1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Garbage collector: finalizers and block profiling.
12 "runtime/internal/atomic"
13 "runtime/internal/sys"
17 // finblock is an array of finalizers to be executed. finblocks are
18 // arranged in a linked list for the finalizer queue.
20 // finblock is allocated from non-GC'd memory, so any heap pointers
21 // must be specially handled. GC currently assumes that the finalizer
22 // queue does not grow during marking (but it can shrink).
23 type finblock struct {
29 fin [(_FinBlockSize - 2*goarch.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
32 var fingStatus atomic.Uint32
34 // finalizer goroutine status.
36 fingUninitialized uint32 = iota
37 fingCreated uint32 = 1 << (iota - 1)
43 var finlock mutex // protects the following variables
44 var fing *g // goroutine that runs finalizers
45 var finq *finblock // list of finalizers that are to be executed
46 var finc *finblock // cache of free blocks
47 var finptrmask [_FinBlockSize / goarch.PtrSize / 8]byte
49 var allfin *finblock // list of all blocks
51 // NOTE: Layout known to queuefinalizer.
52 type finalizer struct {
53 fn *funcval // function to call (may be a heap pointer)
54 arg unsafe.Pointer // ptr to object (may be a heap pointer)
55 nret uintptr // bytes of return values from fn
56 fint *_type // type of first argument of fn
57 ot *ptrtype // type of ptr to object (may be a heap pointer)
60 var finalizer1 = [...]byte{
61 // Each Finalizer is 5 words, ptr ptr INT ptr ptr (INT = uintptr here)
62 // Each byte describes 8 words.
63 // Need 8 Finalizers described by 5 bytes before pattern repeats:
64 // ptr ptr INT ptr ptr
65 // ptr ptr INT ptr ptr
66 // ptr ptr INT ptr ptr
67 // ptr ptr INT ptr ptr
68 // ptr ptr INT ptr ptr
69 // ptr ptr INT ptr ptr
70 // ptr ptr INT ptr ptr
71 // ptr ptr INT ptr ptr
74 // ptr ptr INT ptr ptr ptr ptr INT
75 // ptr ptr ptr ptr INT ptr ptr ptr
76 // ptr INT ptr ptr ptr ptr INT ptr
77 // ptr ptr ptr INT ptr ptr ptr ptr
78 // INT ptr ptr ptr ptr INT ptr ptr
80 // Assumptions about Finalizer layout checked below.
81 1<<0 | 1<<1 | 0<<2 | 1<<3 | 1<<4 | 1<<5 | 1<<6 | 0<<7,
82 1<<0 | 1<<1 | 1<<2 | 1<<3 | 0<<4 | 1<<5 | 1<<6 | 1<<7,
83 1<<0 | 0<<1 | 1<<2 | 1<<3 | 1<<4 | 1<<5 | 0<<6 | 1<<7,
84 1<<0 | 1<<1 | 1<<2 | 0<<3 | 1<<4 | 1<<5 | 1<<6 | 1<<7,
85 0<<0 | 1<<1 | 1<<2 | 1<<3 | 1<<4 | 0<<5 | 1<<6 | 1<<7,
88 // lockRankMayQueueFinalizer records the lock ranking effects of a
89 // function that may call queuefinalizer.
90 func lockRankMayQueueFinalizer() {
91 lockWithRankMayAcquire(&finlock, getLockRank(&finlock))
94 func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
95 if gcphase != _GCoff {
96 // Currently we assume that the finalizer queue won't
97 // grow during marking so we don't have to rescan it
98 // during mark termination. If we ever need to lift
99 // this assumption, we can do it by adding the
100 // necessary barriers to queuefinalizer (which it may
101 // have automatically).
102 throw("queuefinalizer during GC")
106 if finq == nil || finq.cnt == uint32(len(finq.fin)) {
108 finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gcMiscSys))
109 finc.alllink = allfin
111 if finptrmask[0] == 0 {
112 // Build pointer mask for Finalizer array in block.
113 // Check assumptions made in finalizer1 array above.
114 if (unsafe.Sizeof(finalizer{}) != 5*goarch.PtrSize ||
115 unsafe.Offsetof(finalizer{}.fn) != 0 ||
116 unsafe.Offsetof(finalizer{}.arg) != goarch.PtrSize ||
117 unsafe.Offsetof(finalizer{}.nret) != 2*goarch.PtrSize ||
118 unsafe.Offsetof(finalizer{}.fint) != 3*goarch.PtrSize ||
119 unsafe.Offsetof(finalizer{}.ot) != 4*goarch.PtrSize) {
120 throw("finalizer out of sync")
122 for i := range finptrmask {
123 finptrmask[i] = finalizer1[i%len(finalizer1)]
132 f := &finq.fin[finq.cnt]
133 atomic.Xadd(&finq.cnt, +1) // Sync with markroots
140 fingStatus.Or(fingWake)
144 func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) {
145 for fb := allfin; fb != nil; fb = fb.alllink {
146 for i := uint32(0); i < fb.cnt; i++ {
148 callback(f.fn, f.arg, f.nret, f.fint, f.ot)
154 if ok := fingStatus.CompareAndSwap(fingCreated|fingWait|fingWake, fingCreated); ok {
161 // start the finalizer goroutine exactly once
162 if fingStatus.Load() == fingUninitialized && fingStatus.CompareAndSwap(fingUninitialized, fingCreated) {
167 func finalizercommit(gp *g, lock unsafe.Pointer) bool {
168 unlock((*mutex)(lock))
169 // fingStatus should be modified after fing is put into a waiting state
170 // to avoid waking fing in running state, even if it is about to be parked.
171 fingStatus.Or(fingWait)
175 // This is the goroutine that runs all of the finalizers.
193 gopark(finalizercommit, unsafe.Pointer(&finlock), waitReasonFinalizerWait, traceBlockSystemGoroutine, 1)
202 for i := fb.cnt; i > 0; i-- {
206 // The args may be passed in registers or on stack. Even for
207 // the register case, we still need the spill slots.
208 // TODO: revisit if we remove spill slots.
210 // Unfortunately because we can have an arbitrary
211 // amount of returns and it would be complex to try and
212 // figure out how many of those can get passed in registers,
213 // just conservatively assume none of them do.
214 framesz := unsafe.Sizeof((any)(nil)) + f.nret
215 if framecap < framesz {
216 // The frame does not contain pointers interesting for GC,
217 // all not yet finalized objects are stored in finq.
218 // If we do not mark it as FlagNoScan,
219 // the last finalized object is not collected.
220 frame = mallocgc(framesz, nil, true)
225 throw("missing type in runfinq")
229 r = unsafe.Pointer(®s.Ints)
231 // frame is effectively uninitialized
232 // memory. That means we have to clear
233 // it before writing to it to avoid
234 // confusing the write barrier.
235 *(*[2]uintptr)(frame) = [2]uintptr{}
237 switch f.fint.Kind_ & kindMask {
239 // direct use of pointer
240 *(*unsafe.Pointer)(r) = f.arg
242 ityp := (*interfacetype)(unsafe.Pointer(f.fint))
243 // set up with empty interface
244 (*eface)(r)._type = &f.ot.Type
245 (*eface)(r).data = f.arg
246 if len(ityp.Methods) != 0 {
247 // convert to interface with methods
248 // this conversion is guaranteed to succeed - we checked in SetFinalizer
249 (*iface)(r).tab = assertE2I(ityp, (*eface)(r)._type)
252 throw("bad kind in runfinq")
254 fingStatus.Or(fingRunningFinalizer)
255 reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), ®s)
256 fingStatus.And(^fingRunningFinalizer)
258 // Drop finalizer queue heap references
259 // before hiding them from markroot.
260 // This also ensures these will be
261 // clear if we reuse the finalizer.
265 atomic.Store(&fb.cnt, i-1)
277 func isGoPointerWithoutSpan(p unsafe.Pointer) bool {
278 // 0-length objects are okay.
279 if p == unsafe.Pointer(&zerobase) {
283 // Global initializers might be linker-allocated.
284 // var Foo = &Object{}
286 // runtime.SetFinalizer(Foo, nil)
288 // The relevant segments are: noptrdata, data, bss, noptrbss.
289 // We cannot assume they are in any order or even contiguous,
290 // due to external linking.
291 for datap := &firstmoduledata; datap != nil; datap = datap.next {
292 if datap.noptrdata <= uintptr(p) && uintptr(p) < datap.enoptrdata ||
293 datap.data <= uintptr(p) && uintptr(p) < datap.edata ||
294 datap.bss <= uintptr(p) && uintptr(p) < datap.ebss ||
295 datap.noptrbss <= uintptr(p) && uintptr(p) < datap.enoptrbss {
302 // SetFinalizer sets the finalizer associated with obj to the provided
303 // finalizer function. When the garbage collector finds an unreachable block
304 // with an associated finalizer, it clears the association and runs
305 // finalizer(obj) in a separate goroutine. This makes obj reachable again,
306 // but now without an associated finalizer. Assuming that SetFinalizer
307 // is not called again, the next time the garbage collector sees
308 // that obj is unreachable, it will free obj.
310 // SetFinalizer(obj, nil) clears any finalizer associated with obj.
312 // The argument obj must be a pointer to an object allocated by calling
313 // new, by taking the address of a composite literal, or by taking the
314 // address of a local variable.
315 // The argument finalizer must be a function that takes a single argument
316 // to which obj's type can be assigned, and can have arbitrary ignored return
317 // values. If either of these is not true, SetFinalizer may abort the
320 // Finalizers are run in dependency order: if A points at B, both have
321 // finalizers, and they are otherwise unreachable, only the finalizer
322 // for A runs; once A is freed, the finalizer for B can run.
323 // If a cyclic structure includes a block with a finalizer, that
324 // cycle is not guaranteed to be garbage collected and the finalizer
325 // is not guaranteed to run, because there is no ordering that
326 // respects the dependencies.
328 // The finalizer is scheduled to run at some arbitrary time after the
329 // program can no longer reach the object to which obj points.
330 // There is no guarantee that finalizers will run before a program exits,
331 // so typically they are useful only for releasing non-memory resources
332 // associated with an object during a long-running program.
333 // For example, an [os.File] object could use a finalizer to close the
334 // associated operating system file descriptor when a program discards
335 // an os.File without calling Close, but it would be a mistake
336 // to depend on a finalizer to flush an in-memory I/O buffer such as a
337 // [bufio.Writer], because the buffer would not be flushed at program exit.
339 // It is not guaranteed that a finalizer will run if the size of *obj is
340 // zero bytes, because it may share same address with other zero-size
341 // objects in memory. See https://go.dev/ref/spec#Size_and_alignment_guarantees.
343 // It is not guaranteed that a finalizer will run for objects allocated
344 // in initializers for package-level variables. Such objects may be
345 // linker-allocated, not heap-allocated.
347 // Note that because finalizers may execute arbitrarily far into the future
348 // after an object is no longer referenced, the runtime is allowed to perform
349 // a space-saving optimization that batches objects together in a single
350 // allocation slot. The finalizer for an unreferenced object in such an
351 // allocation may never run if it always exists in the same batch as a
352 // referenced object. Typically, this batching only happens for tiny
353 // (on the order of 16 bytes or less) and pointer-free objects.
355 // A finalizer may run as soon as an object becomes unreachable.
356 // In order to use finalizers correctly, the program must ensure that
357 // the object is reachable until it is no longer required.
358 // Objects stored in global variables, or that can be found by tracing
359 // pointers from a global variable, are reachable. For other objects,
360 // pass the object to a call of the [KeepAlive] function to mark the
361 // last point in the function where the object must be reachable.
363 // For example, if p points to a struct, such as os.File, that contains
364 // a file descriptor d, and p has a finalizer that closes that file
365 // descriptor, and if the last use of p in a function is a call to
366 // syscall.Write(p.d, buf, size), then p may be unreachable as soon as
367 // the program enters [syscall.Write]. The finalizer may run at that moment,
368 // closing p.d, causing syscall.Write to fail because it is writing to
369 // a closed file descriptor (or, worse, to an entirely different
370 // file descriptor opened by a different goroutine). To avoid this problem,
371 // call KeepAlive(p) after the call to syscall.Write.
373 // A single goroutine runs all finalizers for a program, sequentially.
374 // If a finalizer must run for a long time, it should do so by starting
377 // In the terminology of the Go memory model, a call
378 // SetFinalizer(x, f) “synchronizes before” the finalization call f(x).
379 // However, there is no guarantee that KeepAlive(x) or any other use of x
380 // “synchronizes before” f(x), so in general a finalizer should use a mutex
381 // or other synchronization mechanism if it needs to access mutable state in x.
382 // For example, consider a finalizer that inspects a mutable field in x
383 // that is modified from time to time in the main program before x
384 // becomes unreachable and the finalizer is invoked.
385 // The modifications in the main program and the inspection in the finalizer
386 // need to use appropriate synchronization, such as mutexes or atomic updates,
387 // to avoid read-write races.
388 func SetFinalizer(obj any, finalizer any) {
390 // debug.sbrk never frees memory, so no finalizers run
391 // (and we don't have the data structures to record them).
397 throw("runtime.SetFinalizer: first argument is nil")
399 if etyp.Kind_&kindMask != kindPtr {
400 throw("runtime.SetFinalizer: first argument is " + toRType(etyp).string() + ", not pointer")
402 ot := (*ptrtype)(unsafe.Pointer(etyp))
404 throw("nil elem type!")
407 if inUserArenaChunk(uintptr(e.data)) {
408 // Arena-allocated objects are not eligible for finalizers.
409 throw("runtime.SetFinalizer: first argument was allocated into an arena")
412 // find the containing object
413 base, _, _ := findObject(uintptr(e.data), 0, 0)
416 if isGoPointerWithoutSpan(e.data) {
419 throw("runtime.SetFinalizer: pointer not in allocated block")
422 if uintptr(e.data) != base {
423 // As an implementation detail we allow to set finalizers for an inner byte
424 // of an object if it could come from tiny alloc (see mallocgc for details).
425 if ot.Elem == nil || ot.Elem.PtrBytes != 0 || ot.Elem.Size_ >= maxTinySize {
426 throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
430 f := efaceOf(&finalizer)
433 // switch to system stack and remove finalizer
435 removefinalizer(e.data)
440 if ftyp.Kind_&kindMask != kindFunc {
441 throw("runtime.SetFinalizer: second argument is " + toRType(ftyp).string() + ", not a function")
443 ft := (*functype)(unsafe.Pointer(ftyp))
445 throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string() + " because dotdotdot")
448 throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string())
450 fint := ft.InSlice()[0]
455 case fint.Kind_&kindMask == kindPtr:
456 if (fint.Uncommon() == nil || etyp.Uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).Elem == ot.Elem {
457 // ok - not same type, but both pointers,
458 // one or the other is unnamed, and same element type, so assignable.
461 case fint.Kind_&kindMask == kindInterface:
462 ityp := (*interfacetype)(unsafe.Pointer(fint))
463 if len(ityp.Methods) == 0 {
464 // ok - satisfies empty interface
467 if itab := assertE2I2(ityp, efaceOf(&obj)._type); itab != nil {
471 throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string())
473 // compute size needed for return parameters
475 for _, t := range ft.OutSlice() {
476 nret = alignUp(nret, uintptr(t.Align_)) + t.Size_
478 nret = alignUp(nret, goarch.PtrSize)
480 // make sure we have a finalizer goroutine
484 if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
485 throw("runtime.SetFinalizer: finalizer already set")
490 // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
494 // KeepAlive marks its argument as currently reachable.
495 // This ensures that the object is not freed, and its finalizer is not run,
496 // before the point in the program where KeepAlive is called.
498 // A very simplified example showing where KeepAlive is required:
500 // type File struct { d int }
501 // d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
502 // // ... do something if err != nil ...
504 // runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
506 // n, err := syscall.Read(p.d, buf[:])
507 // // Ensure p is not finalized until Read returns.
508 // runtime.KeepAlive(p)
509 // // No more uses of p after this point.
511 // Without the KeepAlive call, the finalizer could run at the start of
512 // [syscall.Read], closing the file descriptor before syscall.Read makes
513 // the actual system call.
515 // Note: KeepAlive should only be used to prevent finalizers from
516 // running prematurely. In particular, when used with [unsafe.Pointer],
517 // the rules for valid uses of unsafe.Pointer still apply.
518 func KeepAlive(x any) {
519 // Introduce a use of x that the compiler can't eliminate.
520 // This makes sure x is alive on entry. We need x to be alive
521 // on entry for "defer runtime.KeepAlive(x)"; see issue 21402.