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runtime: make morestack less subtle

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morestack writes the context pointer to gobuf.ctxt, but since
morestack is written in assembly (and has to be very careful with
state), it does *not* invoke the requisite write barrier for this
write. Instead, we patch this up later, in newstack, where we invoke
an explicit write barrier for ctxt.

This already requires some subtle reasoning, and it's going to get a
lot hairier with the hybrid barrier.

Fix this by simplifying the whole mechanism. Instead of writing
gobuf.ctxt in morestack, just pass the value of the context register
to newstack and let it write it to gobuf.ctxt. This is a normal Go
pointer write, so it gets the normal Go write barrier. No subtle
reasoning required.

Updates #17503.

Change-Id: Ia6bf8459bfefc6828f53682ade32c02412e4db63
Reviewed-on: https://go-review.googlesource.com/31550
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
Austin Clements 2016-10-19 18:27:39 -04:00
parent cdccd6a79c
commit bf9c71cb43
9 changed files with 39 additions and 19 deletions
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21
src/runtime/stack.go
View file

@ -925,7 +925,10 @@ func round2(x int32) int32 {
//
// g->atomicstatus will be Grunning or Gscanrunning upon entry.
// If the GC is trying to stop this g then it will set preemptscan to true.
func newstack() {
//
// ctxt is the value of the context register on morestack. newstack
// will write it to g.sched.ctxt.
func newstack(ctxt unsafe.Pointer) {
thisg := getg()
// TODO: double check all gp. shouldn't be getg().
if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {
@ -937,8 +940,13 @@ func newstack() {
traceback(morebuf.pc, morebuf.sp, morebuf.lr, morebuf.g.ptr())
throw("runtime: wrong goroutine in newstack")
}
gp := thisg.m.curg
// Write ctxt to gp.sched. We do this here instead of in
// morestack so it has the necessary write barrier.
gp.sched.ctxt = ctxt
if thisg.m.curg.throwsplit {
gp := thisg.m.curg
// Update syscallsp, syscallpc in case traceback uses them.
morebuf := thisg.m.morebuf
gp.syscallsp = morebuf.sp
@ -951,7 +959,6 @@ func newstack() {
throw("runtime: stack split at bad time")
}
gp := thisg.m.curg
morebuf := thisg.m.morebuf
thisg.m.morebuf.pc = 0
thisg.m.morebuf.lr = 0
@ -1003,14 +1010,6 @@ func newstack() {
throw("runtime: split stack overflow")
}
if gp.sched.ctxt != nil {
// morestack wrote sched.ctxt on its way in here,
// without a write barrier. Run the write barrier now.
// It is not possible to be preempted between then
// and now, so it's okay.
writebarrierptr_nostore((*uintptr)(unsafe.Pointer(&gp.sched.ctxt)), uintptr(gp.sched.ctxt))
}
if preempt {
if gp == thisg.m.g0 {
throw("runtime: preempt g0")