Source file src/runtime/mfinal.go

     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.
     4  
     5  // Garbage collector: finalizers and block profiling.
     6  
     7  package runtime
     8  
     9  import (
    10  	"internal/abi"
    11  	"internal/goarch"
    12  	"internal/goexperiment"
    13  	"runtime/internal/atomic"
    14  	"runtime/internal/sys"
    15  	"unsafe"
    16  )
    17  
    18  // finblock is an array of finalizers to be executed. finblocks are
    19  // arranged in a linked list for the finalizer queue.
    20  //
    21  // finblock is allocated from non-GC'd memory, so any heap pointers
    22  // must be specially handled. GC currently assumes that the finalizer
    23  // queue does not grow during marking (but it can shrink).
    24  type finblock struct {
    25  	_       sys.NotInHeap
    26  	alllink *finblock
    27  	next    *finblock
    28  	cnt     uint32
    29  	_       int32
    30  	fin     [(_FinBlockSize - 2*goarch.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
    31  }
    32  
    33  var fingStatus atomic.Uint32
    34  
    35  // finalizer goroutine status.
    36  const (
    37  	fingUninitialized uint32 = iota
    38  	fingCreated       uint32 = 1 << (iota - 1)
    39  	fingRunningFinalizer
    40  	fingWait
    41  	fingWake
    42  )
    43  
    44  var finlock mutex  // protects the following variables
    45  var fing *g        // goroutine that runs finalizers
    46  var finq *finblock // list of finalizers that are to be executed
    47  var finc *finblock // cache of free blocks
    48  var finptrmask [_FinBlockSize / goarch.PtrSize / 8]byte
    49  
    50  var allfin *finblock // list of all blocks
    51  
    52  // NOTE: Layout known to queuefinalizer.
    53  type finalizer struct {
    54  	fn   *funcval       // function to call (may be a heap pointer)
    55  	arg  unsafe.Pointer // ptr to object (may be a heap pointer)
    56  	nret uintptr        // bytes of return values from fn
    57  	fint *_type         // type of first argument of fn
    58  	ot   *ptrtype       // type of ptr to object (may be a heap pointer)
    59  }
    60  
    61  var finalizer1 = [...]byte{
    62  	// Each Finalizer is 5 words, ptr ptr INT ptr ptr (INT = uintptr here)
    63  	// Each byte describes 8 words.
    64  	// Need 8 Finalizers described by 5 bytes before pattern repeats:
    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
    72  	//	ptr ptr INT ptr ptr
    73  	// aka
    74  	//
    75  	//	ptr ptr INT ptr ptr ptr ptr INT
    76  	//	ptr ptr ptr ptr INT ptr ptr ptr
    77  	//	ptr INT ptr ptr ptr ptr INT ptr
    78  	//	ptr ptr ptr INT ptr ptr ptr ptr
    79  	//	INT ptr ptr ptr ptr INT ptr ptr
    80  	//
    81  	// Assumptions about Finalizer layout checked below.
    82  	1<<0 | 1<<1 | 0<<2 | 1<<3 | 1<<4 | 1<<5 | 1<<6 | 0<<7,
    83  	1<<0 | 1<<1 | 1<<2 | 1<<3 | 0<<4 | 1<<5 | 1<<6 | 1<<7,
    84  	1<<0 | 0<<1 | 1<<2 | 1<<3 | 1<<4 | 1<<5 | 0<<6 | 1<<7,
    85  	1<<0 | 1<<1 | 1<<2 | 0<<3 | 1<<4 | 1<<5 | 1<<6 | 1<<7,
    86  	0<<0 | 1<<1 | 1<<2 | 1<<3 | 1<<4 | 0<<5 | 1<<6 | 1<<7,
    87  }
    88  
    89  // lockRankMayQueueFinalizer records the lock ranking effects of a
    90  // function that may call queuefinalizer.
    91  func lockRankMayQueueFinalizer() {
    92  	lockWithRankMayAcquire(&finlock, getLockRank(&finlock))
    93  }
    94  
    95  func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
    96  	if gcphase != _GCoff {
    97  		// Currently we assume that the finalizer queue won't
    98  		// grow during marking so we don't have to rescan it
    99  		// during mark termination. If we ever need to lift
   100  		// this assumption, we can do it by adding the
   101  		// necessary barriers to queuefinalizer (which it may
   102  		// have automatically).
   103  		throw("queuefinalizer during GC")
   104  	}
   105  
   106  	lock(&finlock)
   107  	if finq == nil || finq.cnt == uint32(len(finq.fin)) {
   108  		if finc == nil {
   109  			finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gcMiscSys))
   110  			finc.alllink = allfin
   111  			allfin = finc
   112  			if finptrmask[0] == 0 {
   113  				// Build pointer mask for Finalizer array in block.
   114  				// Check assumptions made in finalizer1 array above.
   115  				if (unsafe.Sizeof(finalizer{}) != 5*goarch.PtrSize ||
   116  					unsafe.Offsetof(finalizer{}.fn) != 0 ||
   117  					unsafe.Offsetof(finalizer{}.arg) != goarch.PtrSize ||
   118  					unsafe.Offsetof(finalizer{}.nret) != 2*goarch.PtrSize ||
   119  					unsafe.Offsetof(finalizer{}.fint) != 3*goarch.PtrSize ||
   120  					unsafe.Offsetof(finalizer{}.ot) != 4*goarch.PtrSize) {
   121  					throw("finalizer out of sync")
   122  				}
   123  				for i := range finptrmask {
   124  					finptrmask[i] = finalizer1[i%len(finalizer1)]
   125  				}
   126  			}
   127  		}
   128  		block := finc
   129  		finc = block.next
   130  		block.next = finq
   131  		finq = block
   132  	}
   133  	f := &finq.fin[finq.cnt]
   134  	atomic.Xadd(&finq.cnt, +1) // Sync with markroots
   135  	f.fn = fn
   136  	f.nret = nret
   137  	f.fint = fint
   138  	f.ot = ot
   139  	f.arg = p
   140  	unlock(&finlock)
   141  	fingStatus.Or(fingWake)
   142  }
   143  
   144  //go:nowritebarrier
   145  func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) {
   146  	for fb := allfin; fb != nil; fb = fb.alllink {
   147  		for i := uint32(0); i < fb.cnt; i++ {
   148  			f := &fb.fin[i]
   149  			callback(f.fn, f.arg, f.nret, f.fint, f.ot)
   150  		}
   151  	}
   152  }
   153  
   154  func wakefing() *g {
   155  	if ok := fingStatus.CompareAndSwap(fingCreated|fingWait|fingWake, fingCreated); ok {
   156  		return fing
   157  	}
   158  	return nil
   159  }
   160  
   161  func createfing() {
   162  	// start the finalizer goroutine exactly once
   163  	if fingStatus.Load() == fingUninitialized && fingStatus.CompareAndSwap(fingUninitialized, fingCreated) {
   164  		go runfinq()
   165  	}
   166  }
   167  
   168  func finalizercommit(gp *g, lock unsafe.Pointer) bool {
   169  	unlock((*mutex)(lock))
   170  	// fingStatus should be modified after fing is put into a waiting state
   171  	// to avoid waking fing in running state, even if it is about to be parked.
   172  	fingStatus.Or(fingWait)
   173  	return true
   174  }
   175  
   176  // This is the goroutine that runs all of the finalizers.
   177  func runfinq() {
   178  	var (
   179  		frame    unsafe.Pointer
   180  		framecap uintptr
   181  		argRegs  int
   182  	)
   183  
   184  	gp := getg()
   185  	lock(&finlock)
   186  	fing = gp
   187  	unlock(&finlock)
   188  
   189  	for {
   190  		lock(&finlock)
   191  		fb := finq
   192  		finq = nil
   193  		if fb == nil {
   194  			gopark(finalizercommit, unsafe.Pointer(&finlock), waitReasonFinalizerWait, traceBlockSystemGoroutine, 1)
   195  			continue
   196  		}
   197  		argRegs = intArgRegs
   198  		unlock(&finlock)
   199  		if raceenabled {
   200  			racefingo()
   201  		}
   202  		for fb != nil {
   203  			for i := fb.cnt; i > 0; i-- {
   204  				f := &fb.fin[i-1]
   205  
   206  				var regs abi.RegArgs
   207  				// The args may be passed in registers or on stack. Even for
   208  				// the register case, we still need the spill slots.
   209  				// TODO: revisit if we remove spill slots.
   210  				//
   211  				// Unfortunately because we can have an arbitrary
   212  				// amount of returns and it would be complex to try and
   213  				// figure out how many of those can get passed in registers,
   214  				// just conservatively assume none of them do.
   215  				framesz := unsafe.Sizeof((any)(nil)) + f.nret
   216  				if framecap < framesz {
   217  					// The frame does not contain pointers interesting for GC,
   218  					// all not yet finalized objects are stored in finq.
   219  					// If we do not mark it as FlagNoScan,
   220  					// the last finalized object is not collected.
   221  					frame = mallocgc(framesz, nil, true)
   222  					framecap = framesz
   223  				}
   224  
   225  				if f.fint == nil {
   226  					throw("missing type in runfinq")
   227  				}
   228  				r := frame
   229  				if argRegs > 0 {
   230  					r = unsafe.Pointer(&regs.Ints)
   231  				} else {
   232  					// frame is effectively uninitialized
   233  					// memory. That means we have to clear
   234  					// it before writing to it to avoid
   235  					// confusing the write barrier.
   236  					*(*[2]uintptr)(frame) = [2]uintptr{}
   237  				}
   238  				switch f.fint.Kind_ & kindMask {
   239  				case kindPtr:
   240  					// direct use of pointer
   241  					*(*unsafe.Pointer)(r) = f.arg
   242  				case kindInterface:
   243  					ityp := (*interfacetype)(unsafe.Pointer(f.fint))
   244  					// set up with empty interface
   245  					(*eface)(r)._type = &f.ot.Type
   246  					(*eface)(r).data = f.arg
   247  					if len(ityp.Methods) != 0 {
   248  						// convert to interface with methods
   249  						// this conversion is guaranteed to succeed - we checked in SetFinalizer
   250  						(*iface)(r).tab = assertE2I(ityp, (*eface)(r)._type)
   251  					}
   252  				default:
   253  					throw("bad kind in runfinq")
   254  				}
   255  				fingStatus.Or(fingRunningFinalizer)
   256  				reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), &regs)
   257  				fingStatus.And(^fingRunningFinalizer)
   258  
   259  				// Drop finalizer queue heap references
   260  				// before hiding them from markroot.
   261  				// This also ensures these will be
   262  				// clear if we reuse the finalizer.
   263  				f.fn = nil
   264  				f.arg = nil
   265  				f.ot = nil
   266  				atomic.Store(&fb.cnt, i-1)
   267  			}
   268  			next := fb.next
   269  			lock(&finlock)
   270  			fb.next = finc
   271  			finc = fb
   272  			unlock(&finlock)
   273  			fb = next
   274  		}
   275  	}
   276  }
   277  
   278  func isGoPointerWithoutSpan(p unsafe.Pointer) bool {
   279  	// 0-length objects are okay.
   280  	if p == unsafe.Pointer(&zerobase) {
   281  		return true
   282  	}
   283  
   284  	// Global initializers might be linker-allocated.
   285  	//	var Foo = &Object{}
   286  	//	func main() {
   287  	//		runtime.SetFinalizer(Foo, nil)
   288  	//	}
   289  	// The relevant segments are: noptrdata, data, bss, noptrbss.
   290  	// We cannot assume they are in any order or even contiguous,
   291  	// due to external linking.
   292  	for datap := &firstmoduledata; datap != nil; datap = datap.next {
   293  		if datap.noptrdata <= uintptr(p) && uintptr(p) < datap.enoptrdata ||
   294  			datap.data <= uintptr(p) && uintptr(p) < datap.edata ||
   295  			datap.bss <= uintptr(p) && uintptr(p) < datap.ebss ||
   296  			datap.noptrbss <= uintptr(p) && uintptr(p) < datap.enoptrbss {
   297  			return true
   298  		}
   299  	}
   300  	return false
   301  }
   302  
   303  // blockUntilEmptyFinalizerQueue blocks until either the finalizer
   304  // queue is emptied (and the finalizers have executed) or the timeout
   305  // is reached. Returns true if the finalizer queue was emptied.
   306  // This is used by the runtime and sync tests.
   307  func blockUntilEmptyFinalizerQueue(timeout int64) bool {
   308  	start := nanotime()
   309  	for nanotime()-start < timeout {
   310  		lock(&finlock)
   311  		// We know the queue has been drained when both finq is nil
   312  		// and the finalizer g has stopped executing.
   313  		empty := finq == nil
   314  		empty = empty && readgstatus(fing) == _Gwaiting && fing.waitreason == waitReasonFinalizerWait
   315  		unlock(&finlock)
   316  		if empty {
   317  			return true
   318  		}
   319  		Gosched()
   320  	}
   321  	return false
   322  }
   323  
   324  // SetFinalizer sets the finalizer associated with obj to the provided
   325  // finalizer function. When the garbage collector finds an unreachable block
   326  // with an associated finalizer, it clears the association and runs
   327  // finalizer(obj) in a separate goroutine. This makes obj reachable again,
   328  // but now without an associated finalizer. Assuming that SetFinalizer
   329  // is not called again, the next time the garbage collector sees
   330  // that obj is unreachable, it will free obj.
   331  //
   332  // SetFinalizer(obj, nil) clears any finalizer associated with obj.
   333  //
   334  // The argument obj must be a pointer to an object allocated by calling
   335  // new, by taking the address of a composite literal, or by taking the
   336  // address of a local variable.
   337  // The argument finalizer must be a function that takes a single argument
   338  // to which obj's type can be assigned, and can have arbitrary ignored return
   339  // values. If either of these is not true, SetFinalizer may abort the
   340  // program.
   341  //
   342  // Finalizers are run in dependency order: if A points at B, both have
   343  // finalizers, and they are otherwise unreachable, only the finalizer
   344  // for A runs; once A is freed, the finalizer for B can run.
   345  // If a cyclic structure includes a block with a finalizer, that
   346  // cycle is not guaranteed to be garbage collected and the finalizer
   347  // is not guaranteed to run, because there is no ordering that
   348  // respects the dependencies.
   349  //
   350  // The finalizer is scheduled to run at some arbitrary time after the
   351  // program can no longer reach the object to which obj points.
   352  // There is no guarantee that finalizers will run before a program exits,
   353  // so typically they are useful only for releasing non-memory resources
   354  // associated with an object during a long-running program.
   355  // For example, an [os.File] object could use a finalizer to close the
   356  // associated operating system file descriptor when a program discards
   357  // an os.File without calling Close, but it would be a mistake
   358  // to depend on a finalizer to flush an in-memory I/O buffer such as a
   359  // [bufio.Writer], because the buffer would not be flushed at program exit.
   360  //
   361  // It is not guaranteed that a finalizer will run if the size of *obj is
   362  // zero bytes, because it may share same address with other zero-size
   363  // objects in memory. See https://go.dev/ref/spec#Size_and_alignment_guarantees.
   364  //
   365  // It is not guaranteed that a finalizer will run for objects allocated
   366  // in initializers for package-level variables. Such objects may be
   367  // linker-allocated, not heap-allocated.
   368  //
   369  // Note that because finalizers may execute arbitrarily far into the future
   370  // after an object is no longer referenced, the runtime is allowed to perform
   371  // a space-saving optimization that batches objects together in a single
   372  // allocation slot. The finalizer for an unreferenced object in such an
   373  // allocation may never run if it always exists in the same batch as a
   374  // referenced object. Typically, this batching only happens for tiny
   375  // (on the order of 16 bytes or less) and pointer-free objects.
   376  //
   377  // A finalizer may run as soon as an object becomes unreachable.
   378  // In order to use finalizers correctly, the program must ensure that
   379  // the object is reachable until it is no longer required.
   380  // Objects stored in global variables, or that can be found by tracing
   381  // pointers from a global variable, are reachable. For other objects,
   382  // pass the object to a call of the [KeepAlive] function to mark the
   383  // last point in the function where the object must be reachable.
   384  //
   385  // For example, if p points to a struct, such as os.File, that contains
   386  // a file descriptor d, and p has a finalizer that closes that file
   387  // descriptor, and if the last use of p in a function is a call to
   388  // syscall.Write(p.d, buf, size), then p may be unreachable as soon as
   389  // the program enters [syscall.Write]. The finalizer may run at that moment,
   390  // closing p.d, causing syscall.Write to fail because it is writing to
   391  // a closed file descriptor (or, worse, to an entirely different
   392  // file descriptor opened by a different goroutine). To avoid this problem,
   393  // call KeepAlive(p) after the call to syscall.Write.
   394  //
   395  // A single goroutine runs all finalizers for a program, sequentially.
   396  // If a finalizer must run for a long time, it should do so by starting
   397  // a new goroutine.
   398  //
   399  // In the terminology of the Go memory model, a call
   400  // SetFinalizer(x, f) “synchronizes before” the finalization call f(x).
   401  // However, there is no guarantee that KeepAlive(x) or any other use of x
   402  // “synchronizes before” f(x), so in general a finalizer should use a mutex
   403  // or other synchronization mechanism if it needs to access mutable state in x.
   404  // For example, consider a finalizer that inspects a mutable field in x
   405  // that is modified from time to time in the main program before x
   406  // becomes unreachable and the finalizer is invoked.
   407  // The modifications in the main program and the inspection in the finalizer
   408  // need to use appropriate synchronization, such as mutexes or atomic updates,
   409  // to avoid read-write races.
   410  func SetFinalizer(obj any, finalizer any) {
   411  	if debug.sbrk != 0 {
   412  		// debug.sbrk never frees memory, so no finalizers run
   413  		// (and we don't have the data structures to record them).
   414  		return
   415  	}
   416  	e := efaceOf(&obj)
   417  	etyp := e._type
   418  	if etyp == nil {
   419  		throw("runtime.SetFinalizer: first argument is nil")
   420  	}
   421  	if etyp.Kind_&kindMask != kindPtr {
   422  		throw("runtime.SetFinalizer: first argument is " + toRType(etyp).string() + ", not pointer")
   423  	}
   424  	ot := (*ptrtype)(unsafe.Pointer(etyp))
   425  	if ot.Elem == nil {
   426  		throw("nil elem type!")
   427  	}
   428  
   429  	if inUserArenaChunk(uintptr(e.data)) {
   430  		// Arena-allocated objects are not eligible for finalizers.
   431  		throw("runtime.SetFinalizer: first argument was allocated into an arena")
   432  	}
   433  
   434  	// find the containing object
   435  	base, span, _ := findObject(uintptr(e.data), 0, 0)
   436  
   437  	if base == 0 {
   438  		if isGoPointerWithoutSpan(e.data) {
   439  			return
   440  		}
   441  		throw("runtime.SetFinalizer: pointer not in allocated block")
   442  	}
   443  
   444  	// Move base forward if we've got an allocation header.
   445  	if goexperiment.AllocHeaders && !span.spanclass.noscan() && !heapBitsInSpan(span.elemsize) && span.spanclass.sizeclass() != 0 {
   446  		base += mallocHeaderSize
   447  	}
   448  
   449  	if uintptr(e.data) != base {
   450  		// As an implementation detail we allow to set finalizers for an inner byte
   451  		// of an object if it could come from tiny alloc (see mallocgc for details).
   452  		if ot.Elem == nil || ot.Elem.PtrBytes != 0 || ot.Elem.Size_ >= maxTinySize {
   453  			throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
   454  		}
   455  	}
   456  
   457  	f := efaceOf(&finalizer)
   458  	ftyp := f._type
   459  	if ftyp == nil {
   460  		// switch to system stack and remove finalizer
   461  		systemstack(func() {
   462  			removefinalizer(e.data)
   463  		})
   464  		return
   465  	}
   466  
   467  	if ftyp.Kind_&kindMask != kindFunc {
   468  		throw("runtime.SetFinalizer: second argument is " + toRType(ftyp).string() + ", not a function")
   469  	}
   470  	ft := (*functype)(unsafe.Pointer(ftyp))
   471  	if ft.IsVariadic() {
   472  		throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string() + " because dotdotdot")
   473  	}
   474  	if ft.InCount != 1 {
   475  		throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string())
   476  	}
   477  	fint := ft.InSlice()[0]
   478  	switch {
   479  	case fint == etyp:
   480  		// ok - same type
   481  		goto okarg
   482  	case fint.Kind_&kindMask == kindPtr:
   483  		if (fint.Uncommon() == nil || etyp.Uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).Elem == ot.Elem {
   484  			// ok - not same type, but both pointers,
   485  			// one or the other is unnamed, and same element type, so assignable.
   486  			goto okarg
   487  		}
   488  	case fint.Kind_&kindMask == kindInterface:
   489  		ityp := (*interfacetype)(unsafe.Pointer(fint))
   490  		if len(ityp.Methods) == 0 {
   491  			// ok - satisfies empty interface
   492  			goto okarg
   493  		}
   494  		if itab := assertE2I2(ityp, efaceOf(&obj)._type); itab != nil {
   495  			goto okarg
   496  		}
   497  	}
   498  	throw("runtime.SetFinalizer: cannot pass " + toRType(etyp).string() + " to finalizer " + toRType(ftyp).string())
   499  okarg:
   500  	// compute size needed for return parameters
   501  	nret := uintptr(0)
   502  	for _, t := range ft.OutSlice() {
   503  		nret = alignUp(nret, uintptr(t.Align_)) + t.Size_
   504  	}
   505  	nret = alignUp(nret, goarch.PtrSize)
   506  
   507  	// make sure we have a finalizer goroutine
   508  	createfing()
   509  
   510  	systemstack(func() {
   511  		if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
   512  			throw("runtime.SetFinalizer: finalizer already set")
   513  		}
   514  	})
   515  }
   516  
   517  // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
   518  //
   519  //go:noinline
   520  
   521  // KeepAlive marks its argument as currently reachable.
   522  // This ensures that the object is not freed, and its finalizer is not run,
   523  // before the point in the program where KeepAlive is called.
   524  //
   525  // A very simplified example showing where KeepAlive is required:
   526  //
   527  //	type File struct { d int }
   528  //	d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
   529  //	// ... do something if err != nil ...
   530  //	p := &File{d}
   531  //	runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
   532  //	var buf [10]byte
   533  //	n, err := syscall.Read(p.d, buf[:])
   534  //	// Ensure p is not finalized until Read returns.
   535  //	runtime.KeepAlive(p)
   536  //	// No more uses of p after this point.
   537  //
   538  // Without the KeepAlive call, the finalizer could run at the start of
   539  // [syscall.Read], closing the file descriptor before syscall.Read makes
   540  // the actual system call.
   541  //
   542  // Note: KeepAlive should only be used to prevent finalizers from
   543  // running prematurely. In particular, when used with [unsafe.Pointer],
   544  // the rules for valid uses of unsafe.Pointer still apply.
   545  func KeepAlive(x any) {
   546  	// Introduce a use of x that the compiler can't eliminate.
   547  	// This makes sure x is alive on entry. We need x to be alive
   548  	// on entry for "defer runtime.KeepAlive(x)"; see issue 21402.
   549  	if cgoAlwaysFalse {
   550  		println(x)
   551  	}
   552  }
   553  

View as plain text