Messages in this thread | | | Date | Wed, 01 Nov 2006 09:16:05 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH] splice : two smp_mb() can be omitted |
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Eric Dumazet wrote:
>On Tuesday 31 October 2006 10:40, Nick Piggin wrote: > > >>Uh, there is nothing that says mutex_unlock or any unlock >>functions contain an implicit smp_mb(). What is given is that the >>lock and unlock obey aquire and release memory ordering, >>respectively. >> >>a = x; >>xxx_unlock >>b = y; >> >>In this situation, the load of y can be executed before that of x. >>And some architectures will even do so (i386 can, because the >>unlock is an unprefixed store; ia64 can, because it uses a release >>barrier in the unlock). >> > >Hum... it seems your mutex_unlock() i386/x86_64 copy is not same as mine :) >
OK, replace xxx with mutex, and what I've said still holds true for ia64.
>Maybe we could document the fact that mutex_{lock|unlock}() has or has not an >implicit smp_mb(). >
It does not, none of the unlock functions ever have.
>If not, delete smp_mb() calls from include/asm-generic/mutex-dec.h >
They should be deleted (and from mutex-xchg). NOT because there is no need for a memory barrier, but because the atomic_alter_value_and_return_something functions always provide a barrier before and after the operation, as per Documentation/atomic_ops.txt
Again, lock / unlock operations require acquire / release consistency. This is a memory ordering operation. It is not equivalent to smp_mb, though.
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