diff --git a/media/libmediaplayerservice/MediaPlayerService.cpp b/media/libmediaplayerservice/MediaPlayerService.cpp
index c120898..d461af3 100644
--- a/media/libmediaplayerservice/MediaPlayerService.cpp
+++ b/media/libmediaplayerservice/MediaPlayerService.cpp
@@ -2125,6 +2125,7 @@
         // immutable with respect to future writes.
         //
         // It is thus safe for another thread to read the AudioCache.
+        Mutex::Autolock lock(mLock);
         mCommandComplete = true;
         mSignal.signal();
     }