diff --git a/services/audioflinger/AudioFlinger.cpp b/services/audioflinger/AudioFlinger.cpp
index 6256951..93e3963 100644
--- a/services/audioflinger/AudioFlinger.cpp
+++ b/services/audioflinger/AudioFlinger.cpp
@@ -3229,6 +3229,7 @@
 
 void AudioFlinger::DuplicatingThread::addOutputTrack(MixerThread *thread)
 {
+    Mutex::Autolock _l(mLock);
     // FIXME explain this formula
     int frameCount = (3 * mFrameCount * mSampleRate) / thread->sampleRate();
     OutputTrack *outputTrack = new OutputTrack(thread,