situations where newm itself can't safely
// start an OS thread.
var newmHandoff struct {
        lock mutex

        // newm points to a list of M structures that need new OS
        // threads. The list is linked through m.schedlink.
        newm muintptr

        // waiting indicates that wake needs to be notified when an m
        // is put on the list.
        waiting bool
        wake    note

        // haveTemplateThread indicates that the templateThread has
        // been started. This is not protected by lock. Use cas to set
        // to 1.
        haveTemplateThread uint32
}

// Create a new m. It will start off with a call to fn, or else the scheduler.
// fn needs to be static and not a heap allocated closure.
// May run with m.p==nil, so write barriers are not allowed.
//
// id is optional pre-allocated m ID. Omit by passing -1.
//
//go:nowritebarrierrec
func newm(fn func(), pp *p, id int64) {
        // allocm adds a new M to allm, but they do not start until created by
        // the OS in newm1 or the template thread.
        //
        // doAllThreadsSyscall requires that every M in allm will eventually
        // start and be signal-able, even with a STW.
        //
        // Disable preemption here until we start the thread to ensure that
        // newm is not preempted between allocm and starting the new thread,
        // ensuring that anything added to allm is guaranteed to eventually
        // start.
        acquirem()

        mp := allocm(pp, fn, id)
        mp.nextp.set(pp)
        mp.sigmask = initSigmask
        if gp := getg(); gp != nil && gp.m != nil && (gp.m.lockedExt != 0 || gp.m.incgo) && GOOS != "plan9" {
                // We're on a locked M or a thread that may have been
                // started by C. The kernel state of this thread may
                // be strange (the user may have locked it for that
                // purpose). We don't want to clone that into another
                // thread. Instead, ask a known-good thread to create
                // the thread for us.
                //
                // This is disabled on Plan 9. See golang.org/issue/22227.
                //
                // TODO: This may be unnecessary on Windows, which
                // doesn't model thread creation off fork.
                lock(&newmHandoff.lock)
                if newmHandoff.haveTemplateThread == 0 {
                        throw("on a locked thread with no template thread")
                }
                mp.schedlink = newmHandoff.newm
                newmHandoff.newm.set(mp)
                if newmHandoff.waiting {
                        newmHandoff.waiting = false
                        notewakeup(&newmHandoff.wake)
                }
                unlock(&newmHandoff.lock)
                // The M has not started yet, but the template thread does not
                // participate in STW, so it will always process queued Ms and
                // it is safe to releasem.
                releasem(getg().m)
                return
        }
        newm1(mp)
        releasem(getg().m)
}

func newm1(mp *m) {
        if iscgo {
                var ts cgothreadstart
                if _cgo_thread_start == nil {
                        throw("_cgo_thread_start missing")
                }
                ts.g.set(mp.g0)
                ts.tls = (*uint64)(unsafe.Pointer(&mp.tls[0]))
                ts.fn = unsafe.Pointer(abi.FuncPCABI0(mstart))
                if msanenabled {
                        msanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
                }
                if asanenabled {
                        asanwrite(unsafe.Pointer(&ts), unsafe.Sizeof(ts))
                }
                execLock.rlock() // Prevent process clone.
                asmcgocall(_cgo_thread_start, unsafe.Pointer(&ts))
                execLock.runlock()
                return
        }
        execLock.rlock() // Prevent process clone.
        newosproc(mp)
        execLock.runlock()
}

// startTemplateThread starts the template thread if it is not already
// running.
//
// The calling thread must itself be in a known-good state.
func startTemplateThread() {
        if GOARCH == "wasm" { // no threads on wasm yet
                return
        }

        // Disable preemption to guarantee that the template thread will be
        // created before a park once haveTemplateThread is set.
        mp := acquirem()
        if !atomic.Cas(&newmHandoff.haveTemplateThread, 0, 1) {
                releasem(mp)
                return
        }
        newm(templateThread, nil, -1)
        releasem(mp)
}

// templateThread is a thread in a known-good state that exists solely
// to start new threads in known-good states when the calling thread
// may not be in a good state.
//
// Many programs never need this, so templateThread is started lazily
// when we first enter a state that might lead to running on a thread
// in an unknown state.
//
// templateThread runs on an M without a P, so it must not have write
// barriers.
//
//go:nowritebarrierrec
func templateThread() {
        lock(&sched.lock)
        sched.nmsys++
        checkdead()
        unlock(&sched.lock)

        for {
                lock(&newmHandoff.lock)
                for newmHandoff.newm != 0 {
                        newm := newmHandoff.newm.ptr()
                        newmHandoff.newm = 0
                        unlock(&newmHandoff.lock)
                        for newm != nil {
                                next := newm.schedlink.ptr()
                                newm.schedlink = 0
                                newm1(newm)
                                newm = next
                        }
                        lock(&newmHandoff.lock)
                }
                newmHandoff.waiting = true
                noteclear(&newmHandoff.wake)
                unlock(&newmHandoff.lock)
                notesleep(&newmHandoff.wake)
        }
}

// Stops execution of the current m until new work is available.
// Returns with acquired P.
func stopm() {
        gp := getg()

        if gp.m.locks != 0 {
                throw("stopm holding locks")
        }
        if gp.m.p != 0 {
                throw("stopm holding p")
        }
        if gp.m.spinning {
                throw("stopm spinning")
        }

        lock(&sched.lock)
        mput(gp.m)
        unlock(&sched.lock)
        mPark()
        acquirep(gp.m.nextp.ptr())
        gp.m.nextp = 0
}

func mspinning() {
        // startm's caller incremented nmspinning. Set the new M's spinning.
        getg().m.spinning = true
}

// Schedules some M to run the p (creates an M if necessary).
// If p==nil, tries to get an idle P, if no idle P's does nothing.
// May run with m.p==nil, so write barriers are not allowed.
// If spinning is set, the caller has incremented nmspinning and must provide a
// P. startm will set m.spinning in the newly started M.
//
// Callers passing a non-nil P must call from a non-preemptible context. See
// comment on acquirem below.
//
// Argument lockheld indicates whether the caller already acquired the
// scheduler lock. Callers holding the lock when making the call must pass
// true. The lock might be temporarily dropped, but will be reacquired before
// returning.
//
// Must not have write barriers because this may be called without a P.
//
//go:nowritebarrierrec
func startm(pp *p, spinning, lockheld bool) {
        // Disable preemption.
        //
        // Every owned P must have an owner that will eventually stop it in the
        // event of a GC stop request. startm takes transient ownership of a P
        // (either from argument or pidleget below) and transfers ownership to
        // a started M, which will be responsible for performing the stop.
        //
        // Preemption must be disabled during this transient ownership,
        // otherwise the P this is running on may enter GC stop while still
        // holding the transient P, leaving that P in limbo and deadlocking the
        // STW.
        //
        // Callers passing a non-nil P must already be in non-preemptible
        // context, otherwise such preemption could occur on function entry to
        // startm. Callers passing a nil P may be preemptible, so we must
        // disable preemption before acquiring a P from pidleget below.
        mp := acquirem()
        if !lockheld {
                lock(&sched.lock)
        }
        if pp == nil {
                if spinning {
                        // TODO(prattmic): All remaining calls to this function
                        // with _p_ == nil could be cleaned up to find a P
                        // before calling startm.
                        throw("startm: P required for spinning=true")
                }
                pp, _ = pidleget(0)
                if pp == nil {
                        if !lockheld {
                                unlock(&sched.lock)
                        }
                        releasem(mp)
                        return
                }
        }
        nmp := mget()
        if nmp == nil {
                // No M is available, we must drop sched.lock and call newm.
                // However, we already own a P to assign to the M.
                //
                // Once sched.lock is released, another G (e.g., in a syscall),
                // could find no idle P while checkdead finds a runnable G but
                // no running M's because this new M hasn't started yet, thus
                // throwing in an apparent deadlock.
                // This apparent deadlock is possible when startm is called
                // from sysmon, which doesn't count as a running M.
                //
                // Avoid this situation by pre-allocating the ID for the new M,
                // thus marking it as 'running' before we drop sched.lock. This
                // new M will eventually run the scheduler to execute any
                // queued G's.
                id := mReserveID()
                unlock(&sched.lock)

                var fn func()
                if spinning {
                        // The caller incremented nmspinning, so set m.spinning in the new M.
                        fn = mspinning
                }
                newm(fn, pp, id)

                if lockheld {
                        lock(&sched.lock)
                }
                // Ownership transfer of pp committed by start in newm.
                // Preemption is now safe.
                releasem(mp)
                return
        }
        if !lockheld {
                unlock(&sched.lock)
        }
        if nmp.spinning {
                throw("startm: m is spinning")
        }
        if nmp.nextp != 0 {
                throw("startm: m has p")
        }
        if spinning && !runqempty(pp) {
                throw("startm: p has runnable gs")
        }
        // The caller incremented nmspinning, so set m.spinning in the new M.
        nmp.spinning = spinning
        nmp.nextp.set(pp)
        notewakeup(&nmp.park)
        // Ownership transfer of pp committed by wakeup. Preemption is now
        // safe.
        releasem(mp)
}

// Hands off P from syscall or locked M.
// Always runs without a P, so write barriers are not allowed.
//
//go:nowritebarrierrec
func handoffp(pp *p) {
        // handoffp must start an M in any situation where
        // findrunnable would return a G to run on pp.

        // if it has local work, start it straight away
        if !runqempty(pp) || sched.runqsize != 0 {
                startm(pp, false, false)
                return
        }
        // if there's trace work to do, start it straight away
        if (traceEnabled() || traceShuttingDown()) && traceReaderAvailable() != nil {
                startm(pp, false, false)
                return
        }
        // if it has GC work, start it straight away
        if gcBlackenEnabled != 0 && gcMarkWorkAvailable(pp) {
                startm(pp, false, false)
                return
        }
        // no local work, check that there are no spinning/idle M's,
        // otherwise our help is not required
        if sched.nmspinning.Load()+sched.npidle.Load() == 0 && sched.nmspinning.CompareAndSwap(0, 1) { // TODO: fast atomic
                sched.needspinning.Store(0)
                startm(pp, true, false)
                return
        }
        lock(&sched.lock)
        if sched.gcwaiting.Load() {
                pp.status = _Pgcstop
                sched.stopwait--
                if sched.stopwait == 0 {
                        notewakeup(&sched.stopnote)
                }
                unlock(&sched.lock)
                return
        }
        if pp.runSafePointFn != 0 && atomic.Cas(&pp.runSafePointFn, 1, 0) {
                sched.safePointFn(pp)
                sched.safePointWait--
                if sched.safePointWait == 0 {
                        notewakeup(&sched.safePointNote)
                }
        }
        if sched.runqsize != 0 {
                unlock(&sched.lock)
                startm(pp, false, false)
                return
        }
        // If this is the last running P and nobody is polling network,
        // need to wakeup another M to poll network.
        if sched.npidle.Load() == gomaxprocs-1 && sched.lastpoll.Load() != 0 {
                unlock(&sched.lock)
                startm(pp, false, false)
                return
        }

        // The scheduler lock cannot be held when calling wakeNetPoller below
        // because wakeNetPoller may call wakep which may call startm.
        when := nobarrierWakeTime(pp)
        pidleput(pp, 0)
        unlock(&sched.lock)

        if when != 0 {
                wakeNetPoller(when)
        }
}

// Tries to add one more P to execute G's.
// Called when a G is made runnable (newproc, ready).
// Must be called with a P.
func wakep() {
        // Be conservative about spinning threads, only start one if none exist
        // already.
        if sched.nmspinning.Load() != 0 || !sched.nmspinning.CompareAndSwap(0, 1) {
                return
        }

        // Disable preemption until ownership of pp transfers to the next M in
        // startm. Otherwise preemption here would leave pp stuck waiting to
        // enter _Pgcstop.
        //
        // See preemption comment on acquirem in startm for more details.
        mp := acquirem()

        var pp *p
        lock(&sched.lock)
        pp, _ = pidlegetSpinning(0)
        if pp == nil {
                if sched.nmspinning.Add(-1) < 0 {
                        throw("wakep: negative nmspinning")
                }
                unlock(&sched.lock)
                releasem(mp)
                return
        }
        // Since we always have a P, the race in the "No M is available"
        // comment in startm doesn't apply during the small window between the
        // unlock here and lock in startm. A checkdead in between will always
        // see at least one running M (ours).
        unlock(&sched.lock)

        startm(pp, true, false)

        releasem(mp)
}

// Stops execution of the current m that is locked to a g until the g is runnable again.
// Returns with acquired P.
func stoplockedm() {
        gp := getg()

        if gp.m.lockedg == 0 || gp.m.lockedg.ptr().lockedm.ptr() != gp.m {
                throw("stoplockedm: inconsistent locking")
        }
        if gp.m.p != 0 {
                // Schedule another M to run this p.
                pp := releasep()
                handoffp(pp)
        }
        incidlelocked(1)
        // Wait until another thread schedules lockedg again.
        mPark()
        status := readgstatus(gp.m.lockedg.ptr())
        if status&^_Gscan != _Grunnable {
                print("runtime:stoplockedm: lockedg (atomicstatus=", status, ") is not Grunnable or Gscanrunnable\n")
                dumpgstatus(gp.m.lockedg.ptr())
                throw("stoplockedm: not runnable")
        }
        acquirep(gp.m.nextp.ptr())
        gp.m.nextp = 0
}

// Schedules the locked m to run the locked gp.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func startlockedm(gp *g) {
        mp := gp.lockedm.ptr()
        if mp == getg().m {
                throw("startlockedm: locked to me")
        }
        if mp.nextp != 0 {
                throw("startlockedm: m has p")
        }
        // directly handoff current P to the locked m
        incidlelocked(-1)
        pp := releasep()
        mp.nextp.set(pp)
        notewakeup(&mp.park)
        stopm()
}

// Stops the current m for stopTheWorld.
// Returns when the world is restarted.
func gcstopm() {
        gp := getg()

        if !sched.gcwaiting.Load() {
                throw("gcstopm: not waiting for gc")
        }
        if gp.m.spinning {
                gp.m.spinning = false
                // OK to just drop nmspinning here,
                // startTheWorld will unpark threads as necessary.
                if sched.nmspinning.Add(-1) < 0 {
                        throw("gcstopm: negative nmspinning")
                }
        }
        pp := releasep()
        lock(&sched.lock)
        pp.status = _Pgcstop
        sched.stopwait--
        if sched.stopwait == 0 {
                notewakeup(&sched.stopnote)
        }
        unlock(&sched.lock)
        stopm()
}

// Schedules gp to run on the current M.
// If inheritTime is true, gp inherits the remaining time in the
// current time slice. Otherwise, it starts a new time slice.
// Never returns.
//
// Write barriers are allowed because this is called immediately after
// acquiring a P in several places.
//
//go:yeswritebarrierrec
func execute(gp *g, inheritTime bool) {
        mp := getg().m

        if goroutineProfile.active {
                // Make sure that gp has had its stack written out to the goroutine
                // profile, exactly as it was when the goroutine profiler first stopped
                // the world.
                tryRecordGoroutineProfile(gp, osyield)
        }

        // Assign gp.m before entering _Grunning so running Gs have an
        // M.
        mp.curg = gp
        gp.m = mp
        casgstatus(gp, _Grunnable, _Grunning)
        gp.waitsince = 0
        gp.preempt = false
        gp.stackguard0 = gp.stack.lo + stackGuard
        if !inheritTime {
                mp.p.ptr().schedtick++
        }

        // Check whether the profiler needs to be turned on or off.
        hz := sched.profilehz
        if mp.profilehz != hz {
                setThreadCPUProfiler(hz)
        }

        if traceEnabled() {
                // GoSysExit has to happen when we have a P, but before GoStart.
                // So we emit it here.
                if gp.syscallsp != 0 {
                        traceGoSysExit()
                }
                traceGoStart()
        }

        gogo(&gp.sched)
}

// Finds a runnable goroutine to execute.
// Tries to steal from other P's, get g from local or global queue, poll network.
// tryWakeP indicates that the returned goroutine is not normal (GC worker, trace
// reader) so the caller should try to wake a P.
func findRunnable() (gp *g, inheritTime, tryWakeP bool) {
        mp := getg().m

        // The conditions here and in handoffp must agree: if
        // findrunnable would return a G to run, handoffp must start
        // an M.

top:
        pp := mp.p.ptr()
        if sched.gcwaiting.Load() {
                gcstopm()
                goto top
        }
        if pp.runSafePointFn != 0 {
                runSafePointFn()
        }

        // now and pollUntil are saved for work stealing later,
        // which may steal timers. It's important that between now
        // and then, nothing blocks, so these numbers remain mostly
        // relevant.
        now, pollUntil, _ := checkTimers(pp, 0)

        // Try to schedule the trace reader.
        if traceEnabled() || traceShuttingDown() {
                gp := traceReader()
                if gp != nil {
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        traceGoUnpark(gp, 0)
                        return gp, false, true
                }
        }

        // Try to schedule a GC worker.
        if gcBlackenEnabled != 0 {
                gp, tnow := gcController.findRunnableGCWorker(pp, now)
                if gp != nil {
                        return gp, false, true
                }
                now = tnow
        }

        // Check the global runnable queue once in a while to ensure fairness.
        // Otherwise two goroutines can completely occupy the local runqueue
        // by constantly respawning each other.
        if pp.schedtick%61 == 0 && sched.runqsize > 0 {
                lock(&sched.lock)
                gp := globrunqget(pp, 1)
                unlock(&sched.lock)
                if gp != nil {
                        return gp, false, false
                }
        }

        // Wake up the finalizer G.
        if fingStatus.Load()&(fingWait|fingWake) == fingWait|fingWake {
                if gp := wakefing(); gp != nil {
                        ready(gp, 0, true)
                }
        }
        if *cgo_yield != nil {
                asmcgocall(*cgo_yield, nil)
        }

        // local runq
        if gp, inheritTime := runqget(pp); gp != nil {
                return gp, inheritTime, false
        }

        // global runq
        if sched.runqsize != 0 {
                lock(&sched.lock)
                gp := globrunqget(pp, 0)
                unlock(&sched.lock)
                if gp != nil {
                        return gp, false, false
                }
        }

        // Poll network.
        // This netpoll is only an optimization before we resort to stealing.
        // We can safely skip it if there are no waiters or a thread is blocked
        // in netpoll already. If there is any kind of logical race with that
        // blocked thread (e.g. it has already returned from netpoll, but does
        // not set lastpoll yet), this thread will do blocking netpoll below
        // anyway.
        if netpollinited() && netpollWaiters.Load() > 0 && sched.lastpoll.Load() != 0 {
                if list := netpoll(0); !list.empty() { // non-blocking
                        gp := list.pop()
                        injectglist(&list)
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        if traceEnabled() {
                                traceGoUnpark(gp, 0)
                        }
                        return gp, false, false
                }
        }

        // Spinning Ms: steal work from other Ps.
        //
        // Limit the number of spinning Ms to half the number of busy Ps.
        // This is necessary to prevent excessive CPU consumption when
        // GOMAXPROCS>>1 but the program parallelism is low.
        if mp.spinning || 2*sched.nmspinning.Load() < gomaxprocs-sched.npidle.Load() {
                if !mp.spinning {
                        mp.becomeSpinning()
                }

                gp, inheritTime, tnow, w, newWork := stealWork(now)
                if gp != nil {
                        // Successfully stole.
                        return gp, inheritTime, false
                }
                if newWork {
                        // There may be new timer or GC work; restart to
                        // discover.
                        goto top
                }

                now = tnow
                if w != 0 && (pollUntil == 0 || w < pollUntil) {
                        // Earlier timer to wait for.
                        pollUntil = w
                }
        }

        // We have nothing to do.
        //
        // If we're in the GC mark phase, can safely scan and blacken objects,
        // and have work to do, run idle-time marking rather than give up the P.
        if gcBlackenEnabled != 0 && gcMarkWorkAvailable(pp) && gcController.addIdleMarkWorker() {
                node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
                if node != nil {
                        pp.gcMarkWorkerMode = gcMarkWorkerIdleMode
                        gp := node.gp.ptr()
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        if traceEnabled() {
                                traceGoUnpark(gp, 0)
                        }
                        return gp, false, false
                }
                gcController.removeIdleMarkWorker()
        }

        // wasm only:
        // If a callback returned and no other goroutine is awake,
        // then wake event handler goroutine which pauses execution
        // until a callback was triggered.
        gp, otherReady := beforeIdle(now, pollUntil)
        if gp != nil {
                casgstatus(gp, _Gwaiting, _Grunnable)
                if traceEnabled() {
                        traceGoUnpark(gp, 0)
                }
                return gp, false, false
        }
        if otherReady {
                goto top
        }

        // Before we drop our P, make a snapshot of the allp slice,
        // which can change underfoot once we no longer block
        // safe-points. We don't need to snapshot the contents because
        // everything up to cap(allp) is immutable.
        allpSnapshot := allp
        // Also snapshot masks. Value changes are OK, but we can't allow
        // len to change out from under us.
        idlepMaskSnapshot := idlepMask
        timerpMaskSnapshot := timerpMask

        // return P and block
        lock(&sched.lock)
        if sched.gcwaiting.Load() || pp.runSafePointFn != 0 {
                unlock(&sched.lock)
                goto top
        }
        if sched.runqsize != 0 {
                gp := globrunqget(pp, 0)
                unlock(&sched.lock)
                return gp, false, false
        }
        if !mp.spinning && sched.needspinning.Load() == 1 {
                // See "Delicate dance" comment below.
                mp.becomeSpinning()
                unlock(&sched.lock)
                goto top
        }
        if releasep() != pp {
                throw("findrunnable: wrong p")
        }
        now = pidleput(pp, now)
        unlock(&sched.lock)

        // Delicate dance: thread transitions from spinning to non-spinning
        // state, potentially concurrently with submission of new work. We must
        // drop nmspinning first and then check all sources again (with
        // #StoreLoad memory barrier in between). If we do it the other way
        // around, another thread can submit work after we've checked all
        // sources but before we drop nmspinning; as a result nobody will
        // unpark a thread to run the work.
        //
        // This applies to the following sources of work:
        //
        // * Goroutines added to a per-P run queue.
        // * New/modified-earlier timers on a per-P timer heap.
        // * Idle-priority GC work (barring golang.org/issue/19112).
        //
        // If we discover new work below, we need to restore m.spinning as a
        // signal for resetspinning to unpark a new worker thread (because
        // there can be more than one starving goroutine).
        //
        // However, if after discovering new work we also observe no idle Ps
        // (either here or in resetspinning), we have a problem. We may be
        // racing with a non-spinning M in the block above, having found no
        // work and preparing to release its P and park. Allowing that P to go
        // idle will result in loss of work conservation (idle P while there is
        // runnable work). This could result in complete deadlock in the
        // unlikely event that we discover new work (from netpoll) right as we
        // are racing with _all_ other Ps going idle.
        //
        // We use sched.needspinning to synchronize with non-spinning Ms going
        // idle. If needspinning is set when they are about to drop their P,
        // they abort the drop and instead become a new spinning M on our
        // behalf. If we are not racing and the system is truly fully loaded
        // then no spinning threads are required, and the next thread to
        // naturally become spinning will clear the flag.
        //
        // Also see "Worker thread parking/unparking" comment at the top of the
        // file.
        wasSpinning := mp.spinning
        if mp.spinning {
                mp.spinning = false
                if sched.nmspinning.Add(-1) < 0 {
                        throw("findrunnable: negative nmspinning")
                }

                // Note the for correctness, only the last M transitioning from
                // spinning to non-spinning must perform these rechecks to
                // ensure no missed work. However, the runtime has some cases
                // of transient increments of nmspinning that are decremented
                // without going through this path, so we must be conservative
                // and perform the check on all spinning Ms.
                //
                // See https://go.dev/issue/43997.

                // Check all runqueues once again.
                pp := checkRunqsNoP(allpSnapshot, idlepMaskSnapshot)
                if pp != nil {
                        acquirep(pp)
                        mp.becomeSpinning()
                        goto top
                }

                // Check for idle-priority GC work again.
                pp, gp := checkIdleGCNoP()
                if pp != nil {
                        acquirep(pp)
                        mp.becomeSpinning()

                        // Run the idle worker.
                        pp.gcMarkWorkerMode = gcMarkWorkerIdleMode
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        if traceEnabled() {
                                traceGoUnpark(gp, 0)
                        }
                        return gp, false, false
                }

                // Finally, check for timer creation or expiry concurrently with
                // transitioning from spinning to non-spinning.
                //
                // Note that we cannot use checkTimers here because it calls
                // adjusttimers which may need to allocate memory, and that isn't
                // allowed when we don't have an active P.
                pollUntil = checkTimersNoP(allpSnapshot, timerpMaskSnapshot, pollUntil)
        }

        // Poll network until next timer.
        if netpollinited() && (netpollWaiters.Load() > 0 || pollUntil != 0) && sched.lastpoll.Swap(0) != 0 {
                sched.pollUntil.Store(pollUntil)
                if mp.p != 0 {
                        throw("findrunnable: netpoll with p")
                }
                if mp.spinning {
                        throw("findrunnable: netpoll with spinning")
                }
                delay := int64(-1)
                if pollUntil != 0 {
                        if now == 0 {
                                now = nanotime()
                        }
                        delay = pollUntil - now
                        if delay < 0 {
                                delay = 0
                        }
                }
                if faketime != 0 {
                        // When using fake time, just poll.
                        delay = 0
                }
                list := netpoll(delay) // block until new work is available
                // Refresh now again, after potentially blocking.
                now = nanotime()
                sched.pollUntil.Store(0)
                sched.lastpoll.Store(now)
                if faketime != 0 && list.empty() {
                        // Using fake time and nothing is ready; stop M.
                        // When all M's stop, checkdead will call timejump.
                        stopm()
                        goto top
                }
                lock(&sched.lock)
                pp, _ := pidleget(now)
                unlock(&sched.lock)
                if pp == nil {
                        injectglist(&list)
                } else {
                        acquirep(pp)
                        if !list.empty() {
                                gp := list.pop()
                                injectglist(&list)
                                casgstatus(gp, _Gwaiting, _Grunnable)
                                if traceEnabled() {
                                        traceGoUnpark(gp, 0)
                                }
                                return gp, false, false
                        }
                        if wasSpinning {
                                mp.becomeSpinning()
                        }
                        goto top
                }
        } else if pollUntil != 0 && netpollinited() {
                pollerPollUntil := sched.pollUntil.Load()
                if pollerPollUntil == 0 || pollerPollUntil > pollUntil {
                        netpollBreak()
                }
        }
        stopm()
        goto top
}

// pollWork reports whether there is non-background work this P could
// be doing. This is a fairly lightweight check to be used for
// background work loops, like idle GC. It checks a subset of the
// conditions checked by the actual scheduler.
func pollWork() bool {
        if sched.runqsize != 0 {
                return true
        }
        p := getg().m.p.ptr()
        if !runqempty(p) {
                return true
        }
        if netpollinited() && netpollWaiters.Load() > 0 && sched.lastpoll.Load() != 0 {
                if list := netpoll(0); !list.empty() {
                        injectglist(&list)
                        return true
                }
        }
        return false
}

// stealWork attempts to steal a runnable goroutine or timer from any P.
//
// If newWork is true, new work may have been readied.
//
// If now is not 0 it is the current time. stealWork returns the passed time or
// the current time if now was passed as 0.
func stealWork(now int64) (gp *g, inheritTime bool, rnow, pollUntil int64, newWork bool) {
        pp := getg().m.p.ptr()

        ranTimer := false

        const stealTries = 4
        for i := 0; i < stealTries; i++ {
                stealTimersOrRunNextG := i == stealTries-1

                for enum := stealOrder.start(fastrand()); !enum.done(); enum.next() {
                        if sched.gcwaiting.Load() {
                                // GC work may be available.
                                return nil, false, now, pollUntil, true
                        }
                        p2 := allp[enum.position()]
                        if pp == p2 {
                                continue
                        }

                        // Steal timers from p2. This call to checkTimers is the only place
                        // where we might hold a lock on a different P's timers. We do this
                        // once on the last pass before checking runnext because stealing
                        // from the other P's runnext should be the last resort, so if there
                        // are timers to steal do that first.
                        //
                        // We only check timers on one of the stealing iterations because
                        // the time stored in now doesn't change in this loop and checking
                        // the timers for each P more than once with the same value of now
                        // is probably a waste of time.
                        //
                        // timerpMask tells us whether the P may have timers at all. If it
                        // can't, no need to check at all.
                        if stealTimersOrRunNextG && timerpMask.read(enum.position()) {
                                tnow, w, ran := checkTimers(p2, now)
                                now = tnow
                                if w != 0 && (pollUntil == 0 || w < pollUntil) {
                                        pollUntil = w
                                }
                                if ran {
                                        // Running the timers may have
                                        // made an arbitrary number of G's
                                        // ready and added them to this P's
                                        // local run queue. That invalidates
                                        // the assumption of runqsteal
                                        // that it always has room to add
                                        // stolen G's. So check now if there
                                        // is a local G to run.
                                        if gp, inheritTime := runqget(pp); gp != nil {
                                                return gp, inheritTime, now, pollUntil, ranTimer
                                        }
                                        ranTimer = true
                                }
                        }

                        // Don't bother to attempt to steal if p2 is idle.
                        if !idlepMask.read(enum.position()) {
                                if gp := runqsteal(pp, p2, stealTimersOrRunNextG); gp != nil {
                                        return gp, false, now, pollUntil, ranTimer
                                }
                        }
                }
        }

        // No goroutines found to steal. Regardless, running a timer may have
        // made some goroutine ready that we missed. Indicate the next timer to
        // wait for.
        return nil, false, now, pollUntil, ranTimer
}

// Check all Ps for a runnable G to steal.
//
// On entry we have no P. If a G is available to steal and a P is available,
// the P is returned which the caller should acquire and attempt to steal the
// work to.
func checkRunqsNoP(allpSnapshot []*p, idlepMaskSnapshot pMask) *p {
        for id, p2 := range allpSnapshot {
                if !idlepMaskSnapshot.read(uint32(id)) && !runqempty(p2) {
                        lock(&sched.lock)
                        pp, _ := pidlegetSpinning(0)
                        if pp == nil {
                                // Can't get a P, don't bother checking remaining Ps.
                                unlock(&sched.lock)
                                return nil
                        }
                        unlock(&sched.lock)
                        return pp
                }
        }

        // No work available.
        return nil
}

// Check all Ps for a timer expiring sooner than pollUntil.
//
// Returns updated pollUntil value.
func checkTimersNoP(allpSnapshot []*p, timerpMaskSnapshot pMask, pollUntil int64) int64 {
        for id, p2 := range allpSnapshot {
                if timerpMaskSnapshot.read(uint32(id)) {
                        w := nobarrierWakeTime(p2)
                        if w != 0 && (pollUntil == 0 || w < pollUntil) {
                                pollUntil = w
                        }
                }
        }

        return pollUntil
}

// Check for idle-priority GC, without a P on entry.
//
// If some GC work, a P, and a worker G are all available, the P and G will be
// returned. The returned P has not been wired yet.
func checkIdleGCNoP() (*p, *g) {
        // N.B. Since we have no P, gcBlackenEnabled may change at any time; we
        // must check again after acquiring a P. As an optimization, we also check
        // if an idle mark worker is needed at all. This is OK here, because if we
        // observe that one isn't needed, at least one is currently running. Even if
        // it stops running, its own journey into the scheduler should schedule it
        // again, if need be (at which point, this check will pass, if relevant).
        if atomic.Load(&gcBlackenEnabled) == 0 || !gcController.needIdleMarkWorker() {
                return nil, nil
        }
        if !gcMarkWorkAvailable(nil) {
                return nil, nil
        }

        // Work is available; we can start an idle GC worker only if there is
        // an available P and available worker G.
        //
        // We can attempt to acquire these in either order, though both have
        // synchronization concerns (see below). Workers are almost always
        // available (see comment in findRunnableGCWorker for the one case
        // there may be none). Since we're slightly less likely to find a P,
        // check for that first.
        //
        // Synchronization: note that we must hold sched.lock until we are
        // committed to keeping it. Otherwise we cannot put the unnecessary P
        // back in sched.pidle without performing the full set of idle
        // transition checks.
        //
        // If we were to check gcBgMarkWorkerPool first, we must somehow handle
        // the assumption in gcControllerState.findRunnableGCWorker that an
        // empty gcBgMarkWorkerPool is only possible if gcMarkDone is running.
        lock(&sched.lock)
        pp, now := pidlegetSpinning(0)
        if pp == nil {
                unlock(&sched.lock)
                return nil, nil
        }

        // Now that we own a P, gcBlackenEnabled can't change (as it requires STW).
        if gcBlackenEnabled == 0 || !gcController.addIdleMarkWorker() {
                pidleput(pp, now)
                unlock(&sched.lock)
                return nil, nil
        }

        node := (*gcBgMarkWorkerNode)(gcBgMarkWorkerPool.pop())
        if node == nil {
                pidleput(pp, now)
                unlock(&sched.lock)
                gcController.removeIdleMarkWorker()
                return nil, nil
        }

        unlock(&sched.lock)

        return pp, node.gp.ptr()
}

// wakeNetPoller wakes up the thread sleeping in the network poller if it isn't
// going to wake up before the when argument; or it wakes an idle P to service
// timers and the network poller if there isn't one already.
func wakeNetPoller(when int64) {
        if sched.lastpoll.Load() == 0 {
                // In findrunnable we ensure that when polling the pollUntil
                // field is either zero or the time to which the current
                // poll is expected to run. This can have a spurious wakeup
                // but should never miss a wakeup.
                pollerPollUntil := sched.pollUntil.Load()
                if pollerPollUntil == 0 || pollerPollUntil > when {
                        netpollBreak()
                }
        } else {
                // There are no threads in the network poller, try to get
                // one there so it can handle new timers.
                if GOOS != "plan9" { // Temporary workaround - see issue #42303.
                        wakep()
                }
        }
}

func resetspinning() {
        gp := getg()
        if !gp.m.spinning {
                throw("resetspinning: not a spinning m")
        }
        gp.m.spinning = false
        nmspinning := sched.nmspinning.Add(-1)
        if nmspinning < 0 {
                throw("findrunnable: negative nmspinning")
        }
        // M wakeup policy is deliberately somewhat conservative, so check if we
        // need to wakeup another P here. See "Worker thread parking/unparking"
        // comment at the top of the file for details.
        wakep()
}

// injectglist adds each runnable G on the list to some run queue,
// and clears glist. If there is no current P, they are added to the
// global queue, and up to npidle M's are started to run them.
// Otherwise, for each idle P, this adds a G to the global queue
// and starts an M. Any remaining G's are added to the current P's
// local run queue.
// This may temporarily acquire sched.lock.
// Can run concurrently with GC.
func injectglist(glist *gList) {
        if glist.empty() {
                return
        }
        if traceEnabled() {
                for gp := glist.head.ptr(); gp != nil; gp = gp.schedlink.ptr() {
                        traceGoUnpark(gp, 0)
                }
        }

        // Mark all the goroutines as runnable before we put them
        // on the run queues.
        head := glist.head.ptr()
        var tail *g
        qsize := 0
        for gp := head; gp != nil; gp = gp.schedlink.ptr() {
                tail = gp
                qsize++
                casgstatus(gp, _Gwaiting, _Grunnable)
        }

        // Turn the gList into a gQueue.
        var q gQueue
        q.head.set(head)
        q.tail.set(tail)
        *glist = gList{}

        startIdle := func(n int) {
                for i := 0; i < n; i++ {
                        mp := acquirem() // See comment in startm.
                        lock(&sched.lock)

                        pp, _ := pidlegetSpinning(0)
                        if pp == nil {
                                unlock(&sched.lock)
                                releasem(mp)
                                break
                        }

                        startm(pp, false, true)
                        unlock(&sched.lock)
                        releasem(mp)
                }
        }

        pp := getg().m.p.ptr()
        if pp == nil {
                lock(&sched.lock)
                globrunqputbatch(&q, int32(qsize))
                unlock(&sched.lock)
                startIdle(qsize)
                return
        }

        npidle := int(sched.npidle.Load())
        var globq gQueue
        var n int
        for n = 0; n < npidle && !q.empty(); n++ {
                g := q.pop()
                globq.pushBack(g)
        }
        if n > 0 {
                lock(&sched.lock)
                globrunqputbatch(&globq, int32(n))
                unlock(&sched.lock)
                startIdle(n)
                qsize -= n
        }

        if !q.empty() {
                runqputbatch(pp, &q, qsize)
        }
}

// One round of scheduler: find a runnable goroutine and execute it.
// Never returns.
func schedule() {
        mp := getg().m

        if mp.locks != 0 {
                throw("schedule: holding locks")
        }

        if mp.lockedg != 0 {
                stoplockedm()
                execute(mp.lockedg.ptr(), false) // Never returns.
        }

        // We should not schedule away from a g that is executing a cgo call,
        // since the cgo call is using the m's g0 stack.
        if mp.incgo {
                throw("schedule: in cgo")
        }

top:
        pp := mp.p.ptr()
        pp.preempt = false

        // Safety check: if we are spinning, the run queue should be empty.
        // Check this before calling checkTimers, as that might call
        // goready to put a ready goroutine on the local run queue.
        if mp.spinning && (pp.runnext != 0 || pp.runqhead != pp.runqtail) {
                throw("schedule: spinning with local work")
        }

        gp, inheritTime, tryWakeP := findRunnable() // blocks until work is available

        // This thread is going to run a goroutine and is not spinning anymore,
        // so if it was marked as spinning we need to reset it now and potentially
        // start a new spinning M.
        if mp.spinning {
                resetspinning()
        }

        if sched.disable.user && !schedEnabled(gp) {
                // Scheduling of this goroutine is disabled. Put it on
                // the list of pending runnable goroutines for when we
                // re-enable user scheduling and look again.
                lock(&sched.lock)
                if schedEnabled(gp) {
                        // Something re-enabled scheduling while we
                        // were acquiring the lock.
                        unlock(&sched.lock)
                } else {
                        sched.disable.runnable.pushBack(gp)
                        sched.disable.n++
                        unlock(&sched.lock)
                        goto top
                }
        }

        // If about to schedule a not-normal goroutine (a GCworker or tracereader),
        // wake a P if there is one.
        if tryWakeP {
                wakep()
        }
        if gp.lockedm != 0 {
                // Hands off own p to the locked m,
                // then blocks waiting for a new p.
                startlockedm(gp)
                goto top
        }

        execute(gp, inheritTime)
}

// dropg removes the association between m and the current goroutine m->curg (gp for short).
// Typically a caller sets gp's status away from Grunning and then
// immediately calls dropg to finish the job. The caller is also responsible
// for arranging that gp will be restarted using ready at an
// appropriate time. After calling dropg and arranging for gp to be
// readied later, the caller can do other work but eventually should
// call schedule to restart the scheduling of goroutines on this m.
func dropg() {
        gp := getg()

        setMNoWB(&gp.m.curg.m, nil)
        setGNoWB(&gp.m.curg, nil)
}

// checkTimers runs any timers for the P that are ready.
// If now is not 0 it is the current time.
// It returns the passed time or the current time if now was passed as 0.
// and the time when the next timer should run or 0 if there is no next timer,
// and reports whether it ran any timers.
// If the time when the next timer should run is not 0,
// it is always larger than the returned time.
// We pass now in and out to avoid extra calls of nanotime.
//
//go:yeswritebarrierrec
func checkTimers(pp *p, now int64) (rnow, pollUntil int64, ran bool) {
        // If it's not yet time for the first timer, or the first adjusted
        // timer, then there is nothing to do.
        next := pp.timer0When.Load()
        nextAdj := pp.timerModifiedEarliest.Load()
        if next == 0 || (nextAdj != 0 && nextAdj < next) {
                next = nextAdj
        }

        if next == 0 {
                // No timers to run or adjust.
                return now, 0, false
        }

        if now == 0 {
                now = nanotime()
        }
        if now < next {
                // Next timer is not ready to run, but keep going
                // if we would clear deleted timers.
                // This corresponds to the condition below where
                // we decide whether to call clearDeletedTimers.
                if pp != getg().m.p.ptr() || int(pp.deletedTimers.Load()) <= int(pp.numTimers.Load()/4) {
                        return now, next, false
                }
        }

        lock(&pp.timersLock)

        if len(pp.timers) > 0 {
                adjusttimers(pp, now)
                for len(pp.timers) > 0 {
                        // Note that runtimer may temporarily unlock
                        // pp.timersLock.
                        if tw := runtimer(pp, now); tw != 0 {
                                if tw > 0 {
                                        pollUntil = tw
                                }
                                break
                        }
                        ran = true
                }
        }

        // If this is the local P, and there are a lot of deleted timers,
        // clear them out. We only do this for the local P to reduce
        // lock contention on timersLock.
        if pp == getg().m.p.ptr() && int(pp.deletedTimers.Load()) > len(pp.timers)/4 {
                clearDeletedTimers(pp)
        }

        unlock(&pp.timersLock)

        return now, pollUntil, ran
}

func parkunlock_c(gp *g, lock unsafe.Pointer) bool {
        unlock((*mutex)(lock))
        return true
}

// park continuation on g0.
func park_m(gp *g) {
        mp := getg().m

        if traceEnabled() {
                traceGoPark(mp.waitTraceBlockReason, mp.waitTraceSkip)
        }

        // N.B. Not using casGToWaiting here because the waitreason is
        // set by park_m's caller.
        casgstatus(gp, _Grunning, _Gwaiting)
        dropg()

        if fn := mp.waitunlockf; fn != nil {
                ok := fn(gp, mp.waitlock)
                mp.waitunlockf = nil
                mp.waitlock = nil
                if !ok {
                        if traceEnabled() {
                                traceGoUnpark(gp, 2)
                        }
                        casgstatus(gp, _Gwaiting, _Grunnable)
                        execute(gp, true) // Schedule it back, never returns.
                }
        }
        schedule()
}

func goschedImpl(gp *g) {
        status := readgstatus(gp)
        if status&^_Gscan != _Grunning {
                dumpgstatus(gp)
                throw("bad g status")
        }
        casgstatus(gp, _Grunning, _Grunnable)
        dropg()
        lock(&sched.lock)
        globrunqput(gp)
        unlock(&sched.lock)

        schedule()
}

// Gosched continuation on g0.
func gosched_m(gp *g) {
        if traceEnabled() {
                traceGoSched()
        }
        goschedImpl(gp)
}

// goschedguarded is a forbidden-states-avoided version of gosched_m.
func goschedguarded_m(gp *g) {

        if !canPreemptM(gp.m) {
                gogo(&gp.sched) // never return
        }

        if traceEnabled() {
                traceGoSched()
        }
        goschedImpl(gp)
}

func gopreempt_m(gp *g) {
        if traceEnabled() {
                traceGoPreempt()
        }
        goschedImpl(gp)
}

// preemptPark parks gp and puts it in _Gpreempted.
//
//go:systemstack
func preemptPark(gp *g) {
        if traceEnabled() {
                traceGoPark(traceBlockPreempted, 0)
        }
        status := readgstatus(gp)
        if status&^_Gscan != _Grunning {
                dumpgstatus(gp)
                throw("bad g status")
        }

        if gp.asyncSafePoint {
                // Double-check that async preemption does not
                // happen in SPWRITE assembly functions.
                // isAsyncSafePoint must exclude this case.
                f := findfunc(gp.sched.pc)
                if !f.valid() {
                        throw("preempt at unknown pc")
                }
                if f.flag&abi.FuncFlagSPWrite != 0 {
                        println("runtime: unexpected SPWRITE function", funcname(f), "in async preempt")
                        throw("preempt SPWRITE")
                }
        }

        // Transition from _Grunning to _Gscan|_Gpreempted. We can't
        // be in _Grunning when we dropg because then we'd be running
        // without an M, but the moment we're in _Gpreempted,
        // something could claim this G before we've fully cleaned it
        // up. Hence, we set the scan bit to lock down further
        // transitions until we can dropg.
        casGToPreemptScan(gp, _Grunning, _Gscan|_Gpreempted)
        dropg()
        casfrom_Gscanstatus(gp, _Gscan|_Gpreempted, _Gpreempted)
        schedule()
}

// goyield is like Gosched, but it:
// - emits a GoPreempt trace event instead of a GoSched trace event
// - puts the current G on the runq of the current P instead of the globrunq
func goyield() {
        checkTimeouts()
        mcall(goyield_m)
}

func goyield_m(gp *g) {
        if traceEnabled() {
                traceGoPreempt()
        }
        pp := gp.m.p.ptr()
        casgstatus(gp, _Grunning, _Grunnable)
        dropg()
        runqput(pp, gp, false)
        schedule()
}

// Finishes execution of the current goroutine.
func goexit1() {
        if raceenabled {
                racegoend()
        }
        if traceEnabled() {
                traceGoEnd()
        }
        mcall(goexit0)
}

// goexit continuation on g0.
func goexit0(gp *g) {
        mp := getg().m
        pp := mp.p.ptr()

        casgstatus(gp, _Grunning, _Gdead)
        gcController.addScannableStack(pp, -int64(gp.stack.hi-gp.stack.lo))
        if isSystemGoroutine(gp, false) {
                sched.ngsys.Add(-1)
        }
        gp.m = nil
        locked := gp.lockedm != 0
        gp.lockedm = 0
        mp.lockedg = 0
        gp.preemptStop = false
        gp.paniconfault = false
        gp._defer = nil // should be true already but just in case.
        gp._panic = nil // non-nil for Goexit during panic. points at stack-allocated data.
        gp.writebuf = nil
        gp.waitreason = waitReasonZero
        gp.param = nil
        gp.labels = nil
        gp.timer = nil

        if gcBlackenEnabled != 0 && gp.gcAssistBytes > 0 {
                // Flush assist credit to the global pool. This gives
                // better information to pacing if the application is
                // rapidly creating an exiting goroutines.
                assistWorkPerByte := gcController.assistWorkPerByte.Load()
                scanCredit := int64(assistWorkPerByte * float64(gp.gcAssistBytes))
                gcController.bgScanCredit.Add(scanCredit)
                gp.gcAssistBytes = 0
        }

        dropg()

        if GOARCH == "wasm" { // no threads yet on wasm
                gfput(pp, gp)
                schedule() // never returns
        }

        if mp.lockedInt != 0 {
                print("invalid m->lockedInt = ", mp.lockedInt, "\n")
                throw("internal lockOSThread error")
        }
        gfput(pp, gp)
        if locked {
                // The goroutine may have locked this thread because
                // it put it in an unusual kernel state. Kill it
                // rather than returning it to the thread pool.

                // Return to mstart, which will release the P and exit
                // the thread.
                if GOOS != "plan9" { // See golang.org/issue/22227.
                        gogo(&mp.g0.sched)
                } else {
                        // Clear lockedExt on plan9 since we may end up re-using
                        // this thread.
                        mp.lockedExt = 0
                }
        }
        schedule()
}

// save updates getg().sched to refer to pc and sp so that a following
// gogo will restore pc and sp.
//
// save must not have write barriers because invoking a write barrier
// can clobber getg().sched.
//
//go:nosplit
//go:nowritebarrierrec
func save(pc, sp uintptr) {
        gp := getg()

        if gp == gp.m.g0 || gp == gp.m.gsignal {
                // m.g0.sched is special and must describe the context
                // for exiting the thread. mstart1 writes to it directly.
                // m.gsignal.sched should not be used at all.
                // This check makes sure save calls do not accidentally
                // run in contexts where they'd write to system g's.
                throw("save on system g not allowed")
        }

        gp.sched.pc = pc
        gp.sched.sp = sp
        gp.sched.lr = 0
        gp.sched.ret = 0
        // We need to ensure ctxt is zero, but can't have a write
        // barrier here. However, it should always already be zero.
        // Assert that.
        if gp.sched.ctxt != nil {
                badctxt()
        }
}

// The goroutine g is about to enter a system call.
// Record that it's not using the cpu anymore.
// This is called only from the go syscall library and cgocall,
// not from the low-level system calls used by the runtime.
//
// Entersyscall cannot split the stack: the save must
// make g->sched refer to the caller's stack segment, because
// entersyscall is going to return immediately after.
//
// Nothing entersyscall calls can split the stack either.
// We cannot safely move the stack during an active call to syscall,
// because we do not know which of the uintptr arguments are
// really pointers (back into the stack).
// In practice, this means that we make the fast path run through
// entersyscall doing no-split things, and the slow path has to use systemstack
// to run bigger things on the system stack.
//
// reentersyscall is the entry point used by cgo callbacks, where explicitly
// saved SP and PC are restored. This is needed when exitsyscall will be called
// from a function further up in the call stack than the parent, as g->syscallsp
// must always point to a valid stack frame. entersyscall below is the normal
// entry point for syscalls, which obtains the SP and PC from the caller.
//
// Syscall tracing:
// At the start of a syscall we emit traceGoSysCall to capture the stack trace.
// If the syscall does not block, that is it, we do not emit any other events.
// If the syscall blocks (that is, P is retaken), retaker emits traceGoSysBlock;
// when syscall returns we emit traceGoSysExit and when the goroutine starts running
// (potentially instantly, if exitsyscallfast returns true) we emit traceGoStart.
// To ensure that traceGoSysExit is emitted strictly after traceGoSysBlock,
// we remember current value of syscalltick in m (gp.m.syscalltick = gp.m.p.ptr().syscalltick),
// whoever emits traceGoSysBlock increments p.syscalltick afterwards;
// and we wait for the increment before emitting traceGoSysExit.
// Note that the increment is done even if tracing is not enabled,
// because tracing can be enabled in the middle of syscall. We don't want the wait to hang.
//
//go:nosplit
func reentersyscall(pc, sp uintptr) {
        gp := getg()

        // Disable preemption because during this function g is in Gsyscall status,
        // but can have inconsistent g->sched, do not let GC observe it.
        gp.m.locks++

        // Entersyscall must not call any function that might split/grow the stack.
        // (See details in comment above.)
        // Catch calls that might, by replacing the stack guard with something that
        // will trip any stack check and leaving a flag to tell newstack to die.
        gp.stackguard0 = stackPreempt
        gp.throwsplit = true

        // Leave SP around for GC and traceback.
        save(pc, sp)
        gp.syscallsp = sp
        gp.syscallpc = pc
        casgstatus(gp, _Grunning, _Gsyscall)
        if staticLockRanking {
                // When doing static lock ranking casgstatus can call
                // systemstack which clobbers g.sched.
                save(pc, sp)
        }
        if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
                systemstack(func() {
                        print("entersyscall inconsistent ", hex(gp.syscallsp), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
                        throw("entersyscall")
                })
        }

        if traceEnabled() {
                systemstack(traceGoSysCall)
                // systemstack itself clobbers g.sched.{pc,sp} and we might
                // need them later when the G is genuinely blocked in a
                // syscall
                save(pc, sp)
        }

        if sched.sysmonwait.Load() {
                systemstack(entersyscall_sysmon)
                save(pc, sp)
        }

        if gp.m.p.ptr().runSafePointFn != 0 {
                // runSafePointFn may stack split if run on this stack
                systemstack(runSafePointFn)
                save(pc, sp)
        }

        gp.m.syscalltick = gp.m.p.ptr().syscalltick
        pp := gp.m.p.ptr()
        pp.m = 0
        gp.m.oldp.set(pp)
        gp.m.p = 0
        atomic.Store(&pp.status, _Psyscall)
        if sched.gcwaiting.Load() {
                systemstack(entersyscall_gcwait)
                save(pc, sp)
        }

        gp.m.locks--
}

// Standard syscall entry used by the go syscall library and normal cgo calls.
//
// This is exported via linkname to assembly in the syscall package and x/sys.
//
//go:nosplit
//go:linkname entersyscall
func entersyscall() {
        reentersyscall(getcallerpc(), getcallersp())
}

func entersyscall_sysmon() {
        lock(&sched.lock)
        if sched.sysmonwait.Load() {
                sched.sysmonwait.Store(false)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
}

func entersyscall_gcwait() {
        gp := getg()
        pp := gp.m.oldp.ptr()

        lock(&sched.lock)
        if sched.stopwait > 0 && atomic.Cas(&pp.status, _Psyscall, _Pgcstop) {
                if traceEnabled() {
                        traceGoSysBlock(pp)
                        traceProcStop(pp)
                }
                pp.syscalltick++
                if sched.stopwait--; sched.stopwait == 0 {
                        notewakeup(&sched.stopnote)
                }
        }
        unlock(&sched.lock)
}

// The same as entersyscall(), but with a hint that the syscall is blocking.
//
//go:nosplit
func entersyscallblock() {
        gp := getg()

        gp.m.locks++ // see comment in entersyscall
        gp.throwsplit = true
        gp.stackguard0 = stackPreempt // see comment in entersyscall
        gp.m.syscalltick = gp.m.p.ptr().syscalltick
        gp.m.p.ptr().syscalltick++

        // Leave SP around for GC and traceback.
        pc := getcallerpc()
        sp := getcallersp()
        save(pc, sp)
        gp.syscallsp = gp.sched.sp
        gp.syscallpc = gp.sched.pc
        if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
                sp1 := sp
                sp2 := gp.sched.sp
                sp3 := gp.syscallsp
                systemstack(func() {
                        print("entersyscallblock inconsistent ", hex(sp1), " ", hex(sp2), " ", hex(sp3), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
                        throw("entersyscallblock")
                })
        }
        casgstatus(gp, _Grunning, _Gsyscall)
        if gp.syscallsp < gp.stack.lo || gp.stack.hi < gp.syscallsp {
                systemstack(func() {
                        print("entersyscallblock inconsistent ", hex(sp), " ", hex(gp.sched.sp), " ", hex(gp.syscallsp), " [", hex(gp.stack.lo), ",", hex(gp.stack.hi), "]\n")
                        throw("entersyscallblock")
                })
        }

        systemstack(entersyscallblock_handoff)

        // Resave for traceback during blocked call.
        save(getcallerpc(), getcallersp())

        gp.m.locks--
}

func entersyscallblock_handoff() {
        if traceEnabled() {
                traceGoSysCall()
                traceGoSysBlock(getg().m.p.ptr())
        }
        handoffp(releasep())
}

// The goroutine g exited its system call.
// Arrange for it to run on a cpu again.
// This is called only from the go syscall library, not
// from the low-level system calls used by the runtime.
//
// Write barriers are not allowed because our P may have been stolen.
//
// This is exported via linkname to assembly in the syscall package.
//
//go:nosplit
//go:nowritebarrierrec
//go:linkname exitsyscall
func exitsyscall() {
        gp := getg()

        gp.m.locks++ // see comment in entersyscall
        if getcallersp() > gp.syscallsp {
                throw("exitsyscall: syscall frame is no longer valid")
        }

        gp.waitsince = 0
        oldp := gp.m.oldp.ptr()
        gp.m.oldp = 0
        if exitsyscallfast(oldp) {
                // When exitsyscallfast returns success, we have a P so can now use
                // write barriers
                if goroutineProfile.active {
                        // Make sure that gp has had its stack written out to the goroutine
                        // profile, exactly as it was when the goroutine profiler first
                        // stopped the world.
                        systemstack(func() {
                                tryRecordGoroutineProfileWB(gp)
                        })
                }
                if traceEnabled() {
                        if oldp != gp.m.p.ptr() || gp.m.syscalltick != gp.m.p.ptr().syscalltick {
                                systemstack(traceGoStart)
                        }
                }
                // There's a cpu for us, so we can run.
                gp.m.p.ptr().syscalltick++
                // We need to cas the status and scan before resuming...
                casgstatus(gp, _Gsyscall, _Grunning)

                // Garbage collector isn't running (since we are),
                // so okay to clear syscallsp.
                gp.syscallsp = 0
                gp.m.locks--
                if gp.preempt {
                        // restore the preemption request in case we've cleared it in newstack
                        gp.stackguard0 = stackPreempt
                } else {
                        // otherwise restore the real stackGuard, we've spoiled it in entersyscall/entersyscallblock
                        gp.stackguard0 = gp.stack.lo + stackGuard
                }
                gp.throwsplit = false

                if sched.disable.user && !schedEnabled(gp) {
                        // Scheduling of this goroutine is disabled.
                        Gosched()
                }

                return
        }

        if traceEnabled() {
                // Wait till traceGoSysBlock event is emitted.
                // This ensures consistency of the trace (the goroutine is started after it is blocked).
                for oldp != nil && oldp.syscalltick == gp.m.syscalltick {
                        osyield()
                }
                // We can't trace syscall exit right now because we don't have a P.
                // Tracing code can invoke write barriers that cannot run without a P.
                // So instead we remember the syscall exit time and emit the event
                // in execute when we have a P.
                gp.trace.sysExitTime = traceClockNow()
        }

        gp.m.locks--

        // Call the scheduler.
        mcall(exitsyscall0)

        // Scheduler returned, so we're allowed to run now.
        // Delete the syscallsp information that we left for
        // the garbage collector during the system call.
        // Must wait until now because until gosched returns
        // we don't know for sure that the garbage collector
        // is not running.
        gp.syscallsp = 0
        gp.m.p.ptr().syscalltick++
        gp.throwsplit = false
}

//go:nosplit
func exitsyscallfast(oldp *p) bool {
        gp := getg()

        // Freezetheworld sets stopwait but does not retake P's.
        if sched.stopwait == freezeStopWait {
                return false
        }

        // Try to re-acquire the last P.
        if oldp != nil && oldp.status == _Psyscall && atomic.Cas(&oldp.status, _Psyscall, _Pidle) {
                // There's a cpu for us, so we can run.
                wirep(oldp)
                exitsyscallfast_reacquired()
                return true
        }

        // Try to get any other idle P.
        if sched.pidle != 0 {
                var ok bool
                systemstack(func() {
                        ok = exitsyscallfast_pidle()
                        if ok && traceEnabled() {
                                if oldp != nil {
                                        // Wait till traceGoSysBlock event is emitted.
                                        // This ensures consistency of the trace (the goroutine is started after it is blocked).
                                        for oldp.syscalltick == gp.m.syscalltick {
                                                osyield()
                                        }
                                }
                                traceGoSysExit()
                        }
                })
                if ok {
                        return true
                }
        }
        return false
}

// exitsyscallfast_reacquired is the exitsyscall path on which this G
// has successfully reacquired the P it was running on before the
// syscall.
//
//go:nosplit
func exitsyscallfast_reacquired() {
        gp := getg()
        if gp.m.syscalltick != gp.m.p.ptr().syscalltick {
                if traceEnabled() {
                        // The p was retaken and then enter into syscall again (since gp.m.syscalltick has changed).
                        // traceGoSysBlock for this syscall was already emitted,
                        // but here we effectively retake the p from the new syscall running on the same p.
                        systemstack(func() {
                                // Denote blocking of the new syscall.
                                traceGoSysBlock(gp.m.p.ptr())
                                // Denote completion of the current syscall.
                                traceGoSysExit()
                        })
                }
                gp.m.p.ptr().syscalltick++
        }
}

func exitsyscallfast_pidle() bool {
        lock(&sched.lock)
        pp, _ := pidleget(0)
        if pp != nil && sched.sysmonwait.Load() {
                sched.sysmonwait.Store(false)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
        if pp != nil {
                acquirep(pp)
                return true
        }
        return false
}

// exitsyscall slow path on g0.
// Failed to acquire P, enqueue gp as runnable.
//
// Called via mcall, so gp is the calling g from this M.
//
//go:nowritebarrierrec
func exitsyscall0(gp *g) {
        casgstatus(gp, _Gsyscall, _Grunnable)
        dropg()
        lock(&sched.lock)
        var pp *p
        if schedEnabled(gp) {
                pp, _ = pidleget(0)
        }
        var locked bool
        if pp == nil {
                globrunqput(gp)

                // Below, we stoplockedm if gp is locked. globrunqput releases
                // ownership of gp, so we must check if gp is locked prior to
                // committing the release by unlocking sched.lock, otherwise we
                // could race with another M transitioning gp from unlocked to
                // locked.
                locked = gp.lockedm != 0
        } else if sched.sysmonwait.Load() {
                sched.sysmonwait.Store(false)
                notewakeup(&sched.sysmonnote)
        }
        unlock(&sched.lock)
        if pp != nil {
                acquirep(pp)
                execute(gp, false) // Never returns.
        }
        if locked {
                // Wait until another thread schedules gp and so m again.
                //
                // N.B. lockedm must be this M, as this g was running on this M
                // before entersyscall.
                stoplockedm()
                execute(gp, false) // Never returns.
        }
        stopm()
        schedule() // Never returns.
}

// Called from syscall package before fork.
//
//go:linkname syscall_runtime_BeforeFork syscall.runtime_BeforeFork
//go:nosplit
func syscall_runtime_BeforeFork() {
        gp := getg().m.curg

        // Block signals during a fork, so that the child does not run
        // a signal handler before exec if a signal is sent to the process
        // group. See issue #18600.
        gp.m.locks++
        sigsave(&gp.m.sigmask)
        sigblock(false)

        // This function is called before fork in syscall package.
        // Code between fork and exec must not allocate memory nor even try to grow stack.
        // Here we spoil g.stackguard0 to reliably detect any attempts to grow stack.
        // runtime_AfterFork will undo this in parent process, but not in child.
        gp.stackguard0 = stackFork
}

// Called from syscall package after fork in parent.
//
//go:linkname syscall_runtime_AfterFork syscall.runtime_AfterFork
//go:nosplit
func syscall_runtime_AfterFork() {
        gp := getg().m.curg

        // See the comments in beforefork.
        gp.stackguard0 = gp.stack.lo + stackGuard

        msigrestore(gp.m.sigmask)

        gp.m.locks--
}

// inForkedChild is true while manipulating signals in the child process.
// This is used to avoid calling libc functions in case we are using vfork.
var inForkedChild bool

// Called from syscall package after fork in child.
// It resets non-sigignored signals to the default handler, and
// restores the signal mask in preparation for the exec.
//
// Because this might be called during a vfork, and therefore may be
// temporarily sharing address space with the parent process, this must
// not change any global variables or calling into C code that may do so.
//
//go:linkname syscall_runtime_AfterForkInChild syscall.runtime_AfterForkInChild
//go:nosplit
//go:nowritebarrierrec
func syscall_runtime_AfterForkInChild() {
        // It's OK to change the global variable inForkedChild here
        // because we are going to change it back. There is no race here,
        // because if we are sharing address space with the parent process,
        // then the parent process can not be running concurrently.
        inForkedChild = true

        clearSignalHandlers()

        // When we are the child we are the only thread running,
        // so we know that nothing else has changed gp.m.sigmask.
        msigrestore(getg().m.sigmask)

        inForkedChild = false
}

// pendingPreemptSignals is the number of preemption signals
// that have been sent but not received. This is only used on Darwin.
// For #41702.
var pendingPreemptSignals atomic.Int32

// Called from syscall package before Exec.
//
//go:linkname syscall_runtime_BeforeExec syscall.runtime_BeforeExec
func syscall_runtime_BeforeExec() {
        // Prevent thread creation during exec.
        execLock.lock()

        // On Darwin, wait for all pending preemption signals to
        // be received. See issue #41702.
        if GOOS == "darwin" || GOOS == "ios" {
                for pendingPreemptSignals.Load() > 0 {
                        osyield()
                }
        }
}

// Called from syscall package after Exec.
//
//go:linkname syscall_runtime_AfterExec syscall.runtime_AfterExec
func syscall_runtime_AfterExec() {
        execLock.unlock()
}

// Allocate a new g, with a stack big enough for stacksize bytes.
func malg(stacksize int32) *g {
        newg := new(g)
        if stacksize >= 0 {
                stacksize = round2(stackSystem + stacksize)
                systemstack(func() {
                        newg.stack = stackalloc(uint32(stacksize))
                })
                newg.stackguard0 = newg.stack.lo + stackGuard
                newg.stackguard1 = ^uintptr(0)
                // Clear the bottom word of the stack. We record g
                // there on gsignal stack during VDSO on ARM and ARM64.
                *(*uintptr)(unsafe.Pointer(newg.stack.lo)) = 0
        }
        return newg
}

// Create a new g running fn.
// Put it on the queue of g's waiting to run.
// The compiler turns a go statement into a call to this.
func newproc(fn *funcval) {
        gp := getg()
        pc := getcallerpc()
        systemstack(func() {
                newg := newproc1(fn, gp, pc)

                pp := getg().m.p.ptr()
                runqput(pp, newg, true)

                if mainStarted {
                        wakep()
                }
        })
}

// Create a new g in state _Grunnable, starting at fn. callerpc is the
// address of the go statement that created this. The caller is responsible
// for adding the new g to the scheduler.
func newproc1(fn *funcval, callergp *g, callerpc uintptr) *g {
        if fn == nil {
                fatal("go of nil func value")
        }

        mp := acquirem() // disable preemption because we hold M and P in local vars.
        pp := mp.p.ptr()
        newg := gfget(pp)
        if newg == nil {
                newg = malg(stackMin)
                casgstatus(newg, _Gidle, _Gdead)
                allgadd(newg) // publishes with a g->status of Gdead so GC scanner doesn't look at uninitialized stack.
        }
        if newg.stack.hi == 0 {
                throw("newproc1: newg missing stack")
        }

        if readgstatus(newg) != _Gdead {
                throw("newproc1: new g is not Gdead")
        }

        totalSize := uintptr(4*goarch.PtrSize + sys.MinFrameSize) // extra space in case of reads slightly beyond frame
        totalSize = alignUp(totalSize, sys.StackAlign)
        sp := newg.stack.hi - totalSize
        spArg := sp
        if usesLR {
                // caller's LR
                *(*uintptr)(unsafe.Pointer(sp)) = 0
                prepGoExitFrame(sp)
                spArg += sys.MinFrameSize
        }

        memclrNoHeapPointers(unsafe.Pointer(&newg.sched), unsafe.Sizeof(newg.sched))
        newg.sched.sp = sp
        newg.stktopsp = sp
        newg.sched.pc = abi.FuncPCABI0(goexit) + sys.PCQuantum // +PCQuantum so that previous instruction is in same function
        newg.sched.g = guintptr(unsafe.Pointer(newg))
        gostartcallfn(&newg.sched, fn)
        newg.parentGoid = callergp.goid
        newg.gopc = callerpc
        newg.ancestors = saveAncestors(callergp)
        newg.startpc = fn.fn
        if isSystemGoroutine(newg, false) {
                sched.ngsys.Add(1)
        } else {
                // Only user goroutines inherit pprof labels.
                if mp.curg != nil {
                        newg.labels = mp.curg.labels
                }
                if goroutineProfile.active {
                        // A concurrent goroutine profile is running. It should include
                        // exactly the set of goroutines that were alive when the goroutine
                        // profiler first stopped the world. That does not include newg, so
                        // mark it as not needing a profile before transitioning it from
                        // _Gdead.
                        newg.goroutineProfiled.Store(goroutineProfileSatisfied)
                }
        }
        // Track initial transition?
        newg.trackingSeq = uint8(fastrand())
        if newg.trackingSeq%gTrackingPeriod == 0 {
                newg.tracking = true
        }
        casgstatus(newg, _Gdead, _Grunnable)
        gcController.addScannableStack(pp, int64(newg.stack.hi-newg.stack.lo))

        if pp.goidcache == pp.goidcacheend {
                // Sched.goidgen is the last allocated id,
                // this batch must be [sched.goidgen+1, sched.goidgen+GoidCacheBatch].
                // At startup sched.goidgen=0, so main goroutine receives goid=1.
                pp.goidcache = sched.goidgen.Add(_GoidCacheBatch)
                pp.goidcache -= _GoidCacheBatch - 1
                pp.goidcacheend = pp.goidcache + _GoidCacheBatch
        }
        newg.goid = pp.goidcache
        pp.goidcache++
        if raceenabled {
                newg.racectx = racegostart(callerpc)
                if newg.labels != nil {
                        // See note in proflabel.go on labelSync's role in synchronizing
                        // with the reads in the signal handler.
                        racereleasemergeg(newg, unsafe.Pointer(&labelSync))
                }
        }
        if traceEnabled() {
                traceGoCreate(newg, newg.startpc)
        }
        releasem(mp)

        return newg
}

// saveAncestors copies previous ancestors of the given caller g and
// includes info for the current caller into a new set of tracebacks for
// a g being created.
func saveAncestors(callergp *g) *[]ancestorInfo {
        // Copy all prior info, except for the root goroutine (goid 0).
        if debug.tracebackancestors <= 0 || callergp.goid == 0 {
                return nil
        }
        var callerAncestors []ancestorInfo
        if callergp.ancestors != nil {
                callerAncestors = *callergp.ancestors
        }
        n := int32(len(callerAncestors)) + 1
        if n > debug.tracebackancestors {
                n = debug.tracebackancestors
        }
        ancestors := make([]ancestorInfo, n)
        copy(ancestors[1:], callerAncestors)

        var pcs [tracebackInnerFrames]uintptr
        npcs := gcallers(callergp, 0, pcs[:])
        ipcs := make([]uintptr, npcs)
        copy(ipcs, pcs[:])
        ancestors[0] = ancestorInfo{
                pcs:  ipcs,
                goid: callergp.goid,
                gopc: callergp.gopc,
        }

        ancestorsp := new([]ancestorInfo)
        *ancestorsp = ancestors
        return ancestorsp
}

// Put on gfree list.
// If local list is too long, transfer a batch to the global list.
func gfput(pp *p, gp *g) {
        if readgstatus(gp) != _Gdead {
                throw("gfput: bad status (not Gdead)")
        }

        stksize := gp.stack.hi - gp.stack.lo

        if stksize != uintptr(startingStackSize) {
                // non-standard stack size - free it.
                stackfree(gp.stack)
                gp.stack.lo = 0
                gp.stack.hi = 0
                gp.stackguard0 = 0
        }

        pp.gFree.push(gp)
        pp.gFree.n++
        if pp.gFree.n >= 64 {
                var (
                        inc      int32
                        stackQ   gQueue
                        noStackQ gQueue
                )
                for pp.gFree.n >= 32 {
                        gp := pp.gFree.pop()
                        pp.gFree.n--
                        if gp.stack.lo == 0 {
                                noStackQ.push(gp)
                        } else {
                                stackQ.push(gp)
                        }
                        inc++
                }
                lock(&sched.gFree.lock)
                sched.gFree.noStack.pushAll(noStackQ)
                sched.gFree.stack.pushAll(stackQ)
                sched.gFree.n += inc
                unlock(&sched.gFree.lock)
        }
}

// Get from gfree list.
// If local list is empty, grab a batch from global list.
func gfget(pp *p) *g {
retry:
        if pp.gFree.empty() && (!sched.gFree.stack.empty() || !sched.gFree.noStack.empty()) {
                lock(&sched.gFree.lock)
                // Move a batch of free Gs to the P.
                for pp.gFree.n < 32 {
                        // Prefer Gs with stacks.
                        gp := sched.gFree.stack.pop()
                        if gp == nil {
                                gp = sched.gFree.noStack.pop()
                                if gp == nil {
                                        break
                                }
                        }
                        sched.gFree.n--
                        pp.gFree.push(gp)
                        pp.gFree.n++
                }
                unlock(&sched.gFree.lock)
                goto retry
        }
        gp := pp.gFree.pop()
        if gp == nil {
                return nil
        }
        pp.gFree.n--
        if gp.stack.lo != 0 && gp.stack.hi-gp.stack.lo != uintptr(startingStackSize) {
                // Deallocate old stack. We kept it in gfput because it was the
                // right size when the goroutine was put on the free list, but
                // the right size has changed since then.
                systemstack(func() {
                        stackfree(gp.stack)
                        gp.stack.lo = 0
                        gp.stack.hi = 0
                        gp.stackguard0 = 0
                })
        }
        if gp.stack.lo == 0 {
                // Stack was deallocated in gfput or just above. Allocate a new one.
                systemstack(func() {
                        gp.stack = stackalloc(startingStackSize)
                })
                gp.stackguard0 = gp.stack.lo + stackGuard
        } else {
                if raceenabled {
                        racemalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
                }
                if msanenabled {
                        msanmalloc(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
                }
                if asanenabled {
                        asanunpoison(unsafe.Pointer(gp.stack.lo), gp.stack.hi-gp.stack.lo)
                }
        }
        return gp
}

// Purge all cached G's from gfree list to the global list.
func gfpurge(pp *p) {
        var (
                inc      int32
                stackQ   gQueue
                noStackQ gQueue
        )
        for !pp.gFree.empty() {
                gp := pp.gFree.pop()
                pp.gFree.n--
                if gp.stack.lo == 0 {
                        noStackQ.push(gp)
                } else {
                        stackQ.push(gp)
                }
                inc++
        }
        lock(&sched.gFree.lock)
        sched.gFree.noStack.pushAll(noStackQ)
        sched.gFree.stack.pushAll(stackQ)
        sched.gFree.n += inc
        unlock(&sched.gFree.lock)
}

// Breakpoint executes a breakpoint trap.
func Breakpoint() {
        breakpoint()
}

// dolockOSThread is called by LockOSThread and lockOSThread below
// after they modify m.locked. Do not allow preemption during this call,
// or else the m might be different in this function than in the caller.
//
//go:nosplit
func dolockOSThread() {
        if GOARCH == "wasm" {
                return // no threads on wasm yet
        }
        gp := getg()
        gp.m.lockedg.set(gp)
        gp.lockedm.set(gp.m)
}

// LockOSThread wires the calling goroutine to its current operating system thread.
// The calling goroutine will always execute in that thread,
// and no other goroutine will execute in it,
// until the calling goroutine has made as many calls to
// UnlockOSThread as to LockOSThread.
// If the calling goroutine exits without unlocking the thread,
// the thread will be terminated.
//
// All init functions are run on the startup thread. Calling LockOSThread
// from an init function will cause the main function to be invoked on
// that thread.
//
// A goroutine should call LockOSThread before calling OS services or
// non-Go library functions that depend on per-thread state.
//
//go:nosplit
func LockOSThread() {
        if atomic.Load(&newmHandoff.haveTemplateThread) == 0 && GOOS != "plan9" {
                // If we need to start a new thread from the locked
                // thread, we need the template thread. Start it now
                // while we're in a known-good state.
                startTemplateThread()
        }
        gp := getg()
        gp.m.lockedExt++
        if gp.m.lockedExt == 0 {
                gp.m.lockedExt--
                panic("LockOSThread nesting overflow")
        }
        dolockOSThread()
}

//go:nosplit
func lockOSThread() {
        getg().m.lockedInt++
        dolockOSThread()
}

// dounlockOSThread is called by UnlockOSThread and unlockOSThread below
// after they update m->locked. Do not allow preemption during this call,
// or else the m might be in different in this function than in the caller.
//
//go:nosplit
func dounlockOSThread() {
        if GOARCH == "wasm" {
                return // no threads on wasm yet
        }
        gp := getg()
        if gp.m.lockedInt != 0 || gp.m.lockedExt != 0 {
                return
        }
        gp.m.lockedg = 0
        gp.lockedm = 0
}

// UnlockOSThread undoes an earlier call to LockOSThread.
// If this drops the number of active LockOSThread calls on the
// calling goroutine to zero, it unwires the calling goroutine from
// its fixed operating system thread.
// If there are no active LockOSThread calls, this is a no-op.
//
// Before calling UnlockOSThread, the caller must ensure that the OS
// thread is suitable for running other goroutines. If the caller made
// any permanent changes to the state of the thread that would affect
// other goroutines, it should not call this function and thus leave
// the goroutine locked to the OS thread until the goroutine (and
// hence the thread) exits.
//
//go:nosplit
func UnlockOSThread() {
        gp := getg()
        if gp.m.lockedExt == 0 {
                return
        }
        gp.m.lockedExt--
        dounlockOSThread()
}

//go:nosplit
func unlockOSThread() {
        gp := getg()
        if gp.m.lockedInt == 0 {
                systemstack(badunlockosthread)
        }
        gp.m.lockedInt--
        dounlockOSThread()
}

func badunlockosthread() {
        throw("runtime: internal error: misuse of lockOSThread/unlockOSThread")
}

func gcount() int32 {
        n := int32(atomic.Loaduintptr(&allglen)) - sched.gFree.n - sched.ngsys.Load()
        for _, pp := range allp {
                n -= pp.gFree.n
        }

        // All these variables can be changed concurrently, so the result can be inconsistent.
        // But at least the current goroutine is running.
        if n < 1 {
                n = 1
        }
        return n
}

func mcount() int32 {
        return int32(sched.mnext - sched.nmfreed)
}

var prof struct {
        signalLock atomic.Uint32

        // Must hold signalLock to write. Reads may be lock-free, but
        // signalLock should be taken to synchronize with changes.
        hz atomic.Int32
}

func _System()                    { _System() }
func _ExternalCode()              { _ExternalCode() }
func _LostExternalCode()          { _LostExternalCode() }
func _GC()                        { _GC() }
func _LostSIGPROFDuringAtomic64() { _LostSIGPROFDuringAtomic64() }
func _VDSO()                      { _VDSO() }

// Called if we receive a SIGPROF signal.
// Called by the signal handler, may run during STW.
//
//go:nowritebarrierrec
func sigprof(pc, sp, lr uintptr, gp *g, mp *m) {
        if prof.hz.Load() == 0 {
                return
        }

        // If mp.profilehz is 0, then profiling is not enabled for this thread.
        // We must check this to avoid a deadlock between setcpuprofilerate
        // and the call to cpuprof.add, below.
        if mp != nil && mp.profilehz == 0 {
                return
        }

        // On mips{,le}/arm, 64bit atomics are emulated with spinlocks, in
        // runtime/internal/atomic. If SIGPROF arrives while the program is inside
        // the critical section, it creates a deadlock (when writing the sample).
        // As a workaround, create a counter of SIGPROFs while in critical section
        // to store the count, and pass it to sigprof.add() later when SIGPROF is
        // received from somewhere else (with _LostSIGPROFDuringAtomic64 as pc).
        if GOARCH == "mips" || GOARCH == "mipsle" || GOARCH == "arm" {
                if f := findfunc(pc); f.valid() {
                        if hasPrefix(funcname(f), "runtime/internal/atomic") {
                                cpuprof.lostAtomic++
                                return
                        }
                }
                if GOARCH == "arm" && goarm < 7 && GOOS == "linux" && pc&0xffff0000 == 0xffff0000 {
                        // runtime/internal/atomic functions call into kernel
                        // helpers on arm < 7. See
                        // runtime/internal/atomic/sys_linux_arm.s.
                        cpuprof.lostAtomic++
                        return
                }
        }

        // Profiling runs concurrently with GC, so it must not allocate.
        // Set a trap in case the code does allocate.
        // Note that on windows, one thread takes profiles of all the
        // other threads, so mp is usually not getg().m.
        // In fact mp may not even be stopped.
        // See golang.org/issue/17165.
        getg().m.mallocing++

        var u unwinder
        var stk [maxCPUProfStack]uintptr
        n := 0
        if mp.ncgo > 0 && mp.curg != nil && mp.curg.syscallpc != 0 && mp.curg.syscallsp != 0 {
                cgoOff := 0
                // Check cgoCallersUse to make sure that we are not
                // interrupting other code that is fiddling with
                // cgoCallers.  We are running in a signal handler
                // with all signals blocked, so we don't have to worry
                // about any other code interrupting us.
                if mp.cgoCallersUse.Load() == 0 && mp.cgoCallers != nil && mp.cgoCallers[0] != 0 {
                        for cgoOff < len(mp.cgoCallers) && mp.cgoCallers[cgoOff] != 0 {
                                cgoOff++
                        }
                        n += copy(stk[:], mp.cgoCallers[:cgoOff])
                        mp.cgoCallers[0] = 0
                }

                // Collect Go stack that leads to the cgo call.
                u.initAt(mp.curg.syscallpc, mp.curg.syscallsp, 0, mp.curg, unwindSilentErrors)
        } else if usesLibcall() && mp.libcallg != 0 && mp.libcallpc != 0 && mp.libcallsp != 0 {
                // Libcall, i.e. runtime syscall on windows.
                // Collect Go stack that leads to the call.
                u.initAt(mp.libcallpc, mp.libcallsp, 0, mp.libcallg.ptr(), unwindSilentErrors)
        } else if mp != nil && mp.vdsoSP != 0 {
                // VDSO call, e.g. nanotime1 on Linux.
                // Collect Go stack that leads to the call.
                u.initAt(mp.vdsoPC, mp.vdsoSP, 0, gp, unwindSilentErrors|unwindJumpStack)
        } else {
                u.initAt(pc, sp, lr, gp, unwindSilentErrors|unwindTrap|unwindJumpStack)
        }
        n += tracebackPCs(&u, 0, stk[n:])

        if n <= 0 {
                // Normal traceback is impossible or has failed.
                // Account it against abstract "System" or "GC".
                n = 2
                if inVDSOPage(pc) {
                        pc = abi.FuncPCABIInternal(_VDSO) + sys.PCQuantum
                } else if pc > firstmoduledata.etext {
                        // "ExternalCode" is better than "etext".
                        pc = abi.FuncPCABIInternal(_ExternalCode) + sys.PCQuantum
                }
                stk[0] = pc
                if mp.preemptoff != "" {
                        stk[1] = abi.FuncPCABIInternal(_GC) + sys.PCQuantum
                } else {
                        stk[1] = abi.FuncPCABIInternal(_System) + sys.PCQuantum
                }
        }

        if prof.hz.Load() != 0 {
                // Note: it can happen on Windows that we interrupted a system thread
                // with no g, so gp could nil. The other nil checks are done out of
                // caution, but not expected to be nil in practice.
                var tagPtr *unsafe.Pointer
                if gp != nil && gp.m != nil && gp.m.curg != nil {
                        tagPtr = &gp.m.curg.labels
                }
                cpuprof.add(tagPtr, stk[:n])

                gprof := gp
                var pp *p
                if gp != nil && gp.m != nil {
                        if gp.m.curg != nil {
                                gprof = gp.m.curg
                        }
                        pp = gp.m.p.ptr()
                }
                traceCPUSample(gprof, pp, stk[:n])
        }
        getg().m.mallocing--
}

// setcpuprofilerate sets the CPU profiling rate to hz times per second.
// If hz <= 0, setcpuprofilerate turns off CPU profiling.
func setcpuprofilerate(hz int32) {
        // Force sane arguments.
        if hz < 0 {
                hz = 0
        }

        // Disable preemption, otherwise we can be rescheduled to another thread
        // that has profiling enabled.
        gp := getg()
        gp.m.locks++

        // Stop profiler on this thread so that it is safe to lock prof.
        // if a profiling signal came in while we had prof locked,
        // it would deadlock.
        setThreadCPUProfiler(0)

        for !prof.signalLock.CompareAndSwap(0, 1) {
                osyield()
        }
        if prof.hz.Load() != hz {
                setProcessCPUProfiler(hz)
                prof.hz.Store(hz)
        }
        prof.signalLock.Store(0)

        lock(&sched.lock)
        sched.profilehz = hz
        unlock(&sched.lock)

        if hz != 0 {
                setThreadCPUProfiler(hz)
        }

        gp.m.locks--
}

// init initializes pp, which may be a freshly allocated p or a
// previously destroyed p, and transitions it to status _Pgcstop.
func (pp *p) init(id int32) {
        pp.id = id
        pp.status = _Pgcstop
        pp.sudogcache = pp.sudogbuf[:0]
        pp.deferpool = pp.deferpoolbuf[:0]
        pp.wbBuf.reset()
        if pp.mcache == nil {
                if id == 0 {
                        if mcache0 == nil {
                                throw("missing mcache?")
                        }
                        // Use the bootstrap mcache0. Only one P will get
                        // mcache0: the one with ID 0.
                        pp.mcache = mcache0
                } else {
                        pp.mcache = allocmcache()
                }
        }
        if raceenabled && pp.raceprocctx == 0 {
                if id == 0 {
                        pp.raceprocctx = raceprocctx0
                        raceprocctx0 = 0 // bootstrap
                } else {
                        pp.raceprocctx = raceproccreate()
                }
        }
        lockInit(&pp.timersLock, lockRankTimers)

        // This P may get timers when it starts running. Set the mask here
        // since the P may not go through pidleget (notably P 0 on startup).
        timerpMask.set(id)
        // Similarly, we may not go through pidleget before this P starts
        // running if it is P 0 on startup.
        idlepMask.clear(id)
}

// destroy releases all of the resources associated with pp and
// transitions it to status _Pdead.
//
// sched.lock must be held and the world must be stopped.
func (pp *p) destroy() {
        assertLockHeld(&sched.lock)
        assertWorldStopped()

        // Move all runnable goroutines to the global queue
        for pp.runqhead != pp.runqtail {
                // Pop from tail of local queue
                pp.runqtail--
                gp := pp.runq[pp.runqtail%uint32(len(pp.runq))].ptr()
                // Push onto head of global queue
                globrunqputhead(gp)
        }
        if pp.runnext != 0 {
                globrunqputhead(pp.runnext.ptr())
                pp.runnext = 0
        }
        if len(pp.timers) > 0 {
                plocal := getg().m.p.ptr()
                // The world is stopped, but we acquire timersLock to
                // protect against sysmon calling timeSleepUntil.
                // This is the only case where we hold the timersLock of
                // more than one P, so there are no deadlock concerns.
                lock(&plocal.timersLock)
                lock(&pp.timersLock)
                moveTimers(plocal, pp.timers)
                pp.timers = nil
                pp.numTimers.Store(0)
                pp.deletedTimers.Store(0)
                pp.timer0When.Store(0)
                unlock(&pp.timersLock)
                unlock(&plocal.timersLock)
        }
        // Flush p's write barrier buffer.
        if gcphase != _GCoff {
                wbBufFlush1(pp)
                pp.gcw.dispose()
        }
        for i := range pp.sudogbuf {
                pp.sudogbuf[i] = nil
        }
        pp.sudogcache = pp.sudogbuf[:0]
        pp.pinnerCache = nil
        for j := range pp.deferpoolbuf {
                pp.deferpoolbuf[j] = nil
        }
        pp.deferpool = pp.deferpoolbuf[:0]
        systemstack(func() {
                for i := 0; i < pp.mspancache.len; i++ {
                        // Safe to call since the world is stopped.
                        mheap_.spanalloc.free(unsafe.Pointer(pp.mspancache.buf[i]))
                }
                pp.mspancache.len = 0
                lock(&mheap_.lock)
                pp.pcache.flush(&mheap_.pages)
                unlock(&mheap_.lock)
        })
        freemcache(pp.mcache)
        pp.mcache = nil
        gfpurge(pp)
        traceProcFree(pp)
        if raceenabled {
                if pp.timerRaceCtx != 0 {
                        // The race detector code uses a callback to fetch
                        // the proc context, so arrange for that callback
                        // to see the right thing.
                        // This hack only works because we are the only
                        // thread running.
                        mp := getg().m
                        phold := mp.p.ptr()
                        mp.p.set(pp)

                        racectxend(pp.timerRaceCtx)
                        pp.timerRaceCtx = 0

                        mp.p.set(phold)
                }
                raceprocdestroy(pp.raceprocctx)
                pp.raceprocctx = 0
        }
        pp.gcAssistTime = 0
        pp.status = _Pdead
}

// Change number of processors.
//
// sched.lock must be held, and the world must be stopped.
//
// gcworkbufs must not be being modified by either the GC or the write barrier
// code, so the GC must not be running if the number of Ps actually changes.
//
// Returns list of Ps with local work, they need to be scheduled by the caller.
func procresize(nprocs int32) *p {
        assertLockHeld(&sched.lock)
        assertWorldStopped()

        old := gomaxprocs
        if old < 0 || nprocs <= 0 {
                throw("procresize: invalid arg")
        }
        if traceEnabled() {
                traceGomaxprocs(nprocs)
        }

        // update statistics
        now := nanotime()
        if sched.procresizetime != 0 {
                sched.totaltime += int64(old) * (now - sched.procresizetime)
        }
        sched.procresizetime = now

        maskWords := (nprocs + 31) / 32

        // Grow allp if necessary.
        if nprocs > int32(len(allp)) {
                // Synchronize with retake, which could be running
                // concurrently since it doesn't run on a P.
                lock(&allpLock)
                if nprocs <= int32(cap(allp)) {
                        allp = allp[:nprocs]
                } else {
                        nallp := make([]*p, nprocs)
                        // Copy everything up to allp's cap so we
                        // never lose old allocated Ps.
                        copy(nallp, allp[:cap(allp)])
                        allp = nallp
                }

                if maskWords <= int32(cap(idlepMask)) {
                        idlepMask = idlepMask[:maskWords]
                        timerpMask = timerpMask[:maskWords]
                } else {
                        nidlepMask := make([]uint32, maskWords)
                        // No need to copy beyond len, old Ps are irrelevant.
                        copy(nidlepMask, idlepMask)
                        idlepMask = nidlepMask

                        ntimerpMask := make([]uint32, maskWords)
                        copy(ntimerpMask, timerpMask)
                        timerpMask = ntimerpMask
                }
                unlock(&allpLock)
        }

        // initialize new P's
        for i := old; i < nprocs; i++ {
                pp := allp[i]
                if pp == nil {
                        pp = new(p)
                }
                pp.init(i)
                atomicstorep(unsafe.Pointer(&allp[i]), unsafe.Pointer(pp))
        }

        gp := getg()
        if gp.m.p != 0 && gp.m.p.ptr().id < nprocs {
                // continue to use the current P
                gp.m.p.ptr().status = _Prunning
                gp.m.p.ptr().mcache.prepareForSweep()
        } else {
                // release the current P and acquire allp[0].
                //
                // We must do this before destroying our current P
                // because p.destroy itself has write barriers, so we
                // need to do that from a valid P.
                if gp.m.p != 0 {
                        if traceEnabled() {
                                // Pretend that we were descheduled
                                // and then scheduled again to keep
                                // the trace sane.
                                traceGoSched()
                                traceProcStop(gp.m.p.ptr())
                        }
                        gp.m.p.ptr().m = 0
                }
                gp.m.p = 0
                pp := allp[0]
                pp.m = 0
                pp.status = _Pidle
                acquirep(pp)
                if traceEnabled() {
                        traceGoStart()
                }
        }

        // g.m.p is now set, so we no longer need mcache0 for bootstrapping.
        mcache0 = nil

        // release resources from unused P's
        for i := nprocs; i < old; i++ {
                pp := allp[i]
                pp.destroy()
                // can't free P itself because it can be referenced by an M in syscall
        }

        // Trim allp.
        if int32(len(allp)) != nprocs {
                lock(&allpLock)
                allp = allp[:nprocs]
                idlepMask = idlepMask[:maskWords]
                timerpMask = timerpMask[:maskWords]
                unlock(&allpLock)
        }

        var runnablePs *p
        for i := nprocs - 1; i >= 0; i-- {
                pp := allp[i]
                if gp.m.p.ptr() == pp {
                        continue
                }
                pp.status = _Pidle
                if runqempty(pp) {
                        pidleput(pp, now)
                } else {
                        pp.m.set(mget())
                        pp.link.set(runnablePs)
                        runnablePs = pp
                }
        }
        stealOrder.reset(uint32(nprocs))
        var int32p *int32 = &gomaxprocs // make compiler check that gomaxprocs is an int32
        atomic.Store((*uint32)(unsafe.Pointer(int32p)), uint32(nprocs))
        if old != nprocs {
                // Notify the limiter that the amount of procs has changed.
                gcCPULimiter.resetCapacity(now, nprocs)
        }
        return runnablePs
}

// Associate p and the current m.
//
// This function is allowed to have write barriers even if the caller
// isn't because it immediately acquires pp.
//
//go:yeswritebarrierrec
func acquirep(pp *p) {
        // Do the part that isn't allowed to have write barriers.
        wirep(pp)

        // Have p; write barriers now allowed.

        // Perform deferred mcache flush before this P can allocate
        // from a potentially stale mcache.
        pp.mcache.prepareForSweep()

        if traceEnabled() {
                traceProcStart()
        }
}

// wirep is the first step of acquirep, which actually associates the
// current M to pp. This is broken out so we can disallow write
// barriers for this part, since we don't yet have a P.
//
//go:nowritebarrierrec
//go:nosplit
func wirep(pp *p) {
        gp := getg()

        if gp.m.p != 0 {
                throw("wirep: already in go")
        }
        if pp.m != 0 || pp.status != _Pidle {
                id := int64(0)
                if pp.m != 0 {
                        id = pp.m.ptr().id
                }
                print("wirep: p->m=", pp.m, "(", id, ") p->status=", pp.status, "\n")
                throw("wirep: invalid p state")
        }
        gp.m.p.set(pp)
        pp.m.set(gp.m)
        pp.status = _Prunning
}

// Disassociate p and the current m.
func releasep() *p {
        gp := getg()

        if gp.m.p == 0 {
                throw("releasep: invalid arg")
        }
        pp := gp.m.p.ptr()
        if pp.m.ptr() != gp.m || pp.status != _Prunning {
                print("releasep: m=", gp.m, " m->p=", gp.m.p.ptr(), " p->m=", hex(pp.m), " p->status=", pp.status, "\n")
                throw("releasep: invalid p state")
        }
        if traceEnabled() {
                traceProcStop(gp.m.p.ptr())
        }
        gp.m.p = 0
        pp.m = 0
        pp.status = _Pidle
        return pp
}

func incidlelocked(v int32) {
        lock(&sched.lock)
        sched.nmidlelocked += v
        if v > 0 {
                checkdead()
        }
        unlock(&sched.lock)
}

// Check for deadlock situation.
// The check is based on number of running M's, if 0 -> deadlock.
// sched.lock must be held.
func checkdead() {
        assertLockHeld(&sched.lock)

        // For -buildmode=c-shared or -buildmode=c-archive it's OK if
        // there are no running goroutines. The calling program is
        // assumed to be running.
        if islibrary || isarchive {
                return
        }

        // If we are dying because of a signal caught on an already idle thread,
        // freezetheworld will cause all running threads to block.
        // And runtime will essentially enter into deadlock state,
        // except that there is a thread that will call exit soon.
        if panicking.Load() > 0 {
                return
        }

        // If we are not running under cgo, but we have an extra M then account
        // for it. (It is possible to have an extra M on Windows without cgo to
        // accommodate callbacks created by syscall.NewCallback. See issue #6751
        // for details.)
        var run0 int32
        if !iscgo && cgoHasExtraM && extraMLength.Load() > 0 {
                run0 = 1
        }

        run := mcount() - sched.nmidle - sched.nmidlelocked - sched.nmsys
        if run > run0 {
                return
        }
        if run < 0 {
                print("runtime: checkdead: nmidle=", sched.nmidle, " nmidlelocked=", sched.nmidlelocked, " mcount=", mcount(), " nmsys=", sched.nmsys, "\n")
                unlock(&sched.lock)
                throw("checkdead: inconsistent counts")
        }

        grunning := 0
        forEachG(func(gp *g) {
                if isSystemGoroutine(gp, false) {
                        return
                }
                s := readgstatus(gp)
                switch s &^ _Gscan {
                case _Gwaiting,
                        _Gpreempted:
                        grunning++
                case _Grunnable,
                        _Grunning,
                        _Gsyscall:
                        print("runtime: checkdead: find g ", gp.goid, " in status ", s, "\n")
                        unlock(&sched.lock)
                        throw("checkdead: runnable g")
                }
        })
        if grunning == 0 { // possible if main goroutine calls runtime·Goexit()
                unlock(&sched.lock) // unlock so that GODEBUG=scheddetail=1 doesn't hang
                fatal("no goroutines (main called runtime.Goexit) - deadlock!")
        }

        // Maybe jump time forward for playground.
        if faketime != 0 {
                if when := timeSleepUntil(); when < maxWhen {
                        faketime = when

                        // Start an M to steal the timer.
                        pp, _ := pidleget(faketime)
                        if pp == nil {
                                // There should always be a free P since
                                // nothing is running.
                                unlock(&sched.lock)
                                throw("checkdead: no p for timer")
                        }
                        mp := mget()
                        if mp == nil {
                                // There should always be a free M since
                                // nothing is running.
                                unlock(&sched.lock)
                                throw("checkdead: no m for timer")
                        }
                        // M must be spinning to steal. We set this to be
                        // explicit, but since this is the only M it would
                        // become spinning on its own anyways.
                        sched.nmspinning.Add(1)
                        mp.spinning = true
                        mp.nextp.set(pp)
                        notewakeup(&mp.park)
                        return
                }
        }

        // There are no goroutines running, so we can look at the P's.
        for _, pp := range allp {
                if len(pp.timers) > 0 {
                        return
                }
        }

        unlock(&sched.lock) // unlock so that GODEBUG=scheddetail=1 doesn't hang
        fatal("all goroutines are asleep - deadlock!")
}

// forcegcperiod is the maximum time in nanoseconds between garbage
// collections. If we go this long without a garbage collection, one
// is forced to run.
//
// This is a variable for testing purposes. It normally doesn't change.
var forcegcperiod int64 = 2 * 60 * 1e9

// needSysmonWorkaround is true if the workaround for
// golang.org/issue/42515 is needed on NetBSD.
var needSysmonWorkaround bool = false

// Always runs without a P, so write barriers are not allowed.
//
//go:nowritebarrierrec
func sysmon() {
        lock(&sched.lock)
        sched.nmsys++
        checkdead()
        unlock(&sched.lock)

        lasttrace := int64(0)
        idle := 0 // how many cycles in succession we had not wokeup somebody
        delay := uint32(0)

        for {
                if idle == 0 { // start with 20us sleep...
                        delay = 20
                } else if idle > 50 { // start doubling the sleep after 1ms...
                        delay *= 2
                }
                if delay > 10*1000 { // up to 10ms
                        delay = 10 * 1000
                }
                usleep(delay)

                // sysmon should not enter deep sleep if schedtrace is enabled so that
                // it can print that information at the right time.
                //
                // It should also not enter deep sleep if there are any active P's so
                // that it can retake P's from syscalls, preempt long running G's, and
                // poll the network if all P's are busy for long stretches.
                //
                // It should wakeup from deep sleep if any P's become active either due
                // to exiting a syscall or waking up due to a timer expiring so that it
                // can resume performing those duties. If it wakes from a syscall it
                // resets idle and delay as a bet that since it had retaken a P from a
                // syscall before, it may need to do it again shortly after the
                // application starts work again. It does not reset idle when waking
                // from a timer to avoid adding system load to applications that spend
                // most of their time sleeping.
                now := nanotime()
                if debug.schedtrace <= 0 && (sched.gcwaiting.Load() || sched.npidle.Load() == gomaxprocs) {
                        lock(&sched.lock)
                        if sched.gcwaiting.Load() || sched.npidle.Load() == gomaxprocs {
                                syscallWake := false
                                next := timeSleepUntil()
                                if next > now {
                                        sched.sysmonwait.Store(true)
                                        unlock(&sched.lock)
                                        // Make wake-up period small enough
                                        // for the sampling to be correct.
                                        sleep := forcegcperiod / 2
                                        if next-now < sleep {
                                                sleep = next - now
                                        }
                                        shouldRelax := sleep >= osRelaxMinNS
                                        if shouldRelax {
                                                osRelax(true)
                                        }
                                        syscallWake = notetsleep(&sched.sysmonnote, sleep)
                                        if shouldRelax {
                                                osRelax(false)
                                        }
                                        lock(&sched.lock)
                                        sched.sysmonwait.Store(false)
                                        noteclear(&sched.sysmonnote)
                                }
                                if syscallWake {
                                        idle = 0
                                        delay = 20
                                }
                        }
                        unlock(&sched.lock)
                }

                lock(&sched.sysmonlock)
                // Update now in case we blocked on sysmonnote or spent a long time
                // blocked on schedlock or sysmonlock above.
                now = nanotime()

                // trigger libc interceptors if needed
                if *cgo_yield != nil {
                        asmcgocall(*cgo_yield, nil)
                }
                // poll network if not polled for more than 10ms
                lastpoll := sched.lastpoll.Load()
                if netpollinited() && lastpoll != 0 && lastpoll+10*1000*1000 < now {
                        sched.lastpoll.CompareAndSwap(lastpoll, now)
                        list := netpoll(0) // non-blocking - returns list of goroutines
                        if !list.empty() {
                                // Need to decrement number of idle locked M's
                                // (pretending that one more is running) before injectglist.
                                // Otherwise it can lead to the following situation:
                                // injectglist grabs all P's but before it starts M's to run the P's,
                                // another M returns from syscall, finishes running its G,
                                // observes that there is no work to do and no other running M's
                                // and reports deadlock.
                                incidlelocked(-1)
                                injectglist(&list)
                                incidlelocked(1)
                        }
                }
                if GOOS == "netbsd" && needSysmonWorkaround {
                        // netpoll is responsible for waiting for timer
                        // expiration, so we typically don't have to worry
                        // about starting an M to service timers. (Note that
                        // sleep for timeSleepUntil above simply ensures sysmon
                        // starts running again when that timer expiration may
                        // cause Go code to run again).
                        //
                        // However, netbsd has a kernel bug that sometimes
                        // misses netpollBreak wake-ups, which can lead to
                        // unbounded delays servicing timers. If we detect this
                        // overrun, then startm to get something to handle the
                        // timer.
                        //
                        // See issue 42515 and
                        // https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=50094.
                        if next := timeSleepUntil(); next < now {
                                startm(nil, false, false)
                        }
                }
                if scavenger.sysmonWake.Load() != 0 {
                        // Kick the scavenger awake if someone requested it.
                        scavenger.wake()
                }
                // retake P's blocked in syscalls
                // and preempt long running G's
                if retake(now) != 0 {
                        idle = 0
                } else {
                        idle++
                }
                // check if we need to force a GC
                if t := (gcTrigger{kind: gcTriggerTime, now: now}); t.test() && forcegc.idle.Load() {
                        lock(&forcegc.lock)
                        forcegc.idle.Store(false)
                        var list gList
                        list.push(forcegc.g)
                        injectglist(&list)
                        unlock(&forcegc.lock)
                }
                if debug.schedtrace > 0 && lasttrace+int64(debug.schedtrace)*1000000 <= now {
                        lasttrace = now
                        schedtrace(debug.scheddetail > 0)
                }
                unlock(&sched.sysmonlock)
        }
}

type sysmontick struct {
        schedtick   uint32
        schedwhen   int64
        syscalltick uint32
        syscallwhen int64
}

// forcePreemptNS is the time slice given to a G before it is
// preempted.
const forcePreemptNS = 10 * 1000 * 1000 // 10ms

func retake(now int64) uint32 {
        n := 0
        // Prevent allp slice changes. This lock will be completely
        // uncontended unless we're already stopping the world.
        lock(&allpLock)
        // We can't use a range loop over allp because we may
        // temporarily drop the allpLock. Hence, we need to re-fetch
        // allp each time around the loop.
        for i := 0; i < len(allp); i++ {
                pp := allp[i]
                if pp == nil {
                        // This can happen if procresize has grown
                        // allp but not yet created new Ps.
                        continue
                }
                pd := &pp.sysmontick
                s := pp.status
                sysretake := false
                if s == _Prunning || s == _Psyscall {
                        // Preempt G if it's running for too long.
                        t := int64(pp.schedtick)
                        if int64(pd.schedtick) != t {
                                pd.schedtick = uint32(t)
                                pd.schedwhen = now
                        } else if pd.schedwhen+forcePreemptNS <= now {
                                preemptone(pp)
                                // In case of syscall, preemptone() doesn't
                                // work, because there is no M wired to P.
                                sysretake = true
                        }
                }
                if s == _Psyscall {
                        // Retake P from syscall if it's there for more than 1 sysmon tick (at least 20us).
                        t := int64(pp.syscalltick)
                        if !sysretake && int64(pd.syscalltick) != t {
                                pd.syscalltick = uint32(t)
                                pd.syscallwhen = now
                                continue
                        }
                        // On the one hand we don't want to retake Ps if there is no other work to do,
                        // but on the other hand we want to retake them eventually
                        // because they can prevent the sysmon thread from deep sleep.
                        if runqempty(pp) && sched.nmspinning.Load()+sched.npidle.Load() > 0 && pd.syscallwhen+10*1000*1000 > now {
                                continue
                        }
                        // Drop allpLock so we can take sched.lock.
                        unlock(&allpLock)
                        // Need to decrement number of idle locked M's
                        // (pretending that one more is running) before the CAS.
                        // Otherwise the M from which we retake can exit the syscall,
                        // increment nmidle and report deadlock.
                        incidlelocked(-1)
                        if atomic.Cas(&pp.status, s, _Pidle) {
                                if traceEnabled() {
                                        traceGoSysBlock(pp)
                                        traceProcStop(pp)
                                }
                                n++
                                pp.syscalltick++
                                handoffp(pp)
                        }
                        incidlelocked(1)
                        lock(&allpLock)
                }
        }
        unlock(&allpLock)
        return uint32(n)
}

// Tell all goroutines that they have been preempted and they should stop.
// This function is purely best-effort. It can fail to inform a goroutine if a
// processor just started running it.
// No locks need to be held.
// Returns true if preemption request was issued to at least one goroutine.
func preemptall() bool {
        res := false
        for _, pp := range allp {
                if pp.status != _Prunning {
                        continue
                }
                if preemptone(pp) {
                        res = true
                }
        }
        return res
}

// Tell the goroutine running on processor P to stop.
// This function is purely best-effort. It can incorrectly fail to inform the
// goroutine. It can inform the wrong goroutine. Even if it informs the
// correct goroutine, that goroutine might ignore the request if it is
// simultaneously executing newstack.
// No lock needs to be held.
// Returns true if preemption request was issued.
// The actual preemption will happen at some point in the future
// and will be indicated by the gp->status no longer being
// Grunning
func preemptone(pp *p) bool {
        mp := pp.m.ptr()
        if mp == nil || mp == getg().m {
                return false
        }
        gp := mp.curg
        if gp == nil || gp == mp.g0 {
                return false
        }

        gp.preempt = true

        // Every call in a goroutine checks for stack overflow by
        // comparing the current stack pointer to gp->stackguard0.
        // Setting gp->stackguard0 to StackPreempt folds
        // preemption into the normal stack overflow check.
        gp.stackguard0 = stackPreempt

        // Request an async preemption of this P.
        if preemptMSupported && debug.asyncpreemptoff == 0 {
                pp.preempt = true
                preemptM(mp)
        }

        return true
}

var starttime int64

func schedtrace(detailed bool) {
        now := nanotime()
        if starttime == 0 {
                starttime = now
        }

        lock(&sched.lock)
        print("SCHED ", (now-starttime)/1e6, "ms: gomaxprocs=", gomaxprocs, " idleprocs=", sched.npidle.Load(), " threads=", mcount(), " spinningthreads=", sched.nmspinning.Load(), " needspinning=", sched.needspinning.Load(), " idlethreads=", sched.nmidle, " runqueue=", sched.runqsize)
        if detailed {
                print(" gcwaiting=", sched.gcwaiting.Load(), " nmidlelocked=", sched.nmidlelocked, " stopwait=", sched.stopwait, " sysmonwait=", sched.sysmonwait.Load(), "\n")
        }
        // We must be careful while reading data from P's, M's and G's.
        // Even if we hold schedlock, most data can be changed concurrently.
        // E.g. (p->m ? p->m->id : -1) can crash if p->m changes from non-nil to nil.
        for i, pp := range allp {
                mp := pp.m.ptr()
                h := atomic.Load(&pp.runqhead)
                t := atomic.Load(&pp.runqtail)
                if detailed {
                        print("  P", i, ": status=", pp.status, " schedtick=", pp.schedtick, " syscalltick=", pp.syscalltick, " m=")
                        if mp != nil {
                                print(mp.id)
                        } else {
                                print("nil")
                        }
                        print(" runqsize=", t-h, " gfreecnt=", pp.gFree.n, " timerslen=", len(pp.timers), "\n")
                } else {
                        // In non-detailed mode format lengths of per-P run queues as:
                        // [len1 len2 len3 len4]
                        print(" ")
                        if i == 0 {
                                print("[")
                        }
                        print(t - h)
                        if i == len(allp)-1 {
                                print("]\n")
                        }
                }
        }

        if !detailed {
                unlock(&sched.lock)
                return
        }

        for mp := allm; mp != nil; mp = mp.alllink {
                pp := mp.p.ptr()
                print("  M", mp.id, ": p=")
                if pp != nil {
                        print(pp.id)
                } else {
                        print("nil")
                }
                print(" curg=")
                if mp.curg != nil {
                        print(mp.curg.goid)
                } else {
                        print("nil")
                }
                print(" mallocing=", mp.mallocing, " throwing=", mp.throwing, " preemptoff=", mp.preemptoff, " locks=", mp.locks, " dying=", mp.dying, " spinning=", mp.spinning, " blocked=", mp.blocked, " lockedg=")
                if lockedg := mp.lockedg.ptr(); lockedg != nil {
                        print(lockedg.goid)
                } else {
                        print("nil")
                }
                print("\n")
        }

        forEachG(func(gp *g) {
                print("  G", gp.goid, ": status=", readgstatus(gp), "(", gp.waitreason.String(), ") m=")
                if gp.m != nil {
                        print(gp.m.id)
                } else {
                        print("nil")
                }
                print(" lockedm=")
                if lockedm := gp.lockedm.ptr(); lockedm != nil {
                        print(lockedm.id)
                } else {
                        print("nil")
                }
                print("\n")
        })
        unlock(&sched.lock)
}

// schedEnableUser enables or disables the scheduling of user
// goroutines.
//
// This does not stop already running user goroutines, so the caller
// should first stop the world when disabling user goroutines.
func schedEnableUser(enable bool) {
        lock(&sched.lock)
        if sched.disable.user == !enable {
                unlock(&sched.lock)
                return
        }
        sched.disable.user = !enable
        if enable {
                n := sched.disable.n
                sched.disable.n = 0
                globrunqputbatch(&sched.disable.runnable, n)
                unlock(&sched.lock)
                for ; n != 0 && sched.npidle.Load() != 0; n-- {
                        startm(nil, false, false)
                }
        } else {
                unlock(&sched.lock)
        }
}

// schedEnabled reports whether gp should be scheduled. It returns
// false is scheduling of gp is disabled.
//
// sched.lock must be held.
func schedEnabled(gp *g) bool {
        assertLockHeld(&sched.lock)

        if sched.disable.user {
                return isSystemGoroutine(gp, true)
        }
        return true
}

// Put mp on midle list.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func mput(mp *m) {
        assertLockHeld(&sched.lock)

        mp.schedlink = sched.midle
        sched.midle.set(mp)
        sched.nmidle++
        checkdead()
}

// Try to get an m from midle list.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func mget() *m {
        assertLockHeld(&sched.lock)

        mp := sched.midle.ptr()
        if mp != nil {
                sched.midle = mp.schedlink
                sched.nmidle--
        }
        return mp
}

// Put gp on the global runnable queue.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func globrunqput(gp *g) {
        assertLockHeld(&sched.lock)

        sched.runq.pushBack(gp)
        sched.runqsize++
}

// Put gp at the head of the global runnable queue.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func globrunqputhead(gp *g) {
        assertLockHeld(&sched.lock)

        sched.runq.push(gp)
        sched.runqsize++
}

// Put a batch of runnable goroutines on the global runnable queue.
// This clears *batch.
// sched.lock must be held.
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func globrunqputbatch(batch *gQueue, n int32) {
        assertLockHeld(&sched.lock)

        sched.runq.pushBackAll(*batch)
        sched.runqsize += n
        *batch = gQueue{}
}

// Try get a batch of G's from the global runnable queue.
// sched.lock must be held.
func globrunqget(pp *p, max int32) *g {
        assertLockHeld(&sched.lock)

        if sched.runqsize == 0 {
                return nil
        }

        n := sched.runqsize/gomaxprocs + 1
        if n > sched.runqsize {
                n = sched.runqsize
        }
        if max > 0 && n > max {
                n = max
        }
        if n > int32(len(pp.runq))/2 {
                n = int32(len(pp.runq)) / 2
        }

        sched.runqsize -= n

        gp := sched.runq.pop()
        n--
        for ; n > 0; n-- {
                gp1 := sched.runq.pop()
                runqput(pp, gp1, false)
        }
        return gp
}

// pMask is an atomic bitstring with one bit per P.
type pMask []uint32

// read returns true if P id's bit is set.
func (p pMask) read(id uint32) bool {
        word := id / 32
        mask := uint32(1) << (id % 32)
        return (atomic.Load(&p[word]) & mask) != 0
}

// set sets P id's bit.
func (p pMask) set(id int32) {
        word := id / 32
        mask := uint32(1) << (id % 32)
        atomic.Or(&p[word], mask)
}

// clear clears P id's bit.
func (p pMask) clear(id int32) {
        word := id / 32
        mask := uint32(1) << (id % 32)
        atomic.And(&p[word], ^mask)
}

// updateTimerPMask clears pp's timer mask if it has no timers on its heap.
//
// Ideally, the timer mask would be kept immediately consistent on any timer
// operations. Unfortunately, updating a shared global data structure in the
// timer hot path adds too much overhead in applications frequently switching
// between no timers and some timers.
//
// As a compromise, the timer mask is updated only on pidleget / pidleput. A
// running P (returned by pidleget) may add a timer at any time, so its mask
// must be set. An idle P (passed to pidleput) cannot add new timers while
// idle, so if it has no timers at that time, its mask may be cleared.
//
// Thus, we get the following effects on timer-stealing in findrunnable:
//
//   - Idle Ps with no timers when they go idle are never checked in findrunnable
//     (for work- or timer-stealing; this is the ideal case).
//   - Running Ps must always be checked.
//   - Idle Ps whose timers are stolen must continue to be checked until they run
//     again, even after timer expiration.
//
// When the P starts running again, the mask should be set, as a timer may be
// added at any time.
//
// TODO(prattmic): Additional targeted updates may improve the above cases.
// e.g., updating the mask when stealing a timer.
func updateTimerPMask(pp *p) {
        if pp.numTimers.Load() > 0 {
                return
        }

        // Looks like there are no timers, however another P may transiently
        // decrement numTimers when handling a timerModified timer in
        // checkTimers. We must take timersLock to serialize with these changes.
        lock(&pp.timersLock)
        if pp.numTimers.Load() == 0 {
                timerpMask.clear(pp.id)
        }
        unlock(&pp.timersLock)
}

// pidleput puts p on the _Pidle list. now must be a relatively recent call
// to nanotime or zero. Returns now or the current time if now was zero.
//
// This releases ownership of p. Once sched.lock is released it is no longer
// safe to use p.
//
// sched.lock must be held.
//
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func pidleput(pp *p, now int64) int64 {
        assertLockHeld(&sched.lock)

        if !runqempty(pp) {
                throw("pidleput: P has non-empty run queue")
        }
        if now == 0 {
                now = nanotime()
        }
        updateTimerPMask(pp) // clear if there are no timers.
        idlepMask.set(pp.id)
        pp.link = sched.pidle
        sched.pidle.set(pp)
        sched.npidle.Add(1)
        if !pp.limiterEvent.start(limiterEventIdle, now) {
                throw("must be able to track idle limiter event")
        }
        return now
}

// pidleget tries to get a p from the _Pidle list, acquiring ownership.
//
// sched.lock must be held.
//
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func pidleget(now int64) (*p, int64) {
        assertLockHeld(&sched.lock)

        pp := sched.pidle.ptr()
        if pp != nil {
                // Timer may get added at any time now.
                if now == 0 {
                        now = nanotime()
                }
                timerpMask.set(pp.id)
                idlepMask.clear(pp.id)
                sched.pidle = pp.link
                sched.npidle.Add(-1)
                pp.limiterEvent.stop(limiterEventIdle, now)
        }
        return pp, now
}

// pidlegetSpinning tries to get a p from the _Pidle list, acquiring ownership.
// This is called by spinning Ms (or callers than need a spinning M) that have
// found work. If no P is available, this must synchronized with non-spinning
// Ms that may be preparing to drop their P without discovering this work.
//
// sched.lock must be held.
//
// May run during STW, so write barriers are not allowed.
//
//go:nowritebarrierrec
func pidlegetSpinning(now int64) (*p, int64) {
        assertLockHeld(&sched.lock)

        pp, now := pidleget(now)
        if pp == nil {
                // See "Delicate dance" comment in findrunnable. We found work
                // that we cannot take, we must synchronize with non-spinning
                // Ms that may be preparing to drop their P.
                sched.needspinning.Store(1)
                return nil, now
        }

        return pp, now
}

// runqempty reports whether pp has no Gs on its local run queue.
// It never returns true spuriously.
func runqempty(pp *p) bool {
        // Defend against a race where 1) pp has G1 in runqnext but runqhead == runqtail,
        // 2) runqput on pp kicks G1 to the runq, 3) runqget on pp empties runqnext.
        // Simply observing that runqhead == runqtail and then observing that runqnext == nil
        // does not mean the queue is empty.
        for {
                head := atomic.Load(&pp.runqhead)
                tail := atomic.Load(&pp.runqtail)
                runnext := atomic.Loaduintptr((*uintptr)(unsafe.Pointer(&pp.runnext)))
                if tail == atomic.Load(&pp.runqtail) {
                        return head == tail && runnext == 0
                }
        }
}

// To shake out latent assumptions about scheduling order,
// we introduce some randomness into scheduling decisions
// when running with the race detector.
// The need for this was made obvious by changing the
// (deterministic) scheduling order in Go 1.5 and breaking
// many poorly-written tests.
// With the randomness here, as long as the tests pass
// consistently with -race, they shouldn't have latent scheduling
// assumptions.
const randomizeScheduler = raceenabled

// runqput tries to put g on the local runnable queue.
// If next is false, runqput adds g to the tail of the runnable queue.
// If next is true, runqput puts g in the pp.runnext slot.
// If the run queue is full, runnext puts g on the global queue.
// Executed only by the owner P.
func runqput(pp *p, gp *g, next bool) {
        if randomizeScheduler && next && fastrandn(2) == 0 {
                next = false
        }

        if next {
        retryNext:
                oldnext := pp.runnext
                if !pp.runnext.cas(oldnext, guintptr(unsafe.Pointer(gp))) {
                        goto retryNext
                }
                if oldnext == 0 {
                        return
                }
                // Kick the old runnext out to the regular run queue.
                gp = oldnext.ptr()
        }

retry:
        h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with consumers
        t := pp.runqtail
        if t-h < uint32(len(pp.runq)) {
                pp.runq[t%uint32(len(pp.runq))].set(gp)
                atomic.StoreRel(&pp.runqtail, t+1) // store-release, makes the item available for consumption
                return
        }
        if runqputslow(pp, gp, h, t) {
                return
        }
        // the queue is not full, now the put above must succeed
        goto retry
}

// Put g and a batch of work from local runnable queue on global queue.
// Executed only by the owner P.
func runqputslow(pp *p, gp *g, h, t uint32) bool {
        var batch [len(pp.runq)/2 + 1]*g

        // First, grab a batch from local queue.
        n := t - h
        n = n / 2
        if n != uint32(len(pp.runq)/2) {
                throw("runqputslow: queue is not full")
        }
        for i := uint32(0); i < n; i++ {
                batch[i] = pp.runq[(h+i)%uint32(len(pp.runq))].ptr()
        }
        if !atomic.CasRel(&pp.runqhead, h, h+n) { // cas-release, commits consume
                return false
        }
        batch[n] = gp

        if randomizeScheduler {
                for i := uint32(1); i <= n; i++ {
                        j := fastrandn(i + 1)
                        batch[i], batch[j] = batch[j], batch[i]
                }
        }

        // Link the goroutines.
        for i := uint32(0); i < n; i++ {
                batch[i].schedlink.set(batch[i+1])
        }
        var q gQueue
        q.head.set(batch[0])
        q.tail.set(batch[n])

        // Now put the batch on global queue.
        lock(&sched.lock)
        globrunqputbatch(&q, int32(n+1))
        unlock(&sched.lock)
        return true
}

// runqputbatch tries to put all the G's on q on the local runnable queue.
// If the queue is full, they are put on the global queue; in that case
// this will temporarily acquire the scheduler lock.
// Executed only by the owner P.
func runqputbatch(pp *p, q *gQueue, qsize int) {
        h := atomic.LoadAcq(&pp.runqhead)
        t := pp.runqtail
        n := uint32(0)
        for !q.empty() && t-h < uint32(len(pp.runq)) {
                gp := q.pop()
                pp.runq[t%uint32(len(pp.runq))].set(gp)
                t++
                n++
        }
        qsize -= int(n)

        if randomizeScheduler {
                off := func(o uint32) uint32 {
                        return (pp.runqtail + o) % uint32(len(pp.runq))
                }
                for i := uint32(1); i < n; i++ {
                        j := fastrandn(i + 1)
                        pp.runq[off(i)], pp.runq[off(j)] = pp.runq[off(j)], pp.runq[off(i)]
                }
        }

        atomic.StoreRel(&pp.runqtail, t)
        if !q.empty() {
                lock(&sched.lock)
                globrunqputbatch(q, int32(qsize))
                unlock(&sched.lock)
        }
}

// Get g from local runnable queue.
// If inheritTime is true, gp should inherit the remaining time in the
// current time slice. Otherwise, it should start a new time slice.
// Executed only by the owner P.
func runqget(pp *p) (gp *g, inheritTime bool) {
        // If there's a runnext, it's the next G to run.
        next := pp.runnext
        // If the runnext is non-0 and the CAS fails, it could only have been stolen by another P,
        // because other Ps can race to set runnext to 0, but only the current P can set it to non-0.
        // Hence, there's no need to retry this CAS if it fails.
        if next != 0 && pp.runnext.cas(next, 0) {
                return next.ptr(), true
        }

        for {
                h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with other consumers
                t := pp.runqtail
                if t == h {
                        return nil, false
                }
                gp := pp.runq[h%uint32(len(pp.runq))].ptr()
                if atomic.CasRel(&pp.runqhead, h, h+1) { // cas-release, commits consume
                        return gp, false
                }
        }
}

// runqdrain drains the local runnable queue of pp and returns all goroutines in it.
// Executed only by the owner P.
func runqdrain(pp *p) (drainQ gQueue, n uint32) {
        oldNext := pp.runnext
        if oldNext != 0 && pp.runnext.cas(oldNext, 0) {
                drainQ.pushBack(oldNext.ptr())
                n++
        }

retry:
        h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with other consumers
        t := pp.runqtail
        qn := t - h
        if qn == 0 {
                return
        }
        if qn > uint32(len(pp.runq)) { // read inconsistent h and t
                goto retry
        }

        if !atomic.CasRel(&pp.runqhead, h, h+qn) { // cas-release, commits consume
                goto retry
        }

        // We've inverted the order in which it gets G's from the local P's runnable queue
        // and then advances the head pointer because we don't want to mess up the statuses of G's
        // while runqdrain() and runqsteal() are running in parallel.
        // Thus we should advance the head pointer before draining the local P into a gQueue,
        // so that we can update any gp.schedlink only after we take the full ownership of G,
        // meanwhile, other P's can't access to all G's in local P's runnable queue and steal them.
        // See https://groups.google.com/g/golang-dev/c/0pTKxEKhHSc/m/6Q85QjdVBQAJ for more details.
        for i := uint32(0); i < qn; i++ {
                gp := pp.runq[(h+i)%uint32(len(pp.runq))].ptr()
                drainQ.pushBack(gp)
                n++
        }
        return
}

// Grabs a batch of goroutines from pp's runnable queue into batch.
// Batch is a ring buffer starting at batchHead.
// Returns number of grabbed goroutines.
// Can be executed by any P.
func runqgrab(pp *p, batch *[256]guintptr, batchHead uint32, stealRunNextG bool) uint32 {
        for {
                h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with other consumers
                t := atomic.LoadAcq(&pp.runqtail) // load-acquire, synchronize with the producer
                n := t - h
                n = n - n/2
                if n == 0 {
                        if stealRunNextG {
                                // Try to steal from pp.runnext.
                                if next := pp.runnext; next != 0 {
                                        if pp.status == _Prunning {
                                                // Sleep to ensure that pp isn't about to run the g
                                                // we are about to steal.
                                                // The important use case here is when the g running
                                                // on pp ready()s another g and then almost
                                                // immediately blocks. Instead of stealing runnext
                                                // in this window, back off to give pp a chance to
                                                // schedule runnext. This will avoid thrashing gs
                                                // between different Ps.
                                                // A sync chan send/recv takes ~50ns as of time of
                                                // writing, so 3us gives ~50x overshoot.
                                                if GOOS != "windows" && GOOS != "openbsd" && GOOS != "netbsd" {
                                                        usleep(3)
                                                } else {
                                                        // On some platforms system timer granularity is
                                                        // 1-15ms, which is way too much for this
                                                        // optimization. So just yield.
                                                        osyield()
                                                }
                                        }
                                        if !pp.runnext.cas(next, 0) {
                                                continue
                                        }
                                        batch[batchHead%uint32(len(batch))] = next
                                        return 1
                                }
                        }
                        return 0
                }
                if n > uint32(len(pp.runq)/2) { // read inconsistent h and t
                        continue
                }
                for i := uint32(0); i < n; i++ {
                        g := pp.runq[(h+i)%uint32(len(pp.runq))]
                        batch[(batchHead+i)%uint32(len(batch))] = g
                }
                if atomic.CasRel(&pp.runqhead, h, h+n) { // cas-release, commits consume
                        return n
                }
        }
}

// Steal half of elements from local runnable queue of p2
// and put onto local runnable queue of p.
// Returns one of the stolen elements (or nil if failed).
func runqsteal(pp, p2 *p, stealRunNextG bool) *g {
        t := pp.runqtail
        n := runqgrab(p2, &pp.runq, t, stealRunNextG)
        if n == 0 {
                return nil
        }
        n--
        gp := pp.runq[(t+n)%uint32(len(pp.runq))].ptr()
        if n == 0 {
                return gp
        }
        h := atomic.LoadAcq(&pp.runqhead) // load-acquire, synchronize with consumers
        if t-h+n >= uint32(len(pp.runq)) {
                throw("runqsteal: runq overflow")
        }
        atomic.StoreRel(&pp.runqtail, t+n) // store-release, makes the item available for consumption
        return gp
}

// A gQueue is a dequeue of Gs linked through g.schedlink. A G can only
// be on one gQueue or gList at a time.
type gQueue struct {
        head guintptr
        tail guintptr
}

// empty reports whether q is empty.
func (q *gQueue) empty() bool {
        return q.head == 0
}

// push adds gp to the head of q.
func (q *gQueue) push(gp *g) {
        gp.schedlink = q.head
        q.head.set(gp)
        if q.tail == 0 {
                q.tail.set(gp)
        }
}

// pushBack adds gp to the tail of q.
func (q *gQueue) pushBack(gp *g) {
        gp.schedlink = 0
        if q.tail != 0 {
                q.tail.ptr().schedlink.set(gp)
        } else {
                q.head.set(gp)
        }
        q.tail.set(gp)
}

// pushBackAll adds all Gs in q2 to the tail of q. After this q2 must
// not be used.
func (q *gQueue) pushBackAll(q2 gQueue) {
        if q2.tail == 0 {
                return
        }
        q2.tail.ptr().schedlink = 0
        if q.tail != 0 {
                q.tail.ptr().schedlink = q2.head
        } else {
                q.head = q2.head
        }
        q.tail = q2.tail
}

// pop removes and returns the head of queue q. It returns nil if
// q is empty.
func (q *gQueue) pop() *g {
        gp := q.head.ptr()
        if gp != nil {
                q.head = gp.schedlink
                if q.head == 0 {
                        q.tail = 0
                }
        }
        return gp
}

// popList takes all Gs in q and returns them as a gList.
func (q *gQueue) popList() gList {
        stack := gList{q.head}
        *q = gQueue{}
        return stack
}

// A gList is a list of Gs linked through g.schedlink. A G can only be
// on one gQueue or gList at a time.
type gList struct {
        head guintptr
}

// empty reports whether l is empty.
func (l *gList) empty() bool {
        return l.head == 0
}

// push adds gp to the head of l.
func (l *gList) push(gp *g) {
        gp.schedlink = l.head
        l.head.set(gp)
}

// pushAll prepends all Gs in q to l.
func (l *gList) pushAll(q gQueue) {
        if !q.empty() {
                q.tail.ptr().schedlink = l.head
                l.head = q.head
        }
}

// pop removes and returns the head of l. If l is empty, it returns nil.
func (l *gList) pop() *g {
        gp := l.head.ptr()
        if gp != nil {
                l.head = gp.schedlink
        }
        return gp
}

//go:linkname setMaxThreads runtime/debug.setMaxThreads
func setMaxThreads(in int) (out int) {
        lock(&sched.lock)
        out = int(sched.maxmcount)
        if in > 0x7fffffff { // MaxInt32
                sched.maxmcount = 0x7fffffff
        } else {
                sched.maxmcount = int32(in)
        }
        checkmcount()
        unlock(&sched.lock)
        return
}

//go:nosplit
func procPin() int {
        gp := getg()
        mp := gp.m

        mp.locks++
        return int(mp.p.ptr().id)
}

//go:nosplit
func procUnpin() {
        gp := getg()
        gp.m.locks--
}

//go:linkname sync_runtime_procPin sync.runtime_procPin
//go:nosplit
func sync_runtime_procPin() int {
        return procPin()
}

//go:linkname sync_runtime_procUnpin sync.runtime_procUnpin
//go:nosplit
func sync_runtime_procUnpin() {
        procUnpin()
}

//go:linkname sync_atomic_runtime_procPin sync/atomic.runtime_procPin
//go:nosplit
func sync_atomic_runtime_procPin() int {
        return procPin()
}

//go:linkname sync_atomic_runtime_procUnpin sync/atomic.runtime_procUnpin
//go:nosplit
func sync_atomic_runtime_procUnpin() {
        procUnpin()
}

// Active spinning for sync.Mutex.
//
//go:linkname sync_runtime_canSpin sync.runtime_canSpin
//go:nosplit
func sync_runtime_canSpin(i int) bool {
        // sync.Mutex is cooperative, so we are conservative with spinning.
        // Spin only few times and only if running on a multicore machine and
        // GOMAXPROCS>1 and there is at least one other running P and local runq is empty.
        // As opposed to runtime mutex we don't do passive spinning here,
        // because there can be work on global runq or on other Ps.
        if i >= active_spin || ncpu <= 1 || gomaxprocs <= sched.npidle.Load()+sched.nmspinning.Load()+1 {
                return false
        }
        if p := getg().m.p.ptr(); !runqempty(p) {
                return false
        }
        return true
}

//go:linkname sync_runtime_doSpin sync.runtime_doSpin
//go:nosplit
func sync_runtime_doSpin() {
        procyield(active_spin_cnt)
}

var stealOrder randomOrder

// randomOrder/randomEnum are helper types for randomized work stealing.
// They allow to enumerate all Ps in different pseudo-random orders without repetitions.
// The algorithm is based on the fact that if we have X such that X and GOMAXPROCS
// are coprime, then a sequences of (i + X) % GOMAXPROCS gives the required enumeration.
type randomOrder struct {
        count    uint32
        coprimes []uint32
}

type randomEnum struct {
        i     uint32
        count uint32
        pos   uint32
        inc   uint32
}

func (ord *randomOrder) reset(count uint32) {
        ord.count = count
        ord.coprimes = ord.coprimes[:0]
        for i := uint32(1); i <= count; i++ {
                if gcd(i, count) == 1 {
                        ord.coprimes = append(ord.coprimes, i)
                }
        }
}

func (ord *randomOrder) start(i uint32) randomEnum {
        return randomEnum{
                count: ord.count,
                pos:   i % ord.count,
                inc:   ord.coprimes[i/ord.count%uint32(len(ord.coprimes))],
        }
}

func (enum *randomEnum) done() bool {
        return enum.i == enum.count
}

func (enum *randomEnum) next() {
        enum.i++
        enum.pos = (enum.pos + enum.inc) % enum.count
}

func (enum *randomEnum) position() uint32 {
        return enum.pos
}

func gcd(a, b uint32) uint32 {
        for b != 0 {
                a, b = b, a%b
        }
        return a
}

// An initTask represents the set of initializations that need to be done for a package.
// Keep in sync with ../../test/noinit.go:initTask
type initTask struct {
        state uint32 // 0 = uninitialized, 1 = in progress, 2 = done
        nfns  uint32
        // followed by nfns pcs, uintptr sized, one per init function to run
}

// inittrace stores statistics for init functions which are
// updated by malloc and newproc when active is true.
var inittrace tracestat

type tracestat struct {
        active bool   // init tracing activation status
        id     uint64 // init goroutine id
        allocs uint64 // heap allocations
        bytes  uint64 // heap allocated bytes
}

func doInit(ts []*initTask) {
        for _, t := range ts {
                doInit1(t)
        }
}

func doInit1(t *initTask) {
        switch t.state {
        case 2: // fully initialized
                return
        case 1: // initialization in progress
                throw("recursive call during initialization - linker skew")
        default: // not initialized yet
                t.state = 1 // initialization in progress

                var (
                        start  int64
                        before tracestat
                )

                if inittrace.active {
                        start = nanotime()
                        // Load stats non-atomically since tracinit is updated only by this init goroutine.
                        before = inittrace
                }

                if t.nfns == 0 {
                        // We should have pruned all of these in the linker.
                        throw("inittask with no functions")
                }

                firstFunc := add(unsafe.Pointer(t), 8)
                for i := uint32(0); i < t.nfns; i++ {
                        p := add(firstFunc, uintptr(i)*goarch.PtrSize)
                        f := *(*func())(unsafe.Pointer(&p))
                        f()
                }

                if inittrace.active {
                        end := nanotime()
                        // Load stats non-atomically since tracinit is updated only by this init goroutine.
                        after := inittrace

                        f := *(*func())(unsafe.Pointer(&firstFunc))
                        pkg := funcpkgpath(findfunc(abi.FuncPCABIInternal(f)))

                        var sbuf [24]byte
                        print("init ", pkg, " @")
                        print(string(fmtNSAsMS(sbuf[:], uint64(start-runtimeInitTime))), " ms, ")
                        print(string(fmtNSAsMS(sbuf[:], uint64(end-start))), " ms clock, ")
                        print(string(itoa(sbuf[:], after.bytes-before.bytes)), " bytes, ")
                        print(string(itoa(sbuf[:], after.allocs-before.allocs)), " allocs")
                        print("\n")
                }

                t.state = 2 // initialization done
        }
}