Define mb() as compiler barrier on a uniprocessor system

Currently, the mb() is defined as a DMB operation on ARMv6, even for
UP systems. This patch defines mb() as a compiler barrier only. For
the SMP case, the smp_* variants should be used anyway and the patch
defines them as DMB.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

include/asm-arm/system.h

@@ -167,11 +167,25 @@ extern unsigned int user_debug;
 				    : : "r" (0) : "memory")
 #define dmb() __asm__ __volatile__ ("" : : : "memory")
 #endif
-#define mb() dmb()
-#define rmb() mb()
-#define wmb() mb()
-#define read_barrier_depends() do { } while(0)
-#define set_mb(var, value)  do { var = value; mb(); } while (0)
+
+#define mb()			barrier()
+#define rmb()			barrier()
+#define wmb()			barrier()
+#define read_barrier_depends()	do { } while(0)
+
+#ifdef CONFIG_SMP
+#define smp_mb()		dmb()
+#define smp_rmb()		dmb()
+#define smp_wmb()		dmb()
+#define smp_read_barrier_depends()	read_barrier_depends()
+#else
+#define smp_mb()		barrier()
+#define smp_rmb()		barrier()
+#define smp_wmb()		barrier()
+#define smp_read_barrier_depends()	read_barrier_depends()
+#endif /* CONFIG_SMP */
+
+#define set_mb(var, value)	do { var = value; smp_mb(); } while (0)
 #define nop() __asm__ __volatile__("mov\tr0,r0\t@ nop\n\t");
 
 extern unsigned long cr_no_alignment;	/* defined in entry-armv.S */
@@ -243,22 +257,6 @@ static inline void sched_cacheflush(void)
 {
 }
 
-#ifdef CONFIG_SMP
-
-#define smp_mb()		mb()
-#define smp_rmb()		rmb()
-#define smp_wmb()		wmb()
-#define smp_read_barrier_depends()		read_barrier_depends()
-
-#else
-
-#define smp_mb()		barrier()
-#define smp_rmb()		barrier()
-#define smp_wmb()		barrier()
-#define smp_read_barrier_depends()		do { } while(0)
-
-#endif /* CONFIG_SMP */
-
 #if defined(CONFIG_CPU_SA1100) || defined(CONFIG_CPU_SA110)
 /*
  * On the StrongARM, "swp" is terminally broken since it bypasses the