Stowage/go
2
0
Fork 0
mirror of https://github.com/golang/go.git synced 2025-12-08 06:10:04 +00:00
Code Issues Projects Releases Packages Wiki Activity

runtime: ensure m.p is never stale

Browse source 
When a goroutine enters a syscall, its M unwires from its P to allow
the P to be retaken by another M if the syscall is slow. The M retains a
reference to its old P, however, so that if its old P has not been
retaken when the syscall returns, it can quickly reacquire that P.

The implementation, however, was confusing, as it left the reference to
the potentially-retaken P in m.p, which implied that the P was still
wired.

Make the code clearer by enforcing the invariant that m.p is never
stale. entersyscall now moves m.p to m.oldp and sets m.p to 0;
exitsyscall does the reverse, provided m.oldp has not been retaken.

With this scheme in place, the issue described in #27660 (assertion
failures in the race detector) would have resulted in a clean segfault
instead of silently corrupting memory.

Change-Id: Ib3e03623ebed4f410e852a716919fe4538858f0a
Reviewed-on: https://go-review.googlesource.com/c/148899
Run-TryBot: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
This commit is contained in:
Nikhil Benesch 2018-11-09 00:55:13 -05:00 • committed by Dmitry Vyukov
parent e4c1feef74
commit 8e0ec5ec09
2 changed files with 24 additions and 29 deletions
Download patch file Download diff file Expand all files Collapse all files

52
src/runtime/proc.go
View file

@ -2828,8 +2828,11 @@ func reentersyscall(pc, sp uintptr) {
_g_.m.syscalltick = _g_.m.p.ptr().syscalltick
_g_.sysblocktraced = true
_g_.m.mcache = nil
_g_.m.p.ptr().m = 0
atomic.Store(&_g_.m.p.ptr().status, _Psyscall)
pp := _g_.m.p.ptr()
pp.m = 0
_g_.m.oldp.set(pp)
_g_.m.p = 0
atomic.Store(&pp.status, _Psyscall)
if sched.gcwaiting != 0 {
systemstack(entersyscall_gcwait)
save(pc, sp)
@ -2855,7 +2858,7 @@ func entersyscall_sysmon() {
func entersyscall_gcwait() {
_g_ := getg()
_p_ := _g_.m.p.ptr()
_p_ := _g_.m.oldp.ptr()
lock(&sched.lock)
if sched.stopwait > 0 && atomic.Cas(&_p_.status, _Psyscall, _Pgcstop) {
@ -2940,8 +2943,9 @@ func exitsyscall() {
}
_g_.waitsince = 0
oldp := _g_.m.p.ptr()
if exitsyscallfast() {
oldp := _g_.m.oldp.ptr()
_g_.m.oldp = 0
if exitsyscallfast(oldp) {
if _g_.m.mcache == nil {
throw("lost mcache")
}
@ -3011,27 +3015,23 @@ func exitsyscall() {
}
//go:nosplit
func exitsyscallfast() bool {
func exitsyscallfast(oldp *p) bool {
_g_ := getg()
// Freezetheworld sets stopwait but does not retake P's.
if sched.stopwait == freezeStopWait {
_g_.m.mcache = nil
_g_.m.p = 0
return false
}
// Try to re-acquire the last P.
if _g_.m.p != 0 && _g_.m.p.ptr().status == _Psyscall && atomic.Cas(&_g_.m.p.ptr().status, _Psyscall, _Prunning) {
if oldp != nil && oldp.status == _Psyscall && atomic.Cas(&oldp.status, _Psyscall, _Pidle) {
// There's a cpu for us, so we can run.
wirep(oldp)
exitsyscallfast_reacquired()
return true
}
// Try to get any other idle P.
oldp := _g_.m.p.ptr()
_g_.m.mcache = nil
_g_.m.p = 0
if sched.pidle != 0 {
var ok bool
systemstack(func() {
@ -3058,15 +3058,9 @@ func exitsyscallfast() bool {
// has successfully reacquired the P it was running on before the
// syscall.
//
// This function is allowed to have write barriers because exitsyscall
// has acquired a P at this point.
//
//go:yeswritebarrierrec
//go:nosplit
func exitsyscallfast_reacquired() {
_g_ := getg()
_g_.m.mcache = _g_.m.p.ptr().mcache
_g_.m.p.ptr().m.set(_g_.m)
if _g_.m.syscalltick != _g_.m.p.ptr().syscalltick {
if trace.enabled {
// The p was retaken and then enter into syscall again (since _g_.m.syscalltick has changed).
@ -4081,11 +4075,9 @@ func procresize(nprocs int32) *p {
//go:yeswritebarrierrec
func acquirep(_p_ *p) {
// Do the part that isn't allowed to have write barriers.
acquirep1(_p_)
wirep(_p_)
// have p; write barriers now allowed
_g_ := getg()
_g_.m.mcache = _p_.mcache
// Have p; write barriers now allowed.
// Perform deferred mcache flush before this P can allocate
// from a potentially stale mcache.
@ -4096,25 +4088,27 @@ func acquirep(_p_ *p) {
}
}
// acquirep1 is the first step of acquirep, which actually acquires
// _p_. This is broken out so we can disallow write barriers for this
// part, since we don't yet have a P.
// wirep is the first step of acquirep, which actually associates the
// current M to _p_. This is broken out so we can disallow write
// barriers for this part, since we don't yet have a P.
//
//go:nowritebarrierrec
func acquirep1(_p_ *p) {
//go:nosplit
func wirep(_p_ *p) {
_g_ := getg()
if _g_.m.p != 0 || _g_.m.mcache != nil {
throw("acquirep: already in go")
throw("wirep: already in go")
}
if _p_.m != 0 || _p_.status != _Pidle {
id := int64(0)
if _p_.m != 0 {
id = _p_.m.ptr().id
}
print("acquirep: p->m=", _p_.m, "(", id, ") p->status=", _p_.status, "\n")
throw("acquirep: invalid p state")
print("wirep: p->m=", _p_.m, "(", id, ") p->status=", _p_.status, "\n")
throw("wirep: invalid p state")
}
_g_.m.mcache = _p_.mcache
_g_.m.p.set(_p_)
_p_.m.set(_g_.m)
_p_.status = _Prunning

1
src/runtime/runtime2.go
View file

@ -417,6 +417,7 @@ type m struct {
caughtsig guintptr // goroutine running during fatal signal
p puintptr // attached p for executing go code (nil if not executing go code)
nextp puintptr
oldp puintptr // the p that was attached before executing a syscall
id int64
mallocing int32
throwing int32