Re: [tip:locking/core] arch,s390: Convert smp_mb__*()

[Martin Schwidefsky]

Hi Peter,

On Fri, 18 Apr 2014 06:11:38 -0700
tip-bot for Peter Zijlstra <tipbot@xxxxxxxxx> wrote:

> arch,s390: Convert smp_mb__*()
> 
> As per the existing implementation; implement the new one using
> smp_mb().
> 
> AFAICT the s390 compare-and-swap does imply a barrier, however there
> are some immediate ops that seem to be singly-copy atomic and do not
> imply a barrier. One such is the "ni" op (which would be
> and-immediate) which is used for the constant clear_bit
> implementation. Therefore s390 needs full barriers for the
> {before,after} atomic ops.

Good catch, if ni/oi are used this is required. What we want to
add is an #ifdef CONFIG_HAVE_MARCH_Z196_FEATURES. With smp_mb__*
defined as smp_mb() we get additional barriers for older machines
while the atomic ops are defined with compare-and-swap which
already does the barrier before and after the operation.

We can do that with a patch on top of this one, so no hurry.

-- 
blue skies,
   Martin.

"Reality continues to ruin my life." - Calvin.

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