[wasm] Fix possible gc-ing of wasm import code objects

[wasm] Fix possible gc-ing of wasm code objects

If incremental GC starts before imports linking, and sees a wasm
function, it won't revisit that after the imports that function are linked.
As a result, the import code objects may be GC-ed. This change
addresses this issue.

BUG=

Committed: https://crrev.com/462d57ae1faa422cad010f2a74b2a7ed94911edf
Cr-Commit-Position: refs/heads/master@{#37507}

[Mircea Trofin, 2016-07-02 01:39:10]

Description was changed from

==========
[wasm] Hang imports off the instance object.

BUG=
==========

to

==========
[wasm] Fix possible gc-ing of wasm code objects 

If incremental GC starts before imports linking, and sees a wasm 
function, it won't revisit that after the imports that function are linked. 
As a result, the import code objects may be GC-ed. This change 
addresses this issue.

BUG=
==========

[Mircea Trofin, 2016-07-02 01:39:10]

mtrofin@chromium.org changed reviewers:
+ mlippautz@chromium.org

[Mircea Trofin, 2016-07-02 01:53:52]

PTAL - and thanks for the help with this one. Your explanations were 
very helpful to my understanding what the issue was!

[Michael Lippautz, 2016-07-04 07:54:11]

Thanks, the write barrier is the proper fix here! 

However, use UPDATE_WRITE_BARRIER as the WB itself will perform all the
necessary checks that need to be done. Having another guaranteed reference does
not make the barrier obsolete. Essentially PS2 is the right fix.

[Mircea Trofin, 2016-07-04 15:28:44]

On 2016/07/04 07:54:11, Michael Lippautz wrote:
> Thanks, the write barrier is the proper fix here! 
> 
> However, use UPDATE_WRITE_BARRIER as the WB itself will perform all the
> necessary checks that need to be done. Having another guaranteed reference
does
> not make the barrier obsolete. Essentially PS2 is the right fix.

Oh, you mean drop ps#3?

[Michael Lippautz, 2016-07-04 16:55:40]

On 2016/07/04 15:28:44, Mircea Trofin wrote:
> On 2016/07/04 07:54:11, Michael Lippautz wrote:
> > Thanks, the write barrier is the proper fix here! 
> > 
> > However, use UPDATE_WRITE_BARRIER as the WB itself will perform all the
> > necessary checks that need to be done. Having another guaranteed reference
> does
> > not make the barrier obsolete. Essentially PS2 is the right fix.
> 
> Oh, you mean drop ps#3?

Yep, exactly.

[Mircea Trofin, 2016-07-04 17:54:07]

Patchset #3 (id:40001) has been deleted

[Mircea Trofin, 2016-07-04 17:54:39]

On 2016/07/04 16:55:40, Michael Lippautz wrote:
> On 2016/07/04 15:28:44, Mircea Trofin wrote:
> > On 2016/07/04 07:54:11, Michael Lippautz wrote:
> > > Thanks, the write barrier is the proper fix here! 
> > > 
> > > However, use UPDATE_WRITE_BARRIER as the WB itself will perform all the
> > > necessary checks that need to be done. Having another guaranteed reference
> > does
> > > not make the barrier obsolete. Essentially PS2 is the right fix.
> > 
> > Oh, you mean drop ps#3?
> 
> Yep, exactly.

Ptal

[Michael Lippautz, 2016-07-04 18:24:42]

Thanks for persisting! LGTM

[Mircea Trofin, 2016-07-04 19:47:29]

The CQ bit was checked by mtrofin@chromium.org

[commit-bot: I haz the power, 2016-07-04 19:47:37]

CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2016-07-04 20:09:01]

Description was changed from

==========
[wasm] Fix possible gc-ing of wasm code objects 

If incremental GC starts before imports linking, and sees a wasm 
function, it won't revisit that after the imports that function are linked. 
As a result, the import code objects may be GC-ed. This change 
addresses this issue.

BUG=
==========

to

==========
[wasm] Fix possible gc-ing of wasm code objects 

If incremental GC starts before imports linking, and sees a wasm 
function, it won't revisit that after the imports that function are linked. 
As a result, the import code objects may be GC-ed. This change 
addresses this issue.

BUG=
==========

[commit-bot: I haz the power, 2016-07-04 20:09:02]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2016-07-04 20:11:22]

Description was changed from

==========
[wasm] Fix possible gc-ing of wasm code objects 

If incremental GC starts before imports linking, and sees a wasm 
function, it won't revisit that after the imports that function are linked. 
As a result, the import code objects may be GC-ed. This change 
addresses this issue.

BUG=
==========

to

==========
[wasm] Fix possible gc-ing of wasm code objects

If incremental GC starts before imports linking, and sees a wasm
function, it won't revisit that after the imports that function are linked.
As a result, the import code objects may be GC-ed. This change
addresses this issue.

BUG=

Committed: https://crrev.com/462d57ae1faa422cad010f2a74b2a7ed94911edf
Cr-Commit-Position: refs/heads/master@{#37507}
==========

[commit-bot: I haz the power, 2016-07-04 20:11:22]

Patchset 2 (id:??) landed as
https://crrev.com/462d57ae1faa422cad010f2a74b2a7ed94911edf
Cr-Commit-Position: refs/heads/master@{#37507}