Make x86 UP "set_mb()" use a lighter barrier than doing a full

locked "xchg". It only needs a compiler barrier on UP.

include/asm-i386/system.h

@@ -296,13 +296,14 @@ static inline unsigned long __cmpxchg(volatile void *ptr, unsigned long old,
 #define smp_mb()	mb()
 #define smp_rmb()	rmb()
 #define smp_wmb()	wmb()
+#define set_mb(var, value) do { xchg(&var, value); } while (0)
 #else
 #define smp_mb()	barrier()
 #define smp_rmb()	barrier()
 #define smp_wmb()	barrier()
+#define set_mb(var, value) do { var = value; barrier(); } while (0)
 #endif
 
-#define set_mb(var, value) do { xchg(&var, value); } while (0)
 #define set_wmb(var, value) do { var = value; wmb(); } while (0)
 
 /* interrupt control.. */