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@xxxxxxx>
Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
Documentation/memory-barriers.txt | 12 +++++++++++-
include/asm-alpha/barrier.h | 2 +-
include/asm-frv/system.h | 2 +-
3 files changed, 13 insertions(+), 3 deletions(-)
Index: linux-2.6/include/asm-alpha/barrier.h
===================================================================
--- linux-2.6.orig/include/asm-alpha/barrier.h
+++ linux-2.6/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) \
Index: linux-2.6/include/asm-frv/system.h
===================================================================
--- linux-2.6.orig/include/asm-frv/system.h
+++ linux-2.6/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()
Index: linux-2.6/Documentation/memory-barriers.txt
===================================================================
--- linux-2.6.orig/Documentation/memory-barriers.txt
+++ linux-2.6/Documentation/memory-barriers.txt
@@ -994,7 +994,17 @@ The Linux kernel has eight basic CPU mem
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,
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