diff --git a/media/libstagefright/foundation/MediaBufferGroup.cpp b/media/libstagefright/foundation/MediaBufferGroup.cpp
index 8e4d064..cb62d92 100644
--- a/media/libstagefright/foundation/MediaBufferGroup.cpp
+++ b/media/libstagefright/foundation/MediaBufferGroup.cpp
@@ -199,6 +199,7 @@
 }
 
 void MediaBufferGroup::signalBufferReturned(MediaBuffer *) {
+    Mutex::Autolock autoLock(mLock);
     mCondition.signal();
 }