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.
11 "runtime/internal/atomic"
12 "runtime/internal/sys"
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*sys.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 / sys.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*sys.PtrSize ||
99 unsafe.Offsetof(finalizer{}.fn) != 0 ||
100 unsafe.Offsetof(finalizer{}.arg) != sys.PtrSize ||
101 unsafe.Offsetof(finalizer{}.nret) != 2*sys.PtrSize ||
102 unsafe.Offsetof(finalizer{}.fint) != 3*sys.PtrSize ||
103 unsafe.Offsetof(finalizer{}.ot) != 4*sys.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
176 goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
184 for i := fb.cnt; i > 0; i-- {
187 framesz := unsafe.Sizeof((interface{})(nil)) + f.nret
188 if framecap < framesz {
189 // The frame does not contain pointers interesting for GC,
190 // all not yet finalized objects are stored in finq.
191 // If we do not mark it as FlagNoScan,
192 // the last finalized object is not collected.
193 frame = mallocgc(framesz, nil, true)
198 throw("missing type in runfinq")
200 // frame is effectively uninitialized
201 // memory. That means we have to clear
202 // it before writing to it to avoid
203 // confusing the write barrier.
204 *(*[2]uintptr)(frame) = [2]uintptr{}
205 switch f.fint.kind & kindMask {
207 // direct use of pointer
208 *(*unsafe.Pointer)(frame) = f.arg
210 ityp := (*interfacetype)(unsafe.Pointer(f.fint))
211 // set up with empty interface
212 (*eface)(frame)._type = &f.ot.typ
213 (*eface)(frame).data = f.arg
214 if len(ityp.mhdr) != 0 {
215 // convert to interface with methods
216 // this conversion is guaranteed to succeed - we checked in SetFinalizer
217 (*iface)(frame).tab = assertE2I(ityp, (*eface)(frame)._type)
220 throw("bad kind in runfinq")
223 // Pass a dummy RegArgs for now.
225 // TODO(mknyszek): Pass arguments in registers.
227 reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), ®s)
230 // Drop finalizer queue heap references
231 // before hiding them from markroot.
232 // This also ensures these will be
233 // clear if we reuse the finalizer.
237 atomic.Store(&fb.cnt, i-1)
249 // SetFinalizer sets the finalizer associated with obj to the provided
250 // finalizer function. When the garbage collector finds an unreachable block
251 // with an associated finalizer, it clears the association and runs
252 // finalizer(obj) in a separate goroutine. This makes obj reachable again,
253 // but now without an associated finalizer. Assuming that SetFinalizer
254 // is not called again, the next time the garbage collector sees
255 // that obj is unreachable, it will free obj.
257 // SetFinalizer(obj, nil) clears any finalizer associated with obj.
259 // The argument obj must be a pointer to an object allocated by calling
260 // new, by taking the address of a composite literal, or by taking the
261 // address of a local variable.
262 // The argument finalizer must be a function that takes a single argument
263 // to which obj's type can be assigned, and can have arbitrary ignored return
264 // values. If either of these is not true, SetFinalizer may abort the
267 // Finalizers are run in dependency order: if A points at B, both have
268 // finalizers, and they are otherwise unreachable, only the finalizer
269 // for A runs; once A is freed, the finalizer for B can run.
270 // If a cyclic structure includes a block with a finalizer, that
271 // cycle is not guaranteed to be garbage collected and the finalizer
272 // is not guaranteed to run, because there is no ordering that
273 // respects the dependencies.
275 // The finalizer is scheduled to run at some arbitrary time after the
276 // program can no longer reach the object to which obj points.
277 // There is no guarantee that finalizers will run before a program exits,
278 // so typically they are useful only for releasing non-memory resources
279 // associated with an object during a long-running program.
280 // For example, an os.File object could use a finalizer to close the
281 // associated operating system file descriptor when a program discards
282 // an os.File without calling Close, but it would be a mistake
283 // to depend on a finalizer to flush an in-memory I/O buffer such as a
284 // bufio.Writer, because the buffer would not be flushed at program exit.
286 // It is not guaranteed that a finalizer will run if the size of *obj is
289 // It is not guaranteed that a finalizer will run for objects allocated
290 // in initializers for package-level variables. Such objects may be
291 // linker-allocated, not heap-allocated.
293 // A finalizer may run as soon as an object becomes unreachable.
294 // In order to use finalizers correctly, the program must ensure that
295 // the object is reachable until it is no longer required.
296 // Objects stored in global variables, or that can be found by tracing
297 // pointers from a global variable, are reachable. For other objects,
298 // pass the object to a call of the KeepAlive function to mark the
299 // last point in the function where the object must be reachable.
301 // For example, if p points to a struct, such as os.File, that contains
302 // a file descriptor d, and p has a finalizer that closes that file
303 // descriptor, and if the last use of p in a function is a call to
304 // syscall.Write(p.d, buf, size), then p may be unreachable as soon as
305 // the program enters syscall.Write. The finalizer may run at that moment,
306 // closing p.d, causing syscall.Write to fail because it is writing to
307 // a closed file descriptor (or, worse, to an entirely different
308 // file descriptor opened by a different goroutine). To avoid this problem,
309 // call runtime.KeepAlive(p) after the call to syscall.Write.
311 // A single goroutine runs all finalizers for a program, sequentially.
312 // If a finalizer must run for a long time, it should do so by starting
314 func SetFinalizer(obj interface{}, finalizer interface{}) {
316 // debug.sbrk never frees memory, so no finalizers run
317 // (and we don't have the data structures to record them).
323 throw("runtime.SetFinalizer: first argument is nil")
325 if etyp.kind&kindMask != kindPtr {
326 throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
328 ot := (*ptrtype)(unsafe.Pointer(etyp))
330 throw("nil elem type!")
333 // find the containing object
334 base, _, _ := findObject(uintptr(e.data), 0, 0)
337 // 0-length objects are okay.
338 if e.data == unsafe.Pointer(&zerobase) {
342 // Global initializers might be linker-allocated.
343 // var Foo = &Object{}
345 // runtime.SetFinalizer(Foo, nil)
347 // The relevant segments are: noptrdata, data, bss, noptrbss.
348 // We cannot assume they are in any order or even contiguous,
349 // due to external linking.
350 for datap := &firstmoduledata; datap != nil; datap = datap.next {
351 if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata ||
352 datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata ||
353 datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss ||
354 datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss {
358 throw("runtime.SetFinalizer: pointer not in allocated block")
361 if uintptr(e.data) != base {
362 // As an implementation detail we allow to set finalizers for an inner byte
363 // of an object if it could come from tiny alloc (see mallocgc for details).
364 if ot.elem == nil || ot.elem.ptrdata != 0 || ot.elem.size >= maxTinySize {
365 throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
369 f := efaceOf(&finalizer)
372 // switch to system stack and remove finalizer
374 removefinalizer(e.data)
379 if ftyp.kind&kindMask != kindFunc {
380 throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
382 ft := (*functype)(unsafe.Pointer(ftyp))
384 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
387 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
394 case fint.kind&kindMask == kindPtr:
395 if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
396 // ok - not same type, but both pointers,
397 // one or the other is unnamed, and same element type, so assignable.
400 case fint.kind&kindMask == kindInterface:
401 ityp := (*interfacetype)(unsafe.Pointer(fint))
402 if len(ityp.mhdr) == 0 {
403 // ok - satisfies empty interface
406 if iface := assertE2I2(ityp, *efaceOf(&obj)); iface.tab != nil {
410 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
412 // compute size needed for return parameters
414 for _, t := range ft.out() {
415 nret = alignUp(nret, uintptr(t.align)) + uintptr(t.size)
417 nret = alignUp(nret, sys.PtrSize)
419 // make sure we have a finalizer goroutine
423 if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
424 throw("runtime.SetFinalizer: finalizer already set")
429 // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
432 // KeepAlive marks its argument as currently reachable.
433 // This ensures that the object is not freed, and its finalizer is not run,
434 // before the point in the program where KeepAlive is called.
436 // A very simplified example showing where KeepAlive is required:
437 // type File struct { d int }
438 // d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
439 // // ... do something if err != nil ...
441 // runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
443 // n, err := syscall.Read(p.d, buf[:])
444 // // Ensure p is not finalized until Read returns.
445 // runtime.KeepAlive(p)
446 // // No more uses of p after this point.
448 // Without the KeepAlive call, the finalizer could run at the start of
449 // syscall.Read, closing the file descriptor before syscall.Read makes
450 // the actual system call.
451 func KeepAlive(x interface{}) {
452 // Introduce a use of x that the compiler can't eliminate.
453 // This makes sure x is alive on entry. We need x to be alive
454 // on entry for "defer runtime.KeepAlive(x)"; see issue 21402.