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authorIngo Molnar <mingo@elte.hu>2008-06-16 11:28:04 +0200
committerIngo Molnar <mingo@elte.hu>2008-06-16 11:28:04 +0200
commit7aaaec38fcd9ef3172e69f8c19f20113830a8498 (patch)
treeb12a1c359ad53ae10601f77b3438bb27c3c8f337 /Documentation/memory-barriers.txt
parent1ac97018169c5a13feaa90d9671f2d6ba2d9e86e (diff)
parent066519068ad2fbe98c7f45552b1f592903a9c8c8 (diff)
Merge branch 'linus' into x86/kconfigtip-x86-kconfig-2008-06-16_09.28_Mon
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diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
index e5a819a4f0c9..f5b7127f54ac 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,