linux-kernel - Re: [tip:locking/core] arch,s390: Convert smp_mb_

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Message-ID: <20140423093757.2c07c209@mschwide>
Date:	Wed, 23 Apr 2014 09:37:57 +0200
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	mingo@...nel.org, hpa@...or.com, linux-kernel@...r.kernel.org,
	torvalds@...ux-foundation.org, will.deacon@....com,
	peterz@...radead.org, schwidefsky@...ibm.com,
	paulmck@...ux.vnet.ibm.com, gang.chen@...anux.com,
	heiko.carstens@...ibm.com, tglx@...utronix.de
Cc:	tipbot@...or.com, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:locking/core] arch,s390: Convert smp_mb__*()

Hi Peter,

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