diff --git a/services/surfaceflinger/DispSync.cpp b/services/surfaceflinger/DispSync.cpp
index fd46822..2ee7570 100644
--- a/services/surfaceflinger/DispSync.cpp
+++ b/services/surfaceflinger/DispSync.cpp
@@ -509,6 +509,7 @@
 }
 
 nsecs_t DispSync::computeNextRefresh(int periodOffset) const {
+    Mutex::Autolock lock(mMutex);
     nsecs_t now = systemTime(SYSTEM_TIME_MONOTONIC);
     return (((now - mPhase) / mPeriod) + periodOffset + 1) * mPeriod + mPhase;
 }