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Commit 73f10281 authored by Nick Piggin's avatar Nick Piggin Committed by Linus Torvalds
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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: default avatarNick Piggin <npiggin@suse.de>
Acked-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 4ef7e3e9

Documentation/memory-barriers.txt

+11 −1
Original line number Diff line number Diff line
@@ -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,

include/asm-alpha/barrier.h

+1 −1
Original line number Diff line number Diff line
@@ -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) \

include/asm-frv/system.h

+1 −1
Original line number Diff line number Diff line
@@ -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()