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 func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
79 if gcphase != _GCoff {
80 // Currently we assume that the finalizer queue won't
81 // grow during marking so we don't have to rescan it
82 // during mark termination. If we ever need to lift
83 // this assumption, we can do it by adding the
84 // necessary barriers to queuefinalizer (which it may
85 // have automatically).
86 throw("queuefinalizer during GC")
90 if finq == nil || finq.cnt == uint32(len(finq.fin)) {
92 finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gcMiscSys))
95 if finptrmask[0] == 0 {
96 // Build pointer mask for Finalizer array in block.
97 // Check assumptions made in finalizer1 array above.
98 if (unsafe.Sizeof(finalizer{}) != 5*goarch.PtrSize ||
99 unsafe.Offsetof(finalizer{}.fn) != 0 ||
100 unsafe.Offsetof(finalizer{}.arg) != goarch.PtrSize ||
101 unsafe.Offsetof(finalizer{}.nret) != 2*goarch.PtrSize ||
102 unsafe.Offsetof(finalizer{}.fint) != 3*goarch.PtrSize ||
103 unsafe.Offsetof(finalizer{}.ot) != 4*goarch.PtrSize) {
104 throw("finalizer out of sync")
106 for i := range finptrmask {
107 finptrmask[i] = finalizer1[i%len(finalizer1)]
116 f := &finq.fin[finq.cnt]
117 atomic.Xadd(&finq.cnt, +1) // Sync with markroots
128 func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) {
129 for fb := allfin; fb != nil; fb = fb.alllink {
130 for i := uint32(0); i < fb.cnt; i++ {
132 callback(f.fn, f.arg, f.nret, f.fint, f.ot)
140 if fingwait && fingwake {
155 // start the finalizer goroutine exactly once
156 if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
161 // This is the goroutine that runs all of the finalizers
180 goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
189 for i := fb.cnt; i > 0; i-- {
193 // The args may be passed in registers or on stack. Even for
194 // the register case, we still need the spill slots.
195 // TODO: revisit if we remove spill slots.
197 // Unfortunately because we can have an arbitrary
198 // amount of returns and it would be complex to try and
199 // figure out how many of those can get passed in registers,
200 // just conservatively assume none of them do.
201 framesz := unsafe.Sizeof((any)(nil)) + f.nret
202 if framecap < framesz {
203 // The frame does not contain pointers interesting for GC,
204 // all not yet finalized objects are stored in finq.
205 // If we do not mark it as FlagNoScan,
206 // the last finalized object is not collected.
207 frame = mallocgc(framesz, nil, true)
212 throw("missing type in runfinq")
216 r = unsafe.Pointer(®s.Ints)
218 // frame is effectively uninitialized
219 // memory. That means we have to clear
220 // it before writing to it to avoid
221 // confusing the write barrier.
222 *(*[2]uintptr)(frame) = [2]uintptr{}
224 switch f.fint.kind & kindMask {
226 // direct use of pointer
227 *(*unsafe.Pointer)(r) = f.arg
229 ityp := (*interfacetype)(unsafe.Pointer(f.fint))
230 // set up with empty interface
231 (*eface)(r)._type = &f.ot.typ
232 (*eface)(r).data = f.arg
233 if len(ityp.mhdr) != 0 {
234 // convert to interface with methods
235 // this conversion is guaranteed to succeed - we checked in SetFinalizer
236 (*iface)(r).tab = assertE2I(ityp, (*eface)(r)._type)
239 throw("bad kind in runfinq")
242 reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), ®s)
245 // Drop finalizer queue heap references
246 // before hiding them from markroot.
247 // This also ensures these will be
248 // clear if we reuse the finalizer.
252 atomic.Store(&fb.cnt, i-1)
264 // SetFinalizer sets the finalizer associated with obj to the provided
265 // finalizer function. When the garbage collector finds an unreachable block
266 // with an associated finalizer, it clears the association and runs
267 // finalizer(obj) in a separate goroutine. This makes obj reachable again,
268 // but now without an associated finalizer. Assuming that SetFinalizer
269 // is not called again, the next time the garbage collector sees
270 // that obj is unreachable, it will free obj.
272 // SetFinalizer(obj, nil) clears any finalizer associated with obj.
274 // The argument obj must be a pointer to an object allocated by calling
275 // new, by taking the address of a composite literal, or by taking the
276 // address of a local variable.
277 // The argument finalizer must be a function that takes a single argument
278 // to which obj's type can be assigned, and can have arbitrary ignored return
279 // values. If either of these is not true, SetFinalizer may abort the
282 // Finalizers are run in dependency order: if A points at B, both have
283 // finalizers, and they are otherwise unreachable, only the finalizer
284 // for A runs; once A is freed, the finalizer for B can run.
285 // If a cyclic structure includes a block with a finalizer, that
286 // cycle is not guaranteed to be garbage collected and the finalizer
287 // is not guaranteed to run, because there is no ordering that
288 // respects the dependencies.
290 // The finalizer is scheduled to run at some arbitrary time after the
291 // program can no longer reach the object to which obj points.
292 // There is no guarantee that finalizers will run before a program exits,
293 // so typically they are useful only for releasing non-memory resources
294 // associated with an object during a long-running program.
295 // For example, an os.File object could use a finalizer to close the
296 // associated operating system file descriptor when a program discards
297 // an os.File without calling Close, but it would be a mistake
298 // to depend on a finalizer to flush an in-memory I/O buffer such as a
299 // bufio.Writer, because the buffer would not be flushed at program exit.
301 // It is not guaranteed that a finalizer will run if the size of *obj is
304 // It is not guaranteed that a finalizer will run for objects allocated
305 // in initializers for package-level variables. Such objects may be
306 // linker-allocated, not heap-allocated.
308 // A finalizer may run as soon as an object becomes unreachable.
309 // In order to use finalizers correctly, the program must ensure that
310 // the object is reachable until it is no longer required.
311 // Objects stored in global variables, or that can be found by tracing
312 // pointers from a global variable, are reachable. For other objects,
313 // pass the object to a call of the KeepAlive function to mark the
314 // last point in the function where the object must be reachable.
316 // For example, if p points to a struct, such as os.File, that contains
317 // a file descriptor d, and p has a finalizer that closes that file
318 // descriptor, and if the last use of p in a function is a call to
319 // syscall.Write(p.d, buf, size), then p may be unreachable as soon as
320 // the program enters syscall.Write. The finalizer may run at that moment,
321 // closing p.d, causing syscall.Write to fail because it is writing to
322 // a closed file descriptor (or, worse, to an entirely different
323 // file descriptor opened by a different goroutine). To avoid this problem,
324 // call KeepAlive(p) after the call to syscall.Write.
326 // A single goroutine runs all finalizers for a program, sequentially.
327 // If a finalizer must run for a long time, it should do so by starting
330 // In the terminology of the Go memory model, a call
331 // SetFinalizer(x, f) “synchronizes before” the finalization call f(x).
332 // However, there is no guarantee that KeepAlive(x) or any other use of x
333 // “synchronizes before” f(x), so in general a finalizer should use a mutex
334 // or other synchronization mechanism if it needs to access mutable state in x.
335 // For example, consider a finalizer that inspects a mutable field in x
336 // that is modified from time to time in the main program before x
337 // becomes unreachable and the finalizer is invoked.
338 // The modifications in the main program and the inspection in the finalizer
339 // need to use appropriate synchronization, such as mutexes or atomic updates,
340 // to avoid read-write races.
341 func SetFinalizer(obj any, finalizer any) {
343 // debug.sbrk never frees memory, so no finalizers run
344 // (and we don't have the data structures to record them).
350 throw("runtime.SetFinalizer: first argument is nil")
352 if etyp.kind&kindMask != kindPtr {
353 throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
355 ot := (*ptrtype)(unsafe.Pointer(etyp))
357 throw("nil elem type!")
360 // find the containing object
361 base, _, _ := findObject(uintptr(e.data), 0, 0)
364 // 0-length objects are okay.
365 if e.data == unsafe.Pointer(&zerobase) {
369 // Global initializers might be linker-allocated.
370 // var Foo = &Object{}
372 // runtime.SetFinalizer(Foo, nil)
374 // The relevant segments are: noptrdata, data, bss, noptrbss.
375 // We cannot assume they are in any order or even contiguous,
376 // due to external linking.
377 for datap := &firstmoduledata; datap != nil; datap = datap.next {
378 if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata ||
379 datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata ||
380 datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss ||
381 datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss {
385 throw("runtime.SetFinalizer: pointer not in allocated block")
388 if uintptr(e.data) != base {
389 // As an implementation detail we allow to set finalizers for an inner byte
390 // of an object if it could come from tiny alloc (see mallocgc for details).
391 if ot.elem == nil || ot.elem.ptrdata != 0 || ot.elem.size >= maxTinySize {
392 throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
396 f := efaceOf(&finalizer)
399 // switch to system stack and remove finalizer
401 removefinalizer(e.data)
406 if ftyp.kind&kindMask != kindFunc {
407 throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
409 ft := (*functype)(unsafe.Pointer(ftyp))
411 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
414 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
421 case fint.kind&kindMask == kindPtr:
422 if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
423 // ok - not same type, but both pointers,
424 // one or the other is unnamed, and same element type, so assignable.
427 case fint.kind&kindMask == kindInterface:
428 ityp := (*interfacetype)(unsafe.Pointer(fint))
429 if len(ityp.mhdr) == 0 {
430 // ok - satisfies empty interface
433 if iface := assertE2I2(ityp, *efaceOf(&obj)); iface.tab != nil {
437 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
439 // compute size needed for return parameters
441 for _, t := range ft.out() {
442 nret = alignUp(nret, uintptr(t.align)) + uintptr(t.size)
444 nret = alignUp(nret, goarch.PtrSize)
446 // make sure we have a finalizer goroutine
450 if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
451 throw("runtime.SetFinalizer: finalizer already set")
456 // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
460 // KeepAlive marks its argument as currently reachable.
461 // This ensures that the object is not freed, and its finalizer is not run,
462 // before the point in the program where KeepAlive is called.
464 // A very simplified example showing where KeepAlive is required:
466 // type File struct { d int }
467 // d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
468 // // ... do something if err != nil ...
470 // runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
472 // n, err := syscall.Read(p.d, buf[:])
473 // // Ensure p is not finalized until Read returns.
474 // runtime.KeepAlive(p)
475 // // No more uses of p after this point.
477 // Without the KeepAlive call, the finalizer could run at the start of
478 // syscall.Read, closing the file descriptor before syscall.Read makes
479 // the actual system call.
481 // Note: KeepAlive should only be used to prevent finalizers from
482 // running prematurely. In particular, when used with unsafe.Pointer,
483 // the rules for valid uses of unsafe.Pointer still apply.
484 func KeepAlive(x any) {
485 // Introduce a use of x that the compiler can't eliminate.
486 // This makes sure x is alive on entry. We need x to be alive
487 // on entry for "defer runtime.KeepAlive(x)"; see issue 21402.