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cmd/internal/obj: mark split-stack prologue nonpreemptible

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When there are both a synchronous preemption request (by
clobbering the stack guard) and an asynchronous one (by signal),
the running goroutine may observe the synchronous request first
in stack bounds check, and go to the path of calling morestack.
If the preemption signal arrives at this point before the call to
morestack, the goroutine will be asynchronously preempted,
entering the scheduler. When it is resumed, the scheduler clears
the preemption request, unclobbers the stack guard. But the
resumed goroutine will still call morestack, as it is already on
its way. morestack will, as there is no preemption request,
double the stack unnecessarily. If this happens multiple times,
the stack may grow too big, although only a small amount is
actually used.

To fix this, we mark the stack bounds check and the call to
morestack async-nonpreemptible, starting after the memory
instruction (mostly a load, on x86 CMP with memory).

Not done for Wasm as it does not support async preemption.

Fixes #35470.

Change-Id: Ibd7f3d935a3649b80f47539116ec9b9556680cf2
Reviewed-on: https://go-review.googlesource.com/c/go/+/207350
Reviewed-by: David Chase <drchase@google.com>
This commit is contained in:
Cherry Zhang 2019-11-14 20:23:17 -05:00
parent c3f149250e
commit b2482e4817
7 changed files with 94 additions and 40 deletions
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10
src/cmd/internal/obj/mips/obj0.go
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@ -677,6 +677,12 @@ func (c *ctxt0) stacksplit(p *obj.Prog, framesize int32) *obj.Prog {
p.To.Type = obj.TYPE_REG
p.To.Reg = REG_R1
// Mark the stack bound check and morestack call async nonpreemptible.
// If we get preempted here, when resumed the preemption request is
// cleared, but we'll still call morestack, which will double the stack
// unnecessarily. See issue #35470.
p = c.ctxt.StartUnsafePoint(p, c.newprog)
var q *obj.Prog
if framesize <= objabi.StackSmall {
// small stack: SP < stackguard
@ -796,7 +802,7 @@ func (c *ctxt0) stacksplit(p *obj.Prog, framesize int32) *obj.Prog {
p.Mark |= LABEL
}
p = c.ctxt.EmitEntryLiveness(c.cursym, p, c.newprog)
p = c.ctxt.EmitEntryStackMap(c.cursym, p, c.newprog)
// JAL runtime.morestack(SB)
p = obj.Appendp(p, c.newprog)
@ -812,6 +818,8 @@ func (c *ctxt0) stacksplit(p *obj.Prog, framesize int32) *obj.Prog {
}
p.Mark |= BRANCH
p = c.ctxt.EndUnsafePoint(p, c.newprog, -1)
// JMP start
p = obj.Appendp(p, c.newprog)