Index: src/mutex_unix.c
==================================================================
--- src/mutex_unix.c
+++ src/mutex_unix.c
@@ -91,10 +91,11 @@
 void sqlite3MemoryBarrier(void){
 #if defined(SQLITE_MEMORY_BARRIER)
   SQLITE_MEMORY_BARRIER;
 #elif defined(__GNUC__) && GCC_VERSION>=4001000
   __sync_synchronize();
+# define sqlite3MemoryBarrier_IS_RELIABLE 1
 #endif
 }
 
 /*
 ** Initialize and deinitialize the mutex subsystem.

Index: src/mutex_w32.c
==================================================================
--- src/mutex_w32.c
+++ src/mutex_w32.c
@@ -85,12 +85,14 @@
 void sqlite3MemoryBarrier(void){
 #if defined(SQLITE_MEMORY_BARRIER)
   SQLITE_MEMORY_BARRIER;
 #elif defined(__GNUC__)
   __sync_synchronize();
+# define sqlite3MemoryBarrier_IS_RELIABLE 1
 #elif MSVC_VERSION>=1300
   _ReadWriteBarrier();
+# define sqlite3MemoryBarrier_IS_RELIABLE 1
 #elif defined(MemoryBarrier)
   MemoryBarrier();
 #endif
 }
 

Index: src/os_unix.c
==================================================================
--- src/os_unix.c
+++ src/os_unix.c
@@ -4891,12 +4891,14 @@
   UNUSED_PARAMETER(fd);
   sqlite3MemoryBarrier();         /* compiler-defined memory barrier */
   assert( fd->pMethods->xLock==nolockLock 
        || unixFileMutexNotheld((unixFile*)fd) 
   );
+#ifndef sqlite3MemoryBarrier_IS_RELIABLE
   unixEnterMutex();               /* Also mutex, for redundancy */
   unixLeaveMutex();
+#endif
 }
 
 /*
 ** Close a connection to shared-memory.  Delete the underlying 
 ** storage if deleteFlag is true.

Index: src/os_win.c
==================================================================
--- src/os_win.c
+++ src/os_win.c
@@ -4172,12 +4172,14 @@
 static void winShmBarrier(
   sqlite3_file *fd          /* Database holding the shared memory */
 ){
   UNUSED_PARAMETER(fd);
   sqlite3MemoryBarrier();   /* compiler-defined memory barrier */
+#ifndef sqlite3MemoryBarrier_IS_RELIABLE
   winShmEnterMutex();       /* Also mutex, for redundancy */
   winShmLeaveMutex();
+#endif
 }
 
 /*
 ** This function is called to obtain a pointer to region iRegion of the
 ** shared-memory associated with the database file fd. Shared-memory regions