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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