8dc2499aa6
gotools/ * Makefile.am (go_cmd_cgo_files): Add ast_go118.go (check-go-tool): Copy golang.org/x/tools directories. * Makefile.in: Regenerate. Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/384695
417 lines
13 KiB
Go
417 lines
13 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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// Garbage collector: finalizers and block profiling.
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package runtime
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import (
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"internal/goarch"
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"runtime/internal/atomic"
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"unsafe"
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)
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// finblock is an array of finalizers to be executed. finblocks are
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// arranged in a linked list for the finalizer queue.
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//
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// finblock is allocated from non-GC'd memory, so any heap pointers
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// must be specially handled. GC currently assumes that the finalizer
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// queue does not grow during marking (but it can shrink).
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//
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//go:notinheap
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type finblock struct {
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alllink *finblock
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next *finblock
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cnt uint32
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_ int32
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fin [(_FinBlockSize - 2*goarch.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
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}
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var finlock mutex // protects the following variables
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var fing *g // goroutine that runs finalizers
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var finq *finblock // list of finalizers that are to be executed
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var finc *finblock // cache of free blocks
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var finptrmask [_FinBlockSize / goarch.PtrSize / 8]byte
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var fingwait bool
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var fingwake bool
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var allfin *finblock // list of all blocks
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// NOTE: Layout known to queuefinalizer.
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type finalizer struct {
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fn *funcval // function to call (may be a heap pointer)
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arg unsafe.Pointer // ptr to object (may be a heap pointer)
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ft *functype // type of fn (unlikely, but may be a heap pointer)
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ot *ptrtype // type of ptr to object (may be a heap pointer)
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}
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func queuefinalizer(p unsafe.Pointer, fn *funcval, ft *functype, ot *ptrtype) {
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if gcphase != _GCoff {
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// Currently we assume that the finalizer queue won't
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// grow during marking so we don't have to rescan it
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// during mark termination. If we ever need to lift
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// this assumption, we can do it by adding the
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// necessary barriers to queuefinalizer (which it may
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// have automatically).
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throw("queuefinalizer during GC")
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}
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lock(&finlock)
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if finq == nil || finq.cnt == uint32(len(finq.fin)) {
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if finc == nil {
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finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gcMiscSys))
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finc.alllink = allfin
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allfin = finc
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if finptrmask[0] == 0 {
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// Build pointer mask for Finalizer array in block.
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// We allocate values of type finalizer in
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// finblock values. Since these values are
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// allocated by persistentalloc, they require
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// special scanning during GC. finptrmask is a
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// pointer mask to use while scanning.
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// Since all the values in finalizer are
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// pointers, just turn all bits on.
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for i := range finptrmask {
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finptrmask[i] = 0xff
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}
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}
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}
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block := finc
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finc = block.next
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block.next = finq
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finq = block
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}
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f := &finq.fin[finq.cnt]
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atomic.Xadd(&finq.cnt, +1) // Sync with markroots
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f.fn = fn
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f.ft = ft
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f.ot = ot
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f.arg = p
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fingwake = true
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unlock(&finlock)
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}
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//go:nowritebarrier
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func iterate_finq(callback func(*funcval, unsafe.Pointer, *functype, *ptrtype)) {
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for fb := allfin; fb != nil; fb = fb.alllink {
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for i := uint32(0); i < fb.cnt; i++ {
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f := &fb.fin[i]
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callback(f.fn, f.arg, f.ft, f.ot)
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}
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}
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}
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func wakefing() *g {
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var res *g
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lock(&finlock)
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if fingwait && fingwake {
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fingwait = false
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fingwake = false
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res = fing
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}
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unlock(&finlock)
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return res
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}
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var (
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fingCreate uint32
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)
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func createfing() {
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// start the finalizer goroutine exactly once
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if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
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expectSystemGoroutine()
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go runfinq()
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}
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}
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// This is the goroutine that runs all of the finalizers
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func runfinq() {
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setSystemGoroutine()
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var (
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ef eface
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ifac iface
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)
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gp := getg()
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gp.isFinalizerGoroutine = true
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for {
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lock(&finlock)
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fb := finq
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finq = nil
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if fb == nil {
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fing = gp
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fingwait = true
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goparkunlock(&finlock, waitReasonFinalizerWait, traceEvGoBlock, 1)
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continue
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}
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unlock(&finlock)
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for fb != nil {
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for i := fb.cnt; i > 0; i-- {
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f := &fb.fin[i-1]
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if f.ft == nil {
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throw("missing type in runfinq")
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}
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fint := f.ft.in[0]
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var param unsafe.Pointer
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switch fint.kind & kindMask {
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case kindPtr:
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// direct use of pointer
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param = unsafe.Pointer(&f.arg)
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case kindInterface:
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ityp := (*interfacetype)(unsafe.Pointer(fint))
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if len(ityp.methods) == 0 {
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// set up with empty interface
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ef._type = &f.ot.typ
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ef.data = f.arg
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param = unsafe.Pointer(&ef)
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} else {
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// convert to interface with methods
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// this conversion is guaranteed to succeed - we checked in SetFinalizer
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ifac.tab = getitab(fint, &f.ot.typ, true)
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ifac.data = f.arg
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param = unsafe.Pointer(&ifac)
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}
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default:
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throw("bad kind in runfinq")
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}
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// This is not a system goroutine while
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// running the actual finalizer.
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// This matters because we want this
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// goroutine to appear in a stack dump
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// if the finalizer crashes.
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// The gc toolchain handles this using
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// a global variable fingRunning,
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// but we don't need that.
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gp.isSystemGoroutine = false
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reflectcall(f.ft, f.fn, false, false, ¶m, nil)
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gp.isSystemGoroutine = true
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// Drop finalizer queue heap references
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// before hiding them from markroot.
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// This also ensures these will be
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// clear if we reuse the finalizer.
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f.fn = nil
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f.arg = nil
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f.ot = nil
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atomic.Store(&fb.cnt, i-1)
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}
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next := fb.next
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lock(&finlock)
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fb.next = finc
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finc = fb
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unlock(&finlock)
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fb = next
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}
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}
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}
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// SetFinalizer sets the finalizer associated with obj to the provided
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// finalizer function. When the garbage collector finds an unreachable block
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// with an associated finalizer, it clears the association and runs
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// finalizer(obj) in a separate goroutine. This makes obj reachable again,
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// but now without an associated finalizer. Assuming that SetFinalizer
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// is not called again, the next time the garbage collector sees
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// that obj is unreachable, it will free obj.
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//
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// SetFinalizer(obj, nil) clears any finalizer associated with obj.
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//
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// The argument obj must be a pointer to an object allocated by calling
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// new, by taking the address of a composite literal, or by taking the
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// address of a local variable.
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// The argument finalizer must be a function that takes a single argument
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// to which obj's type can be assigned, and can have arbitrary ignored return
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// values. If either of these is not true, SetFinalizer may abort the
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// program.
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//
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// Finalizers are run in dependency order: if A points at B, both have
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// finalizers, and they are otherwise unreachable, only the finalizer
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// for A runs; once A is freed, the finalizer for B can run.
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// If a cyclic structure includes a block with a finalizer, that
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// cycle is not guaranteed to be garbage collected and the finalizer
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// is not guaranteed to run, because there is no ordering that
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// respects the dependencies.
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//
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// The finalizer is scheduled to run at some arbitrary time after the
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// program can no longer reach the object to which obj points.
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// There is no guarantee that finalizers will run before a program exits,
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// so typically they are useful only for releasing non-memory resources
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// associated with an object during a long-running program.
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// For example, an os.File object could use a finalizer to close the
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// associated operating system file descriptor when a program discards
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// an os.File without calling Close, but it would be a mistake
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// to depend on a finalizer to flush an in-memory I/O buffer such as a
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// bufio.Writer, because the buffer would not be flushed at program exit.
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//
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// It is not guaranteed that a finalizer will run if the size of *obj is
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// zero bytes.
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//
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// It is not guaranteed that a finalizer will run for objects allocated
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// in initializers for package-level variables. Such objects may be
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// linker-allocated, not heap-allocated.
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//
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// A finalizer may run as soon as an object becomes unreachable.
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// In order to use finalizers correctly, the program must ensure that
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// the object is reachable until it is no longer required.
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// Objects stored in global variables, or that can be found by tracing
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// pointers from a global variable, are reachable. For other objects,
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// pass the object to a call of the KeepAlive function to mark the
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// last point in the function where the object must be reachable.
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//
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// For example, if p points to a struct, such as os.File, that contains
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// a file descriptor d, and p has a finalizer that closes that file
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// descriptor, and if the last use of p in a function is a call to
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// syscall.Write(p.d, buf, size), then p may be unreachable as soon as
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// the program enters syscall.Write. The finalizer may run at that moment,
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// closing p.d, causing syscall.Write to fail because it is writing to
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// a closed file descriptor (or, worse, to an entirely different
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// file descriptor opened by a different goroutine). To avoid this problem,
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// call runtime.KeepAlive(p) after the call to syscall.Write.
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//
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// A single goroutine runs all finalizers for a program, sequentially.
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// If a finalizer must run for a long time, it should do so by starting
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// a new goroutine.
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func SetFinalizer(obj any, finalizer any) {
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if debug.sbrk != 0 {
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// debug.sbrk never frees memory, so no finalizers run
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// (and we don't have the data structures to record them).
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return
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}
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e := efaceOf(&obj)
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etyp := e._type
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if etyp == nil {
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throw("runtime.SetFinalizer: first argument is nil")
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}
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if etyp.kind&kindMask != kindPtr {
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throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
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}
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ot := (*ptrtype)(unsafe.Pointer(etyp))
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if ot.elem == nil {
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throw("nil elem type!")
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}
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// find the containing object
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base, _, _ := findObject(uintptr(e.data), 0, 0, false)
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if base == 0 {
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// 0-length objects are okay.
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if e.data == unsafe.Pointer(&zerobase) {
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return
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}
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// Global initializers might be linker-allocated.
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// var Foo = &Object{}
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// func main() {
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// runtime.SetFinalizer(Foo, nil)
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// }
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// The relevant segments are: noptrdata, data, bss, noptrbss.
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// We cannot assume they are in any order or even contiguous,
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// due to external linking.
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//
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// For gccgo we have no reliable way to detect them,
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// so we just return.
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return
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}
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if uintptr(e.data) != base {
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// As an implementation detail we allow to set finalizers for an inner byte
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// of an object if it could come from tiny alloc (see mallocgc for details).
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if ot.elem == nil || ot.elem.ptrdata != 0 || ot.elem.size >= maxTinySize {
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throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
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}
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}
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f := efaceOf(&finalizer)
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ftyp := f._type
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if ftyp == nil {
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// switch to system stack and remove finalizer
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systemstack(func() {
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removefinalizer(e.data)
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})
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return
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}
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if ftyp.kind&kindMask != kindFunc {
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throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
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}
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ft := (*functype)(unsafe.Pointer(ftyp))
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if ft.dotdotdot {
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throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
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}
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if len(ft.in) != 1 {
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throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
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}
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fint := ft.in[0]
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switch {
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case fint == etyp:
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// ok - same type
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goto okarg
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case fint.kind&kindMask == kindPtr:
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if (fint.uncommontype == nil || etyp.uncommontype == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
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// ok - not same type, but both pointers,
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// one or the other is unnamed, and same element type, so assignable.
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goto okarg
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}
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case fint.kind&kindMask == kindInterface:
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ityp := (*interfacetype)(unsafe.Pointer(fint))
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if len(ityp.methods) == 0 {
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// ok - satisfies empty interface
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goto okarg
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}
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if getitab(fint, etyp, true) == nil {
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goto okarg
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}
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}
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throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
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okarg:
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// make sure we have a finalizer goroutine
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createfing()
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systemstack(func() {
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data := f.data
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if !isDirectIface(ftyp) {
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data = *(*unsafe.Pointer)(data)
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}
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if !addfinalizer(e.data, (*funcval)(data), ft, ot) {
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throw("runtime.SetFinalizer: finalizer already set")
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}
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})
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}
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// Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
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//go:noinline
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// KeepAlive marks its argument as currently reachable.
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// This ensures that the object is not freed, and its finalizer is not run,
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// before the point in the program where KeepAlive is called.
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//
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// A very simplified example showing where KeepAlive is required:
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// type File struct { d int }
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// d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
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// // ... do something if err != nil ...
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// p := &File{d}
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// runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
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// var buf [10]byte
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// n, err := syscall.Read(p.d, buf[:])
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// // Ensure p is not finalized until Read returns.
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// runtime.KeepAlive(p)
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// // No more uses of p after this point.
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//
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// Without the KeepAlive call, the finalizer could run at the start of
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// syscall.Read, closing the file descriptor before syscall.Read makes
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// the actual system call.
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//
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// Note: KeepAlive should only be used to prevent finalizers from
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// running prematurely. In particular, when used with unsafe.Pointer,
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// the rules for valid uses of unsafe.Pointer still apply.
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func KeepAlive(x any) {
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// Introduce a use of x that the compiler can't eliminate.
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// This makes sure x is alive on entry. We need x to be alive
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// on entry for "defer runtime.KeepAlive(x)"; see issue 21402.
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if cgoAlwaysFalse {
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println(x)
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}
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}
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