mips/atomic: Fix cmpxchg64 barriers

There were no memory barriers on the 32bit implementation of
cmpxchg64(). Fix this.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>

arch/mips/include/asm/cmpxchg.h

@@ -290,10 +290,13 @@ static inline unsigned long __cmpxchg64(volatile void *ptr,
 	 * will cause a build error unless cpu_has_64bits is a		\
 	 * compile-time constant 1.					\
 	 */								\
-	if (cpu_has_64bits && kernel_uses_llsc)				\
+	if (cpu_has_64bits && kernel_uses_llsc) {			\
+		smp_mb__before_llsc();					\
 		__res = __cmpxchg64((ptr), __old, __new);		\
-	else								\
+		smp_llsc_mb();						\
+	} else {							\
 		__res = __cmpxchg64_unsupported();			\
+	}								\
 									\
 	__res;								\
 })