Access current frame counter carefully and remove m_cachedSampleFrame.

Source/modules/webaudio/AudioDestinationNode.cpp

 /*
  * Copyright (C) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1.  Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2.  Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 13 matching lines @@
 
 #include "config.h"
 #if ENABLE(WEB_AUDIO)
 #include "modules/webaudio/AudioDestinationNode.h"
 
 #include "modules/webaudio/AudioContext.h"
 #include "modules/webaudio/AudioNodeInput.h"
 #include "modules/webaudio/AudioNodeOutput.h"
 #include "platform/audio/AudioUtilities.h"
 #include "platform/audio/DenormalDisabler.h"
+#include "wtf/Atomics.h"
 
 namespace blink {
 
 AudioDestinationHandler::AudioDestinationHandler(AudioNode& node, float sampleRate)
     : AudioHandler(NodeTypeDestination, node, sampleRate)
     , m_currentSampleFrame(0)
 {
     addInput();
 }
 
@@ skipping 39 matching lines @@
         destinationBus->copyFrom(*renderedBus);
     }
 
     // Process nodes which need a little extra help because they are not connected to anything, but still need to process.
     context()->deferredTaskHandler().processAutomaticPullNodes(numberOfFrames);
 
     // Let the context take care of any business at the end of each render quantum.
     context()->handlePostRenderTasks();
 
     // Advance current sample-frame.
-    m_currentSampleFrame += numberOfFrames;
+    atomicAdd(&m_currentSampleFrame, numberOfFrames);

[tkent, 2015/05/22 00:47:12]

I don't think using atomicAdd() here is correct.
atomicAdd() is implemented by InterlockedExchangeAdd() on Windows, and 
InterlockedExchangeAdd() avoid races with other Interlocked* functions.  So,
accessing m_currentSampleFrame directly from the main thread is not safe.
We have only one audio thread for a single AudioDestinationHandler.  We don't
need to use atomicAdd() here.

Instead, we should do:

AudioDestinationHandler::render():
    size_t sampleFrame = m_currentSampleFrame + numberOfFrames;
    releaseStore(&m_currentSampleFrame, sampleFrame);

AudioDestinationHandler::rcurrentSampleFrame():
    return acquireLoad(&m_currentSampleFrame);

[Raymond Toy, 2015/05/22 16:13:48]

Done.  Thanks for the hint.

 }
 
 // ----------------------------------------------------------------
 
 AudioDestinationNode::AudioDestinationNode(AudioContext& context)
     : AudioNode(context)
 {
 }
 
 AudioDestinationHandler& AudioDestinationNode::audioDestinationHandler() const
 {
     return static_cast<AudioDestinationHandler&>(handler());
 }
 
 unsigned long AudioDestinationNode::maxChannelCount() const
 {
     return audioDestinationHandler().maxChannelCount();
 }
 
 } // namespace blink
 
 #endif // ENABLE(WEB_AUDIO)