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runtime: remove write barriers from newstack, gogo

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Currently, newstack and gogo have write barriers for maintaining the
context register saved in g.sched.ctxt. This is troublesome, because
newstack can be called from go:nowritebarrierrec places that can't
allow write barriers. It happens to be benign because g.sched.ctxt
will always be nil on entry to newstack *and* it so happens the
incoming ctxt will also always be nil in these contexts (I
think/hope), but this is playing with fire. It's also desirable to
mark newstack go:nowritebarrierrec to prevent any other, non-benign
write barriers from creeping in, but we can't do that right now
because of this one write barrier.

Fix all of this by observing that g.sched.ctxt is really just a saved
live pointer register. Hence, we can shade it when we scan g's stack
and otherwise move it back and forth between the actual context
register and g.sched.ctxt without write barriers. This means we can
save it in morestack along with all of the other g.sched, eliminate
the save from newstack along with its troublesome write barrier, and
eliminate the shenanigans in gogo to invoke the write barrier when
restoring it.

Once we've done all of this, we can mark newstack
go:nowritebarrierrec.

Fixes #22385.
For #22460.

Change-Id: I43c24958e3f6785b53c1350e1e83c2844e0d1522
Reviewed-on: https://go-review.googlesource.com/72553
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
This commit is contained in:
Austin Clements 2017-10-22 21:37:05 -04:00
parent ca0f303f2b
commit 3beaf26e4f
12 changed files with 41 additions and 150 deletions
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12
src/runtime/stack.go
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@ -913,9 +913,12 @@ 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.
//
// ctxt is the value of the context register on morestack. newstack
// will write it to g.sched.ctxt.
func newstack(ctxt unsafe.Pointer) {
// This must be nowritebarrierrec because it can be called as part of
// stack growth from other nowritebarrierrec functions, but the
// compiler doesn't check this.
//
//go:nowritebarrierrec
func newstack() {
thisg := getg()
// TODO: double check all gp. shouldn't be getg().
if thisg.m.morebuf.g.ptr().stackguard0 == stackFork {
@ -929,9 +932,6 @@ func newstack(ctxt unsafe.Pointer) {
}
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 {
// Update syscallsp, syscallpc in case traceback uses them.