Protect AudioScheduledSoureNode::m_startTime and m_endTime.

Source/modules/webaudio/AudioScheduledSourceNode.h

 /*
  * Copyright (C) 2012 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
@@ skipping 85 matching lines @@
 
     // Called when we have no more sound to play or the stop() time has been reached. No onEnded
     // event is called.
     virtual void finishWithoutOnEnded();
 
     // Like finishWithoutOnEnded(), but an onEnded (if specified) is called.
     virtual void finish();
 
     void notifyEnded();
 
+    // Both |m_startTime| and |m_endTime| are accessed from both the main thread and the audio
+    // thread.  Use the setters and getters to protect the access to these!
+
+    double startTime() const
+    {
+        return acquireLoad(&m_startTime);

[tkent, 2015/08/07 01:10:03]

I'm not sure acquireLoad and releaseStore are enough for 64 bit value on 32 bit
platforms.

[Raymond Toy, 2015/08/07 15:45:00]

Hmm. I perhaps erroneously assumed floating-point reads/writes would be atomic
64-bit operations.  Do you have an alternative?  If this is not enough, I guess
a more coarse-grained approach will be needed. 

Atomics.h has support for long long, which should be 64 bits.  If that works,
floats and doubles should also work, right?

[tkent, 2015/08/11 00:45:19]

Alternative would be a normal mutex, or a spin lock.
It's possible that they block an audio rendering thread, but the block time must
be very short. 

+    }
+
+    void setStartTime(double newStartTime)
+    {
+        releaseStore(&m_startTime, newStartTime);
+    }
+
+    double endTime() const
+    {
+        return acquireLoad(&m_endTime);
+    }
+
+    void setEndTime(double newEndTime)
+    {
+        releaseStore(&m_endTime, newEndTime);
+    }
+
+    bool m_hasEndedListener;
+
+    static const double UnknownTime;
+private:
     // m_startTime is the time to start playing based on the context's timeline (0 or a time less than the context's current time means "now").
     double m_startTime; // in seconds
 
     // m_endTime is the time to stop playing based on the context's timeline (0 or a time less than the context's current time means "now").
     // If it hasn't been set explicitly, then the sound will not stop playing (if looping) or will stop when the end of the AudioBuffer
     // has been reached.
     double m_endTime; // in seconds
 
-    bool m_hasEndedListener;
-
-    static const double UnknownTime;
-private:
     // This is accessed by both the main thread and audio thread.  Use the setter and getter to
     // protect the access to this!
     int m_playbackState;
 };
 
 class AudioScheduledSourceNode : public AudioSourceNode {
 public:
     void start(ExceptionState&);
     void start(double when, ExceptionState&);
     void stop(ExceptionState&);
     void stop(double when, ExceptionState&);
 
     EventListener* onended();
     void setOnended(PassRefPtrWillBeRawPtr<EventListener>);
 
 protected:
     explicit AudioScheduledSourceNode(AbstractAudioContext&);
     AudioScheduledSourceHandler& audioScheduledSourceHandler() const;
 };
 
 } // namespace blink
 
 #endif // AudioScheduledSourceNode_h