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| author |   Anton Vorontsov <cbouatmailru@gmail.com> | 2008-07-30 02:05:23 +0400 |
|---|---|---|
| committer | Anton Vorontsov <cbouatmailru@gmail.com> | 2008-07-30 02:05:23 +0400 |
| commit | 9fec6060d9e48ed7db0dac0e16d0f0f0e615b7f6 (patch) | |
| tree | 74b41f31a08f6500ff3dfcf64ba21e2d9a8e87e5 /Documentation/memory-barriers.txt | |
| parent | power_supply: Sharp SL-6000 (tosa) batteries support (diff) | |
| parent | Linux 2.6.27-rc1 (diff) | |
| download | linux-9fec6060d9e48ed7db0dac0e16d0f0f0e615b7f6.tar.xz linux-9fec6060d9e48ed7db0dac0e16d0f0f0e615b7f6.zip | |
Merge branch 'master' of /home/cbou/linux-2.6
Conflicts:
drivers/power/Kconfig
drivers/power/Makefile
Diffstat (limited to 'Documentation/memory-barriers.txt')
| -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, |