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 (file)
--- a/Documentation/memory-barriers.txt
+++ b/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,

diff --git a/include/asm-alpha/barrier.h b/include/asm-alpha/barrier.h
index 384dc08..ac78eba 100644 (file)
--- a/include/asm-alpha/barrier.h
+++ b/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) \

diff --git a/include/asm-frv/system.h b/include/asm-frv/system.h
index cb307f8..d3a12a9 100644 (file)
--- a/include/asm-frv/system.h
+++ b/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()