Add a lock around getting and setting of source_ to avoid possible compiler reorderings.

media/audio/mac/audio_output_mac.h

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef MEDIA_AUDIO_MAC_AUDIO_OUTPUT_MAC_H_
 #define MEDIA_AUDIO_MAC_AUDIO_OUTPUT_MAC_H_
 
 #include <AudioToolbox/AudioFormat.h>
 #include <AudioToolbox/AudioQueue.h>
 #include <AudioUnit/AudioUnit.h>
 
+#include "base/synchronization/lock.h"
 #include "media/audio/audio_io.h"
 #include "media/audio/audio_parameters.h"
 
 class AudioManagerMac;
 
 // Implementation of AudioOuputStream for Mac OS X using the audio queue service
 // present in OS 10.5 and later. Audioqueue is the successor to the SoundManager
 // services but it is supported in 64 bits.
 class PCMQueueOutAudioOutputStream : public AudioOutputStream {
  public:
@@ skipping 23 matching lines @@
   void SwizzleLayout(Format* b, uint32 filled);
   // Check and move channels if surround sound layout needs adjusted.
   bool CheckForAdjustedLayout(Channels input_channel, Channels output_channel);
 
   // The OS calls back here when an audio buffer has been processed.
   static void RenderCallback(void* p_this, AudioQueueRef queue,
                              AudioQueueBufferRef buffer);
   // Called when an error occurs.
   void HandleError(OSStatus err);
 
+  // Atomic operations for setting/getting the source callback.
+  void SetSource(AudioSourceCallback* source);
+  AudioSourceCallback* GetSource();
+
   // Structure that holds the stream format details such as bitrate.
   AudioStreamBasicDescription format_;
   // Handle to the OS audio queue object.
   AudioQueueRef audio_queue_;
   // Array of pointers to the OS managed audio buffers.
   AudioQueueBufferRef buffer_[kNumBuffers];
+  // Mutex for the |source_| to implment atomic set and get.
+  // It is important to NOT wait on any other locks while this is held.
+  base::Lock source_lock_;
   // Pointer to the object that will provide the audio samples.
   AudioSourceCallback* source_;
   // Our creator, the audio manager needs to be notified when we close.
   AudioManagerMac* manager_;
   // Packet size in bytes.
   uint32 packet_size_;
   // Number of bytes for making a silence buffer.
   int silence_bytes_;
   // Volume level from 0 to 1.
   float volume_;
@@ skipping 11 matching lines @@
   int num_core_channels_;
   // A flag to determine if swizzle is needed from source to device layouts.
   bool should_swizzle_;
   // A flag to determine if downmix is needed from source to device layouts.
   bool should_down_mix_;
 
   DISALLOW_COPY_AND_ASSIGN(PCMQueueOutAudioOutputStream);
 };
 
 #endif  // MEDIA_AUDIO_MAC_AUDIO_OUTPUT_MAC_H_