ParallelHelperPool::runFunctionInParallel() shouldn't allocate, and ParallelHelperPool.h shouldn't be included everywhere

• Component: JavaScriptCore
• Status: RESOLVED INVALID

[Filip Pizlo]

Patch forthcoming.

[Filip Pizlo]

Created attachment 262084 [details]
the patch

[Saam Barati]

Comment on attachment 262084 [details]
the patch

r=me

[Filip Pizlo]

Landed in http://trac.webkit.org/changeset/190324

[Alexey Proskuryakov]

I suspect that this caused flaky crashes, rdar://problem/22916304

[WebKit Commit Bot]

Re-opened since this is blocked by bug 149671

[Filip Pizlo]

(In reply to comment #4)
> I suspect that this caused flaky crashes, rdar://problem/22916304

Yeah, this needs to be rolled out, and I believe that I understand now why the patch is wrong.

The observation behind this patch is that the task cannot run again after runFunctionInParallel returns.  That's true.  But the way that ParallelHelperPool::helperThreadBody works, it will notify runFunctionInParallel that it's no longer going to run the task *before* it derefs the task.  The sequence is:

1) get a task and ref it.
2) run the task.
3) notify task complection.
     ---> at this point, runFunctionInParallel can return.
4) deref the task.
     ---> at this point, the helper thread body will crash if runFunctionInParallel returned.

We could still find other ways to avoid allocating the task.  But I think that it's not worth it if it will be complicated.  The easiest mental model is to treat the task as a proper ref-counted object, in which case a stray RefPtr referencing the task after it cannot run anymore is fine.  If we wanted to allow stack-allocation of the task, then we'd have to change our mental model to: it's illegal to have a RefPtr to the task after we have consensus that the task cannot run anymore.  I don't want to have to think that hard about this code.

[Filip Pizlo]

See https://bugs.webkit.org/show_bug.cgi?id=149635#c6.