Add a mutex lock to AudioPowerMonitor::Reset() to placate TSAN.

Add a mutex lock to AudioPowerMonitor::Reset() to placate TSAN.

Since Reset() is not a high-performance code path, it's acceptable to
add the extra mutex lock while resetting all the member variables.  By
design, it was not actually harmful that ReadCurrentPowerAndClip() was
making a racey read.

BUG=418290
Committed: https://crrev.com/a63a57f288c10e746c5623b86446a663eb5d683d
Cr-Commit-Position: refs/heads/master@{#297545}

[miu, 2014-09-30 20:09:13]

miu@chromium.org changed reviewers:
+ dalecurtis@chromium.org

[miu, 2014-09-30 20:09:14]

One liner to fix a TSAN race bug.  PTAL.

[DaleCurtis, 2014-09-30 20:10:50]

https://codereview.chromium.org/611373003/diff/1/media/audio/audio_power_moni...
File media/audio/audio_power_monitor.cc (right):

https://codereview.chromium.org/611373003/diff/1/media/audio/audio_power_moni...
media/audio/audio_power_monitor.cc:57: average_power_ = std::max(0.0f,
std::min(1.0f, sum_power / num_channels));
average power is set outside of the lock, so it won't help.

[miu, 2014-09-30 21:14:22]

https://codereview.chromium.org/611373003/diff/1/media/audio/audio_power_moni...
File media/audio/audio_power_monitor.cc (right):

https://codereview.chromium.org/611373003/diff/1/media/audio/audio_power_moni...
media/audio/audio_power_monitor.cc:57: average_power_ = std::max(0.0f,
std::min(1.0f, sum_power / num_channels));
On 2014/09/30 20:10:50, DaleCurtis wrote:
> average power is set outside of the lock, so it won't help.

The root cause of the bug is a read/write race between ReadCurrentPowerAndClip()
and Reset(), and not between Scan() and Reset().  (Also, the header file
documentation explicitly states that Reset() should not be called while
Scan()ning is running).

Good point, though.  I've updated the code to reflect this.

[DaleCurtis, 2014-09-30 21:22:45]

OIC, my bad, lgtm!

[miu, 2014-09-30 21:57:33]

The CQ bit was checked by miu@chromium.org

[commit-bot: I haz the power, 2014-09-30 21:58:40]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/611373003/20001

[commit-bot: I haz the power, 2014-09-30 23:36:49]

Committed patchset #2 (id:20001) as d391c014f602975a4b45ed30770136792d743dc8

[commit-bot: I haz the power, 2014-09-30 23:38:12]

Patchset 2 (id:??) landed as
https://crrev.com/a63a57f288c10e746c5623b86446a663eb5d683d
Cr-Commit-Position: refs/heads/master@{#297545}