Fix a logical race when adding an awakable to a wait set.

Fix a logical race when adding an awakable to a wait set.

If the wait set is being woken on one thread and have an awakable added
on another (i.e. during MojoWait on the wait set), it is possible for
the awakable to be added after checking the signals state, but before
adding to the awakables list. In this case, the added awakable won't get
woken up. The fix is to ensure that checking the signals state and
adding to the awakables list is an atomic operation.

Committed: https://crrev.com/e388b5966b6a02215e744269b7add8e7e36a6a0c
Cr-Commit-Position: refs/heads/master@{#376089}

[Anand Mistry (off Chromium), 2016-02-18 01:50:07]

amistry@chromium.org changed reviewers:
+ rockot@chromium.org

[Ken Rockot(use gerrit already), 2016-02-18 03:35:24]

lgtm

[Anand Mistry (off Chromium), 2016-02-18 03:41:15]

The CQ bit was checked by amistry@chromium.org

[commit-bot: I haz the power, 2016-02-18 03:42:14]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1708953002/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1708953002/1

[commit-bot: I haz the power, 2016-02-18 04:18:43]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2016-02-18 04:20:53]

Description was changed from

==========
Fix a logical race when adding an awakable to a wait set.

If the wait set is being woken on one thread and have an awakable added
on another (i.e. during MojoWait on the wait set), it is possible for
the awakable to be added after checking the signals state, but before
adding to the awakables list. In this case, the added awakable won't get
woken up. The fix is to ensure that checking the signals state and
adding to the awakables list is an atomic operation.
==========

to

==========
Fix a logical race when adding an awakable to a wait set.

If the wait set is being woken on one thread and have an awakable added
on another (i.e. during MojoWait on the wait set), it is possible for
the awakable to be added after checking the signals state, but before
adding to the awakables list. In this case, the added awakable won't get
woken up. The fix is to ensure that checking the signals state and
adding to the awakables list is an atomic operation.

Committed: https://crrev.com/e388b5966b6a02215e744269b7add8e7e36a6a0c
Cr-Commit-Position: refs/heads/master@{#376089}
==========

[commit-bot: I haz the power, 2016-02-18 04:20:54]

Patchset 1 (id:??) landed as
https://crrev.com/e388b5966b6a02215e744269b7add8e7e36a6a0c
Cr-Commit-Position: refs/heads/master@{#376089}