golang mcheckmark 代码
文件路径：/src/runtime/mcheckmark.go
/ Copyright 2020 The Go Authors. All rights reserved.
/ Use of this source code is governed by a BSD-style
/ license that can be found in the LICENSE file.
/ GC checkmarks
/
/ In a concurrent garbage collector, one worries about failing to mark
/ a live object due to mutations without write barriers or bugs in the
/ collector implementation. As a sanity check, the GC has a 'checkmark'
/ mode that retraverses the object graph with the world stopped, to make
/ sure that everything that should be marked is marked.
package runtime
import (
"internal/goarch"
"runtime/internal/atomic"
"unsafe"
)
/ A checkmarksMap stores the GC marks in "checkmarks" mode. It is a
/ per-arena bitmap with a bit for every word in the arena. The mark
/ is stored on the bit corresponding to the first word of the marked
/ allocation.
/
/go:notinheap
type checkmarksMap [heapArenaBytes / goarch.PtrSize / 8]uint8
/ If useCheckmark is true, marking of an object uses the checkmark
/ bits instead of the standard mark bits.
var useCheckmark = false
/ startCheckmarks prepares for the checkmarks phase.
/
/ The world must be stopped.
func startCheckmarks() {
assertWorldStopped()
/ Clear all checkmarks.
for _, ai := range mheap_.allArenas {
arena := mheap_.arenas[ai.l1()][ai.l2()]
bitmap := arena.checkmarks
if bitmap == nil {
/ Allocate bitmap on first use.
bitmap = (*checkmarksMap)(persistentalloc(unsafe.Sizeof(*bitmap), 0, &memstats.gcMiscSys))
if bitmap == nil {
throw("out of memory allocating checkmarks bitmap")
}
arena.checkmarks = bitmap
} else {
/ Otherwise clear the existing bitmap.
for i := range bitmap {
bitmap[i] = 0
}
}
}
/ Enable checkmarking.
useCheckmark = true
}
/ endCheckmarks ends the checkmarks phase.
func endCheckmarks() {
if gcMarkWorkAvailable(nil) {
throw("GC work not flushed")
}
useCheckmark = false
}
/ setCheckmark throws if marking object is a checkmarks violation,
/ and otherwise sets obj's checkmark. It returns true if obj was
/ already checkmarked.
func setCheckmark(obj, base, off uintptr, mbits markBits) bool {
if !mbits.isMarked() {
printlock()
print("runtime: checkmarks found unexpected unmarked object obj=", hex(obj), "\n")
print("runtime: found obj at *(", hex(base), "+", hex(off), ")\n")
/ Dump the source (base) object
gcDumpObject("base", base, off)
/ Dump the object
gcDumpObject("obj", obj, ^uintptr(0))
getg().m.traceback = 2
throw("checkmark found unmarked object")
}
ai := arenaIndex(obj)
arena := mheap_.arenas[ai.l1()][ai.l2()]
arenaWord := (obj / heapArenaBytes / 8) % uintptr(len(arena.checkmarks))
mask := byte(1 << ((obj / heapArenaBytes) % 8))
bytep := &arena.checkmarks[arenaWord]
if atomic.Load8(bytep)&mask != 0 {
/ Already checkmarked.
return true
}
atomic.Or8(bytep, mask)
return false
}