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| author | Nick Piggin <npiggin@suse.de> | 2008-05-14 06:35:11 +0200 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2008-05-14 10:05:18 -0700 |
| commit | 73f10281ea96d7e8b4fc1c5d755a7c8eb484155b (patch) | |
| tree | d75ab7ba0f571ce0c08d30a04f1da7c781b02092 /Documentation | |
| parent | 4ef7e3e90f56c74b2a17e12d49ed35c3767d66c2 (diff) | |
| download | lwn-73f10281ea96d7e8b4fc1c5d755a7c8eb484155b.tar.gz lwn-73f10281ea96d7e8b4fc1c5d755a7c8eb484155b.zip | |
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')
| -rw-r--r-- | Documentation/memory-barriers.txt | 12 |
1 files changed, 11 insertions, 1 deletions
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, |