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>
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index e5a819a..f5b7127 100644
--- a/Documentation/memory-barriers.txt
+++ b/Documentation/memory-barriers.txt
@@ -994,7 +994,17 @@
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,
diff --git a/include/asm-alpha/barrier.h b/include/asm-alpha/barrier.h
index 384dc08..ac78eba 100644
--- a/include/asm-alpha/barrier.h
+++ b/include/asm-alpha/barrier.h
@@ -24,7 +24,7 @@
#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) \
diff --git a/include/asm-frv/system.h b/include/asm-frv/system.h
index cb307f8..d3a12a9 100644
--- a/include/asm-frv/system.h
+++ b/include/asm-frv/system.h
@@ -179,7 +179,7 @@
#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()