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runtime: replace func-based write barrier skipping with type-based

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This CL revises CL 7504 to use explicitly uintptr types for the
struct fields that are going to be updated sometimes without
write barriers. The result is that the fields are now updated *always*
without write barriers.

This approach has two important properties:

1) Now the GC never looks at the field, so if the missing reference
could cause a problem, it will do so all the time, not just when the
write barrier is missed at just the right moment.

2) Now a write barrier never happens for the field, avoiding the
(correct) detection of inconsistent write barriers when GODEBUG=wbshadow=1.

Change-Id: Iebd3962c727c0046495cc08914a8dc0808460e0e
Reviewed-on: https://go-review.googlesource.com/9019
Reviewed-by: Austin Clements <austin@google.com>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Russ Cox 2015-04-17 00:21:30 -04:00
parent c776592a4f
commit 181e26b9fa
21 changed files with 263 additions and 278 deletions
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101
src/runtime/runtime2.go
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@ -87,8 +87,28 @@ type eface struct {
data unsafe.Pointer
}
// The guintptr, muintptr, and puintptr are all used to bypass write barriers.
// It is particularly important to avoid write barriers when the current P has
// been released, because the GC thinks the world is stopped, and an
// unexpected write barrier would not be synchronized with the GC,
// which can lead to a half-executed write barrier that has marked the object
// but not queued it. If the GC skips the object and completes before the
// queuing can occur, it will incorrectly free the object.
//
// We tried using special assignment functions invoked only when not
// holding a running P, but then some updates to a particular memory
// word went through write barriers and some did not. This breaks the
// write barrier shadow checking mode, and it is also scary: better to have
// a word that is completely ignored by the GC than to have one for which
// only a few updates are ignored.
//
// Gs, Ms, and Ps are always reachable via true pointers in the
// allgs, allm, and allp lists or (during allocation before they reach those lists)
// from stack variables.
// A guintptr holds a goroutine pointer, but typed as a uintptr
// to bypass write barriers. It is used in the Gobuf goroutine state.
// to bypass write barriers. It is used in the Gobuf goroutine state
// and in scheduling lists that are manipulated without a P.
//
// The Gobuf.g goroutine pointer is almost always updated by assembly code.
// In one of the few places it is updated by Go code - func save - it must be
@ -107,41 +127,18 @@ type eface struct {
// alternate arena. Using guintptr doesn't make that problem any worse.
type guintptr uintptr
func (gp guintptr) ptr() *g {
return (*g)(unsafe.Pointer(gp))
}
func (gp guintptr) ptr() *g { return (*g)(unsafe.Pointer(gp)) }
func (gp *guintptr) set(g *g) { *gp = guintptr(unsafe.Pointer(g)) }
// ps, ms, gs, and mcache are structures that must be manipulated at a level
// lower than that of the normal Go language. For example the routine that
// stops the world removes the p from the m structure informing the GC that
// this P is stopped and then it moves the g to the global runnable queue.
// If write barriers were allowed to happen at this point not only does
// the GC think the thread is stopped but the underlying structures
// like a p or m are not in a state that is not coherent enough to
// support the write barrier actions.
// This is particularly painful since a partially executed write barrier
// may mark the object but be delinquent in informing the GC that the
// object needs to be scanned.
type puintptr uintptr
// setGNoWriteBarriers does *gdst = gval without a write barrier.
func setGNoWriteBarrier(gdst **g, gval *g) {
*(*uintptr)(unsafe.Pointer(gdst)) = uintptr(unsafe.Pointer(gval))
}
func (pp puintptr) ptr() *p { return (*p)(unsafe.Pointer(pp)) }
func (pp *puintptr) set(p *p) { *pp = puintptr(unsafe.Pointer(p)) }
// setMNoWriteBarriers does *mdst = mval without a write barrier.
func setMNoWriteBarrier(mdst **m, mval *m) {
*(*uintptr)(unsafe.Pointer(mdst)) = uintptr(unsafe.Pointer(mval))
}
type muintptr uintptr
// setPNoWriteBarriers does *pdst = pval without a write barrier.
func setPNoWriteBarrier(pdst **p, pval *p) {
*(*uintptr)(unsafe.Pointer(pdst)) = uintptr(unsafe.Pointer(pval))
}
// setMcacheNoWriteBarriers does *mcachedst = mcacheval without a write barrier.
func setMcacheNoWriteBarrier(mcachedst **mcache, mcacheval *mcache) {
*(*uintptr)(unsafe.Pointer(mcachedst)) = uintptr(unsafe.Pointer(mcacheval))
}
func (mp muintptr) ptr() *m { return (*m)(unsafe.Pointer(mp)) }
func (mp *muintptr) set(m *m) { *mp = muintptr(unsafe.Pointer(m)) }
type gobuf struct {
// The offsets of sp, pc, and g are known to (hard-coded in) libmach.
@ -224,7 +221,7 @@ type g struct {
goid int64
waitsince int64 // approx time when the g become blocked
waitreason string // if status==gwaiting
schedlink *g
schedlink guintptr
preempt bool // preemption signal, duplicates stackguard0 = stackpreempt
paniconfault bool // panic (instead of crash) on unexpected fault address
preemptscan bool // preempted g does scan for gc
@ -263,11 +260,11 @@ type m struct {
procid uint64 // for debuggers, but offset not hard-coded
gsignal *g // signal-handling g
tls [4]uintptr // thread-local storage (for x86 extern register)
mstartfn uintptr // TODO: type as func(); note: this is a non-heap allocated func()
curg *g // current running goroutine
caughtsig *g // goroutine running during fatal signal
p *p // attached p for executing go code (nil if not executing go code)
nextp *p
mstartfn func()
curg *g // current running goroutine
caughtsig guintptr // goroutine running during fatal signal
p puintptr // attached p for executing go code (nil if not executing go code)
nextp puintptr
id int32
mallocing int32
throwing int32
@ -286,7 +283,7 @@ type m struct {
ncgo int32 // number of cgo calls currently in progress
park note
alllink *m // on allm
schedlink *m
schedlink muintptr
machport uint32 // return address for mach ipc (os x)
mcache *mcache
lockedg *g
@ -315,7 +312,7 @@ type m struct {
libcall libcall
libcallpc uintptr // for cpu profiler
libcallsp uintptr
libcallg *g
libcallg guintptr
//#endif
//#ifdef GOOS_solaris
perrno *int32 // pointer to tls errno
@ -336,10 +333,10 @@ type p struct {
id int32
status uint32 // one of pidle/prunning/...
link *p
schedtick uint32 // incremented on every scheduler call
syscalltick uint32 // incremented on every system call
m *m // back-link to associated m (nil if idle)
link puintptr
schedtick uint32 // incremented on every scheduler call
syscalltick uint32 // incremented on every system call
m muintptr // back-link to associated m (nil if idle)
mcache *mcache
deferpool [5][]*_defer // pool of available defer structs of different sizes (see panic.go)
@ -379,19 +376,19 @@ type schedt struct {
goidgen uint64
midle *m // idle m's waiting for work
nmidle int32 // number of idle m's waiting for work
nmidlelocked int32 // number of locked m's waiting for work
mcount int32 // number of m's that have been created
maxmcount int32 // maximum number of m's allowed (or die)
midle muintptr // idle m's waiting for work
nmidle int32 // number of idle m's waiting for work
nmidlelocked int32 // number of locked m's waiting for work
mcount int32 // number of m's that have been created
maxmcount int32 // maximum number of m's allowed (or die)
pidle *p // idle p's
pidle puintptr // idle p's
npidle uint32
nmspinning uint32
// Global runnable queue.
runqhead *g
runqtail *g
runqhead guintptr
runqtail guintptr
runqsize int32
// Global cache of dead G's.