[patch] powerpc: rmp_wmb lwsync optimisation

[Nick Piggin]

smp_rmb can be lwsync if possible. Clarify the comment.

Signed-off-by: Nick Piggin <npiggin at suse.de>
---
Index: linux-2.6/arch/powerpc/include/asm/system.h
===================================================================
--- linux-2.6.orig/arch/powerpc/include/asm/system.h	2008-11-01 23:56:39.000000000 +1100
+++ linux-2.6/arch/powerpc/include/asm/system.h	2008-11-02 00:02:46.000000000 +1100
@@ -23,15 +23,17 @@
  * read_barrier_depends() prevents data-dependent loads being reordered
  *	across this point (nop on PPC).
  *
- * We have to use the sync instructions for mb(), since lwsync doesn't
- * order loads with respect to previous stores.  Lwsync is fine for
- * rmb(), though. Note that rmb() actually uses a sync on 32-bit
- * architectures.
+ * *mb() variants without smp_ prefix must order all types of memory
+ * operations with one another. sync is the only instruction sufficient
+ * to do this.
  *
- * For wmb(), we use sync since wmb is used in drivers to order
- * stores to system memory with respect to writes to the device.
- * However, smp_wmb() can be a lighter-weight lwsync or eieio barrier
- * on SMP since it is only used to order updates to system memory.
+ * For the smp_ barriers, ordering is for cacheable memory operations
+ * only. We have to use the sync instruction for smp_mb(), since lwsync
+ * doesn't order loads with respect to previous stores.  Lwsync can be
+ * used for smp_rmb() and smp_wmb().
+ *
+ * However, on 32-bit, lwsync is actually just a sync, in which case smp_wmb()
+ * can be a lighter-weight eieio barrier.
  */
 #define mb()   __asm__ __volatile__ ("sync" : : : "memory")
 #define rmb()  __asm__ __volatile__ ("sync" : : : "memory")
@@ -51,7 +53,7 @@
 #endif
 
 #define smp_mb()	mb()
-#define smp_rmb()	rmb()
+#define smp_rmb()	__asm__ __volatile__(LWSYNC_ON_SMP : : : "memory")
 #define smp_wmb()	__asm__ __volatile__ (__stringify(SMPWMB) : : :"memory")
 #define smp_read_barrier_depends()	read_barrier_depends()
 #else