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"
16 // finblock is an array of finalizers to be executed. finblocks are
17 // arranged in a linked list for the finalizer queue.
19 // finblock is allocated from non-GC'd memory, so any heap pointers
20 // must be specially handled. GC currently assumes that the finalizer
21 // queue does not grow during marking (but it can shrink).
24 type finblock struct {
29 fin [(_FinBlockSize - 2*goarch.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
32 var finlock mutex // protects the following variables
33 var fing *g // goroutine that runs finalizers
34 var finq *finblock // list of finalizers that are to be executed
35 var finc *finblock // cache of free blocks
36 var finptrmask [_FinBlockSize / goarch.PtrSize / 8]byte
39 var allfin *finblock // list of all blocks
41 // NOTE: Layout known to queuefinalizer.
42 type finalizer struct {
43 fn *funcval // function to call (may be a heap pointer)
44 arg unsafe.Pointer // ptr to object (may be a heap pointer)
45 nret uintptr // bytes of return values from fn
46 fint *_type // type of first argument of fn
47 ot *ptrtype // type of ptr to object (may be a heap pointer)
50 var finalizer1 = [...]byte{
51 // Each Finalizer is 5 words, ptr ptr INT ptr ptr (INT = uintptr here)
52 // Each byte describes 8 words.
53 // Need 8 Finalizers described by 5 bytes before pattern repeats:
54 // ptr ptr INT ptr ptr
55 // ptr ptr INT ptr ptr
56 // ptr ptr INT ptr ptr
57 // ptr ptr INT ptr ptr
58 // ptr ptr INT ptr ptr
59 // ptr ptr INT ptr ptr
60 // ptr ptr INT ptr ptr
61 // ptr ptr INT ptr ptr
64 // ptr ptr INT ptr ptr ptr ptr INT
65 // ptr ptr ptr ptr INT ptr ptr ptr
66 // ptr INT ptr ptr ptr ptr INT ptr
67 // ptr ptr ptr INT ptr ptr ptr ptr
68 // INT ptr ptr ptr ptr INT ptr ptr
70 // Assumptions about Finalizer layout checked below.
71 1<<0 | 1<<1 | 0<<2 | 1<<3 | 1<<4 | 1<<5 | 1<<6 | 0<<7,
72 1<<0 | 1<<1 | 1<<2 | 1<<3 | 0<<4 | 1<<5 | 1<<6 | 1<<7,
73 1<<0 | 0<<1 | 1<<2 | 1<<3 | 1<<4 | 1<<5 | 0<<6 | 1<<7,
74 1<<0 | 1<<1 | 1<<2 | 0<<3 | 1<<4 | 1<<5 | 1<<6 | 1<<7,
75 0<<0 | 1<<1 | 1<<2 | 1<<3 | 1<<4 | 0<<5 | 1<<6 | 1<<7,
78 // lockRankMayQueueFinalizer records the lock ranking effects of a
79 // function that may call queuefinalizer.
80 func lockRankMayQueueFinalizer() {
81 lockWithRankMayAcquire(&finlock, getLockRank(&finlock))
84 func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
85 if gcphase != _GCoff {
86 // Currently we assume that the finalizer queue won't
87 // grow during marking so we don't have to rescan it
88 // during mark termination. If we ever need to lift
89 // this assumption, we can do it by adding the
90 // necessary barriers to queuefinalizer (which it may
91 // have automatically).
92 throw("queuefinalizer during GC")
96 if finq == nil || finq.cnt == uint32(len(finq.fin)) {
98 finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gcMiscSys))
101 if finptrmask[0] == 0 {
102 // Build pointer mask for Finalizer array in block.
103 // Check assumptions made in finalizer1 array above.
104 if (unsafe.Sizeof(finalizer{}) != 5*goarch.PtrSize ||
105 unsafe.Offsetof(finalizer{}.fn) != 0 ||
106 unsafe.Offsetof(finalizer{}.arg) != goarch.PtrSize ||
107 unsafe.Offsetof(finalizer{}.nret) != 2*goarch.PtrSize ||
108 unsafe.Offsetof(finalizer{}.fint) != 3*goarch.PtrSize ||
109 unsafe.Offsetof(finalizer{}.ot) != 4*goarch.PtrSize) {
110 throw("finalizer out of sync")
112 for i := range finptrmask {
113 finptrmask[i] = finalizer1[i%len(finalizer1)]
122 f := &finq.fin[finq.cnt]
123 atomic.Xadd(&finq.cnt, +1) // Sync with markroots
134 func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) {
135 for fb := allfin; fb != nil; fb = fb.alllink {
136 for i := uint32(0); i < fb.cnt; i++ {
138 callback(f.fn, f.arg, f.nret, f.fint, f.ot)
146 if fingwait && fingwake {
161 // start the finalizer goroutine exactly once
162 if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
167 // This is the goroutine that runs all of the finalizers
186 goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
195 for i := fb.cnt; i > 0; i-- {
199 // The args may be passed in registers or on stack. Even for
200 // the register case, we still need the spill slots.
201 // TODO: revisit if we remove spill slots.
203 // Unfortunately because we can have an arbitrary
204 // amount of returns and it would be complex to try and
205 // figure out how many of those can get passed in registers,
206 // just conservatively assume none of them do.
207 framesz := unsafe.Sizeof((any)(nil)) + f.nret
208 if framecap < framesz {
209 // The frame does not contain pointers interesting for GC,
210 // all not yet finalized objects are stored in finq.
211 // If we do not mark it as FlagNoScan,
212 // the last finalized object is not collected.
213 frame = mallocgc(framesz, nil, true)
218 throw("missing type in runfinq")
222 r = unsafe.Pointer(®s.Ints)
224 // frame is effectively uninitialized
225 // memory. That means we have to clear
226 // it before writing to it to avoid
227 // confusing the write barrier.
228 *(*[2]uintptr)(frame) = [2]uintptr{}
230 switch f.fint.kind & kindMask {
232 // direct use of pointer
233 *(*unsafe.Pointer)(r) = f.arg
235 ityp := (*interfacetype)(unsafe.Pointer(f.fint))
236 // set up with empty interface
237 (*eface)(r)._type = &f.ot.typ
238 (*eface)(r).data = f.arg
239 if len(ityp.mhdr) != 0 {
240 // convert to interface with methods
241 // this conversion is guaranteed to succeed - we checked in SetFinalizer
242 (*iface)(r).tab = assertE2I(ityp, (*eface)(r)._type)
245 throw("bad kind in runfinq")
248 reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), ®s)
251 // Drop finalizer queue heap references
252 // before hiding them from markroot.
253 // This also ensures these will be
254 // clear if we reuse the finalizer.
258 atomic.Store(&fb.cnt, i-1)
270 // SetFinalizer sets the finalizer associated with obj to the provided
271 // finalizer function. When the garbage collector finds an unreachable block
272 // with an associated finalizer, it clears the association and runs
273 // finalizer(obj) in a separate goroutine. This makes obj reachable again,
274 // but now without an associated finalizer. Assuming that SetFinalizer
275 // is not called again, the next time the garbage collector sees
276 // that obj is unreachable, it will free obj.
278 // SetFinalizer(obj, nil) clears any finalizer associated with obj.
280 // The argument obj must be a pointer to an object allocated by calling
281 // new, by taking the address of a composite literal, or by taking the
282 // address of a local variable.
283 // The argument finalizer must be a function that takes a single argument
284 // to which obj's type can be assigned, and can have arbitrary ignored return
285 // values. If either of these is not true, SetFinalizer may abort the
288 // Finalizers are run in dependency order: if A points at B, both have
289 // finalizers, and they are otherwise unreachable, only the finalizer
290 // for A runs; once A is freed, the finalizer for B can run.
291 // If a cyclic structure includes a block with a finalizer, that
292 // cycle is not guaranteed to be garbage collected and the finalizer
293 // is not guaranteed to run, because there is no ordering that
294 // respects the dependencies.
296 // The finalizer is scheduled to run at some arbitrary time after the
297 // program can no longer reach the object to which obj points.
298 // There is no guarantee that finalizers will run before a program exits,
299 // so typically they are useful only for releasing non-memory resources
300 // associated with an object during a long-running program.
301 // For example, an os.File object could use a finalizer to close the
302 // associated operating system file descriptor when a program discards
303 // an os.File without calling Close, but it would be a mistake
304 // to depend on a finalizer to flush an in-memory I/O buffer such as a
305 // bufio.Writer, because the buffer would not be flushed at program exit.
307 // It is not guaranteed that a finalizer will run if the size of *obj is
310 // It is not guaranteed that a finalizer will run for objects allocated
311 // in initializers for package-level variables. Such objects may be
312 // linker-allocated, not heap-allocated.
314 // A finalizer may run as soon as an object becomes unreachable.
315 // In order to use finalizers correctly, the program must ensure that
316 // the object is reachable until it is no longer required.
317 // Objects stored in global variables, or that can be found by tracing
318 // pointers from a global variable, are reachable. For other objects,
319 // pass the object to a call of the KeepAlive function to mark the
320 // last point in the function where the object must be reachable.
322 // For example, if p points to a struct, such as os.File, that contains
323 // a file descriptor d, and p has a finalizer that closes that file
324 // descriptor, and if the last use of p in a function is a call to
325 // syscall.Write(p.d, buf, size), then p may be unreachable as soon as
326 // the program enters syscall.Write. The finalizer may run at that moment,
327 // closing p.d, causing syscall.Write to fail because it is writing to
328 // a closed file descriptor (or, worse, to an entirely different
329 // file descriptor opened by a different goroutine). To avoid this problem,
330 // call KeepAlive(p) after the call to syscall.Write.
332 // A single goroutine runs all finalizers for a program, sequentially.
333 // If a finalizer must run for a long time, it should do so by starting
336 // In the terminology of the Go memory model, a call
337 // SetFinalizer(x, f) “synchronizes before” the finalization call f(x).
338 // However, there is no guarantee that KeepAlive(x) or any other use of x
339 // “synchronizes before” f(x), so in general a finalizer should use a mutex
340 // or other synchronization mechanism if it needs to access mutable state in x.
341 // For example, consider a finalizer that inspects a mutable field in x
342 // that is modified from time to time in the main program before x
343 // becomes unreachable and the finalizer is invoked.
344 // The modifications in the main program and the inspection in the finalizer
345 // need to use appropriate synchronization, such as mutexes or atomic updates,
346 // to avoid read-write races.
347 func SetFinalizer(obj any, finalizer any) {
349 // debug.sbrk never frees memory, so no finalizers run
350 // (and we don't have the data structures to record them).
356 throw("runtime.SetFinalizer: first argument is nil")
358 if etyp.kind&kindMask != kindPtr {
359 throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
361 ot := (*ptrtype)(unsafe.Pointer(etyp))
363 throw("nil elem type!")
366 // find the containing object
367 base, _, _ := findObject(uintptr(e.data), 0, 0)
370 // 0-length objects are okay.
371 if e.data == unsafe.Pointer(&zerobase) {
375 // Global initializers might be linker-allocated.
376 // var Foo = &Object{}
378 // runtime.SetFinalizer(Foo, nil)
380 // The relevant segments are: noptrdata, data, bss, noptrbss.
381 // We cannot assume they are in any order or even contiguous,
382 // due to external linking.
383 for datap := &firstmoduledata; datap != nil; datap = datap.next {
384 if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata ||
385 datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata ||
386 datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss ||
387 datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss {
391 throw("runtime.SetFinalizer: pointer not in allocated block")
394 if uintptr(e.data) != base {
395 // As an implementation detail we allow to set finalizers for an inner byte
396 // of an object if it could come from tiny alloc (see mallocgc for details).
397 if ot.elem == nil || ot.elem.ptrdata != 0 || ot.elem.size >= maxTinySize {
398 throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
402 f := efaceOf(&finalizer)
405 // switch to system stack and remove finalizer
407 removefinalizer(e.data)
412 if ftyp.kind&kindMask != kindFunc {
413 throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
415 ft := (*functype)(unsafe.Pointer(ftyp))
417 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
420 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
427 case fint.kind&kindMask == kindPtr:
428 if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
429 // ok - not same type, but both pointers,
430 // one or the other is unnamed, and same element type, so assignable.
433 case fint.kind&kindMask == kindInterface:
434 ityp := (*interfacetype)(unsafe.Pointer(fint))
435 if len(ityp.mhdr) == 0 {
436 // ok - satisfies empty interface
439 if iface := assertE2I2(ityp, *efaceOf(&obj)); iface.tab != nil {
443 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
445 // compute size needed for return parameters
447 for _, t := range ft.out() {
448 nret = alignUp(nret, uintptr(t.align)) + uintptr(t.size)
450 nret = alignUp(nret, goarch.PtrSize)
452 // make sure we have a finalizer goroutine
456 if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
457 throw("runtime.SetFinalizer: finalizer already set")
462 // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
466 // KeepAlive marks its argument as currently reachable.
467 // This ensures that the object is not freed, and its finalizer is not run,
468 // before the point in the program where KeepAlive is called.
470 // A very simplified example showing where KeepAlive is required:
472 // type File struct { d int }
473 // d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
474 // // ... do something if err != nil ...
476 // runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
478 // n, err := syscall.Read(p.d, buf[:])
479 // // Ensure p is not finalized until Read returns.
480 // runtime.KeepAlive(p)
481 // // No more uses of p after this point.
483 // Without the KeepAlive call, the finalizer could run at the start of
484 // syscall.Read, closing the file descriptor before syscall.Read makes
485 // the actual system call.
487 // Note: KeepAlive should only be used to prevent finalizers from
488 // running prematurely. In particular, when used with unsafe.Pointer,
489 // the rules for valid uses of unsafe.Pointer still apply.
490 func KeepAlive(x any) {
491 // Introduce a use of x that the compiler can't eliminate.
492 // This makes sure x is alive on entry. We need x to be alive
493 // on entry for "defer runtime.KeepAlive(x)"; see issue 21402.