Use atomic operation to read the length of a fixed array.

Use atomic operation to read the length of a fixed array.

This fixes a race where
- mutator changes the fixed array length by trimming it,
- sweeper thread reads the length of the fixed array.

Also rename FROM_GC and FROM_MUTATOR to be more precise.

BUG=chromium:462908
LOG=NO

Committed: https://crrev.com/aae2c01740364454b807613b8d9d66a33ee45073
Cr-Commit-Position: refs/heads/master@{#27869}

[ulan, 2015-03-26 13:46:05]

ulan@chromium.org changed reviewers:
+ hpayer@chromium.org

[ulan, 2015-03-26 13:46:06]

PTAL

[Erik Corry, 2015-03-26 14:39:29]

erik.corry@gmail.com changed reviewers:
+ erik.corry@gmail.com

[Erik Corry, 2015-03-26 14:39:30]

The description looks wrong. These are synchronized operations, not atomic ones.

[ulan, 2015-03-26 15:00:11]

On 2015/03/26 14:39:30, Erik Corry wrote:
> The description looks wrong. These are synchronized operations, not atomic
ones.

synchronized_length performs base::Acquire_Load, which is an atomic load.

[Hannes Payer (out of office), 2015-03-26 16:27:24]

lgtm, one nit

https://codereview.chromium.org/1034163002/diff/1/src/heap/heap.h
File src/heap/heap.h (right):

https://codereview.chromium.org/1034163002/diff/1/src/heap/heap.h#newcode748
src/heap/heap.h:748: enum InvocationMode { SWEEPING_IS_OFF, SWEEPING_STARTED };
Can we call it SEQUENTIAL_TO_SWEEPER and CONCURRENT_TO_SWEEPER.

[ulan, 2015-04-16 08:00:20]

The CQ bit was checked by ulan@chromium.org

[ulan, 2015-04-16 08:00:20]

The patchset sent to the CQ was uploaded after l-g-t-m from hpayer@chromium.org
Link to the patchset: https://codereview.chromium.org/1034163002/#ps40001
(title: "Fix compile error")

[commit-bot: I haz the power, 2015-04-16 08:00:25]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1034163002/40001

[commit-bot: I haz the power, 2015-04-16 08:39:24]

Committed patchset #3 (id:40001)

[commit-bot: I haz the power, 2015-04-16 08:39:33]

Patchset 3 (id:??) landed as
https://crrev.com/aae2c01740364454b807613b8d9d66a33ee45073
Cr-Commit-Position: refs/heads/master@{#27869}