Less lock acquisitions for AudioMixer.

webrtc/modules/audio_mixer/audio_mixer_impl.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 114 matching lines @@
                                  const AudioFrameList& audioFrameList) const;
 
   bool LimitMixedAudio(AudioFrame* mixedAudio) const;
 
   // Output level functions for VoEVolumeControl.
   int GetOutputAudioLevel() override;
 
   int GetOutputAudioLevelFullRange() override;
 
   std::unique_ptr<CriticalSectionWrapper> crit_;
-  std::unique_ptr<CriticalSectionWrapper> cb_crit_;

[kwiberg-webrtc, 2016/08/31 09:10:36]

It's troubling that there aren't lock annotations on the member variables that
tell the compiler what the locks are supposed to protect. To prevent mistakes,
it's probably best to start by adding GUARDED_BY annotations everywhere...

(base/thread_checker.h seems to have corresponding annotations for thread
checkers.)

 
   int32_t id_;
 
   // The current sample frequency and sample size when mixing.
   Frequency output_frequency_;
   size_t sample_size_;
 
   // List of all audio sources. Note all lists are disjunct
   MixerAudioSourceList audio_source_list_;  // May be mixed.
 
@@ skipping 12 matching lines @@
 
   // Used for inhibiting saturation in mixing.
   std::unique_ptr<AudioProcessing> limiter_;
 
   // Measures audio level for the combined signal.
   voe::AudioLevel audio_level_;
 };
 }  // namespace webrtc
 
 #endif  // WEBRTC_MODULES_AUDIO_MIXER_AUDIO_MIXER_IMPL_H_