109 lines
3 KiB
Go
109 lines
3 KiB
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, forStack bool) bool {
|
|
if !mbits.isMarked() {
|
|
// Stack scanning is conservative, so we can see a
|
|
// reference to an object not previously found.
|
|
// Assume the object was correctly not marked and
|
|
// ignore the pointer.
|
|
if forStack {
|
|
return false
|
|
}
|
|
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
|
|
}
|