code review 118050043: runtime: set sweepgen atomically before updating span state

runtime: set sweepgen atomically before updating span state

Sweepone may be running while a new span is allocating.  It
must not see the state updated while the sweepgen is unset.

Fixes issue 8399

[khr, 2014-07-21 21:39:34]

Hello golang-codereviews@googlegroups.com,

I'd like you to review this change to
https://khr%40golang.org@code.google.com/p/go/

[dvyukov, 2014-07-22 09:26:59]

LGTM with nit

https://codereview.appspot.com/118050043/diff/40001/src/pkg/runtime/mgc0.c
File src/pkg/runtime/mgc0.c (right):

https://codereview.appspot.com/118050043/diff/40001/src/pkg/runtime/mgc0.c#ne...
src/pkg/runtime/mgc0.c:1257: if(runtime·atomicload(&s->sweepgen) != sg) {
this does not need to be atomic, because it's running during stop-the-world and
nobody must be allocating from heap (otherwise other bad things will happen)

[khr, 2014-07-24 21:37:37]

*** Submitted as https://code.google.com/p/go/source/detail?r=cf71e2f3f8b5 ***

runtime: set sweepgen atomically before updating span state

Sweepone may be running while a new span is allocating.  It
must not see the state updated while the sweepgen is unset.

Fixes issue 8399

LGTM=dvyukov
R=golang-codereviews, dvyukov
CC=golang-codereviews
https://codereview.appspot.com/118050043

[gobot, 2014-07-24 22:17:27]

This CL appears to have broken the openbsd-386-rootbsd builder.
See http://build.golang.org/log/98a02959ba2807e1e90d010fc1b8e36681544baa