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read_barrier_depends arch fixlets

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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>
This commit is contained in:
Nick Piggin 2008-05-14 06:35:11 +02:00 committed by Linus Torvalds
parent 4ef7e3e90f
commit 73f10281ea
3 changed files with 13 additions and 3 deletions
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12
Documentation/memory-barriers.txt
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@ -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,

2
include/asm-alpha/barrier.h
View file

@ -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) \

2
include/asm-frv/system.h
View file

@ -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()