path: root/Documentation
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authorNick Piggin <npiggin@suse.de>2008-05-14 06:35:11 +0200
committerLinus Torvalds <torvalds@linux-foundation.org>2008-05-14 10:05:18 -0700
commit73f10281ea96d7e8b4fc1c5d755a7c8eb484155b (patch)
treed75ab7ba0f571ce0c08d30a04f1da7c781b02092 /Documentation
parent4ef7e3e90f56c74b2a17e12d49ed35c3767d66c2 (diff)
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>
Diffstat (limited to 'Documentation')
1 files changed, 11 insertions, 1 deletions
diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index e5a819a4f0c..f5b7127f54a 100644
--- 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,