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// 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"
"runtime/internal/sys"
"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.
type checkmarksMap struct {
_ sys.NotInHeap
b [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.b {
bitmap.b[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.b))
mask := byte(1 << ((obj / heapArenaBytes) % 8))
bytep := &arena.checkmarks.b[arenaWord]
if atomic.Load8(bytep)&mask != 0 {
// Already checkmarked.
return true
}
atomic.Or8(bytep, mask)
return false
}