diff --git a/include/utils/threads.h b/include/utils/threads.h
index 41e5766..0bd69cf 100644
--- a/include/utils/threads.h
+++ b/include/utils/threads.h
@@ -526,6 +526,7 @@
     Thread& operator=(const Thread&);
     static  int             _threadLoop(void* user);
     const   bool            mCanCallJava;
+    // always hold mLock when reading or writing
             thread_id_t     mThread;
     mutable Mutex           mLock;
             Condition       mThreadExitedCondition;