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  	"runtime/internal/atomic"
    13  	"unsafe"
    14  )
    15  
    16  // finblock is an array of finalizers to be executed. finblocks are
    17  // arranged in a linked list for the finalizer queue.
    18  //
    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).
    22  //
    23  //go:notinheap
    24  type finblock struct {
    25  	alllink *finblock
    26  	next    *finblock
    27  	cnt     uint32
    28  	_       int32
    29  	fin     [(_FinBlockSize - 2*goarch.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
    30  }
    31  
    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
    37  var fingwait bool
    38  var fingwake bool
    39  var allfin *finblock // list of all blocks
    40  
    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)
    48  }
    49  
    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
    62  	// aka
    63  	//
    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
    69  	//
    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,
    76  }
    77  
    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")
    87  	}
    88  
    89  	lock(&finlock)
    90  	if finq == nil || finq.cnt == uint32(len(finq.fin)) {
    91  		if finc == nil {
    92  			finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gcMiscSys))
    93  			finc.alllink = allfin
    94  			allfin = finc
    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")
   105  				}
   106  				for i := range finptrmask {
   107  					finptrmask[i] = finalizer1[i%len(finalizer1)]
   108  				}
   109  			}
   110  		}
   111  		block := finc
   112  		finc = block.next
   113  		block.next = finq
   114  		finq = block
   115  	}
   116  	f := &finq.fin[finq.cnt]
   117  	atomic.Xadd(&finq.cnt, +1) // Sync with markroots
   118  	f.fn = fn
   119  	f.nret = nret
   120  	f.fint = fint
   121  	f.ot = ot
   122  	f.arg = p
   123  	fingwake = true
   124  	unlock(&finlock)
   125  }
   126  
   127  //go:nowritebarrier
   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++ {
   131  			f := &fb.fin[i]
   132  			callback(f.fn, f.arg, f.nret, f.fint, f.ot)
   133  		}
   134  	}
   135  }
   136  
   137  func wakefing() *g {
   138  	var res *g
   139  	lock(&finlock)
   140  	if fingwait && fingwake {
   141  		fingwait = false
   142  		fingwake = false
   143  		res = fing
   144  	}
   145  	unlock(&finlock)
   146  	return res
   147  }
   148  
   149  var (
   150  	fingCreate  uint32
   151  	fingRunning bool
   152  )
   153  
   154  func createfing() {
   155  	// start the finalizer goroutine exactly once
   156  	if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
   157  		go runfinq()
   158  	}
   159  }
   160  
   161  // This is the goroutine that runs all of the finalizers
   162  func runfinq() {
   163  	var (
   164  		frame    unsafe.Pointer
   165  		framecap uintptr
   166  		argRegs  int
   167  	)
   168  
   169  	gp := getg()
   170  	lock(&finlock)
   171  	fing = gp
   172  	unlock(&finlock)
   173  
   174  	for {
   175  		lock(&finlock)
   176  		fb := finq
   177  		finq = nil
   178  		if fb == nil {
   179  			fingwait = true
   180  			goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
   181  			continue
   182  		}
   183  		argRegs = intArgRegs
   184  		unlock(&finlock)
   185  		if raceenabled {
   186  			racefingo()
   187  		}
   188  		for fb != nil {
   189  			for i := fb.cnt; i > 0; i-- {
   190  				f := &fb.fin[i-1]
   191  
   192  				var regs abi.RegArgs
   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.
   196  				//
   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)
   208  					framecap = framesz
   209  				}
   210  
   211  				if f.fint == nil {
   212  					throw("missing type in runfinq")
   213  				}
   214  				r := frame
   215  				if argRegs > 0 {
   216  					r = unsafe.Pointer(&regs.Ints)
   217  				} else {
   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{}
   223  				}
   224  				switch f.fint.kind & kindMask {
   225  				case kindPtr:
   226  					// direct use of pointer
   227  					*(*unsafe.Pointer)(r) = f.arg
   228  				case kindInterface:
   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)
   237  					}
   238  				default:
   239  					throw("bad kind in runfinq")
   240  				}
   241  				fingRunning = true
   242  				reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz), uint32(framesz), &regs)
   243  				fingRunning = false
   244  
   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.
   249  				f.fn = nil
   250  				f.arg = nil
   251  				f.ot = nil
   252  				atomic.Store(&fb.cnt, i-1)
   253  			}
   254  			next := fb.next
   255  			lock(&finlock)
   256  			fb.next = finc
   257  			finc = fb
   258  			unlock(&finlock)
   259  			fb = next
   260  		}
   261  	}
   262  }
   263  
   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.
   271  //
   272  // SetFinalizer(obj, nil) clears any finalizer associated with obj.
   273  //
   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
   280  // program.
   281  //
   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.
   289  //
   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.
   300  //
   301  // It is not guaranteed that a finalizer will run if the size of *obj is
   302  // zero bytes.
   303  //
   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.
   307  //
   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.
   315  //
   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.
   325  //
   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
   328  // a new goroutine.
   329  //
   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) {
   342  	if debug.sbrk != 0 {
   343  		// debug.sbrk never frees memory, so no finalizers run
   344  		// (and we don't have the data structures to record them).
   345  		return
   346  	}
   347  	e := efaceOf(&obj)
   348  	etyp := e._type
   349  	if etyp == nil {
   350  		throw("runtime.SetFinalizer: first argument is nil")
   351  	}
   352  	if etyp.kind&kindMask != kindPtr {
   353  		throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
   354  	}
   355  	ot := (*ptrtype)(unsafe.Pointer(etyp))
   356  	if ot.elem == nil {
   357  		throw("nil elem type!")
   358  	}
   359  
   360  	// find the containing object
   361  	base, _, _ := findObject(uintptr(e.data), 0, 0)
   362  
   363  	if base == 0 {
   364  		// 0-length objects are okay.
   365  		if e.data == unsafe.Pointer(&zerobase) {
   366  			return
   367  		}
   368  
   369  		// Global initializers might be linker-allocated.
   370  		//	var Foo = &Object{}
   371  		//	func main() {
   372  		//		runtime.SetFinalizer(Foo, nil)
   373  		//	}
   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 {
   382  				return
   383  			}
   384  		}
   385  		throw("runtime.SetFinalizer: pointer not in allocated block")
   386  	}
   387  
   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")
   393  		}
   394  	}
   395  
   396  	f := efaceOf(&finalizer)
   397  	ftyp := f._type
   398  	if ftyp == nil {
   399  		// switch to system stack and remove finalizer
   400  		systemstack(func() {
   401  			removefinalizer(e.data)
   402  		})
   403  		return
   404  	}
   405  
   406  	if ftyp.kind&kindMask != kindFunc {
   407  		throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
   408  	}
   409  	ft := (*functype)(unsafe.Pointer(ftyp))
   410  	if ft.dotdotdot() {
   411  		throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
   412  	}
   413  	if ft.inCount != 1 {
   414  		throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
   415  	}
   416  	fint := ft.in()[0]
   417  	switch {
   418  	case fint == etyp:
   419  		// ok - same type
   420  		goto okarg
   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.
   425  			goto okarg
   426  		}
   427  	case fint.kind&kindMask == kindInterface:
   428  		ityp := (*interfacetype)(unsafe.Pointer(fint))
   429  		if len(ityp.mhdr) == 0 {
   430  			// ok - satisfies empty interface
   431  			goto okarg
   432  		}
   433  		if iface := assertE2I2(ityp, *efaceOf(&obj)); iface.tab != nil {
   434  			goto okarg
   435  		}
   436  	}
   437  	throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
   438  okarg:
   439  	// compute size needed for return parameters
   440  	nret := uintptr(0)
   441  	for _, t := range ft.out() {
   442  		nret = alignUp(nret, uintptr(t.align)) + uintptr(t.size)
   443  	}
   444  	nret = alignUp(nret, goarch.PtrSize)
   445  
   446  	// make sure we have a finalizer goroutine
   447  	createfing()
   448  
   449  	systemstack(func() {
   450  		if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
   451  			throw("runtime.SetFinalizer: finalizer already set")
   452  		}
   453  	})
   454  }
   455  
   456  // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
   457  //
   458  //go:noinline
   459  
   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.
   463  //
   464  // A very simplified example showing where KeepAlive is required:
   465  //
   466  //	type File struct { d int }
   467  //	d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
   468  //	// ... do something if err != nil ...
   469  //	p := &File{d}
   470  //	runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
   471  //	var buf [10]byte
   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.
   476  //
   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.
   480  //
   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.
   488  	if cgoAlwaysFalse {
   489  		println(x)
   490  	}
   491  }
   492  

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