read_barrier_depends arch fixlets

read_barrie_depends has always been a noop (not a compiler barrier) on all
architectures except SMP alpha. This brings UP alpha and frv into line with all
other architectures, and fixes incorrect documentation.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/memory-barriers.txt

@@ -994,7 +994,17 @@ The Linux kernel has eight basic CPU memory barriers:
 	DATA DEPENDENCY	read_barrier_depends()	smp_read_barrier_depends()
 
 
-All CPU memory barriers unconditionally imply compiler barriers.
+All memory barriers except the data dependency barriers imply a compiler
+barrier. Data dependencies do not impose any additional compiler ordering.
+
+Aside: In the case of data dependencies, the compiler would be expected to
+issue the loads in the correct order (eg. `a[b]` would have to load the value
+of b before loading a[b]), however there is no guarantee in the C specification
+that the compiler may not speculate the value of b (eg. is equal to 1) and load
+a before b (eg. tmp = a[1]; if (b != 1) tmp = a[b]; ). There is also the
+problem of a compiler reloading b after having loaded a[b], thus having a newer
+copy of b than a[b]. A consensus has not yet been reached about these problems,
+however the ACCESS_ONCE macro is a good place to start looking.
 
 SMP memory barriers are reduced to compiler barriers on uniprocessor compiled
 systems because it is assumed that a CPU will appear to be self-consistent,

include/asm-alpha/barrier.h

@@ -24,7 +24,7 @@ __asm__ __volatile__("mb": : :"memory")
 #define smp_mb()	barrier()
 #define smp_rmb()	barrier()
 #define smp_wmb()	barrier()
-#define smp_read_barrier_depends()	barrier()
+#define smp_read_barrier_depends()	do { } while (0)
 #endif
 
 #define set_mb(var, value) \

include/asm-frv/system.h

@@ -179,7 +179,7 @@ do {							\
 #define mb()			asm volatile ("membar" : : :"memory")
 #define rmb()			asm volatile ("membar" : : :"memory")
 #define wmb()			asm volatile ("membar" : : :"memory")
-#define read_barrier_depends()	barrier()
+#define read_barrier_depends()	do { } while (0)
 
 #ifdef CONFIG_SMP
 #define smp_mb()			mb()