Index: src/os_unix.c
==================================================================
--- src/os_unix.c
+++ src/os_unix.c
@@ -4994,16 +4994,20 @@
 */
 static void unixShmBarrier(
   sqlite3_file *fd                /* Database file holding the shared memory */
 ){
   UNUSED_PARAMETER(fd);
+#ifdef SQLITE_MUTEXFREE_SHMLOCK
+  __sync_synchronize();
+#else
   sqlite3MemoryBarrier();         /* compiler-defined memory barrier */
   assert( fd->pMethods->xLock==nolockLock 
        || unixFileMutexNotheld((unixFile*)fd) 
   );
   unixEnterMutex();               /* Also mutex, for redundancy */
   unixLeaveMutex();
+#endif
 }
 
 /*
 ** Close a connection to shared-memory.  Delete the underlying 
 ** storage if deleteFlag is true.