linux-kernel - Re: [RFC v2 5/7] powerpc: atomic: Implement cmpxchg{,64}_* and atomic{,64}_cmpxchg

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Date:	Thu, 1 Oct 2015 14:36:26 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Boqun Feng <boqun.feng@...il.com>
Cc:	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
	Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Michael Ellerman <mpe@...erman.id.au>,
	Thomas Gleixner <tglx@...utronix.de>,
	Will Deacon <will.deacon@....com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Waiman Long <waiman.long@...com>
Subject: Re: [RFC v2 5/7] powerpc: atomic: Implement cmpxchg{,64}_* and
 atomic{,64}_cmpxchg_* variants

On Thu, Oct 01, 2015 at 02:27:15PM +0200, Peter Zijlstra wrote:
> On Wed, Sep 16, 2015 at 11:49:33PM +0800, Boqun Feng wrote:
> > Unlike other atomic operation variants, cmpxchg{,64}_acquire and
> > atomic{,64}_cmpxchg_acquire don't have acquire semantics if the cmp part
> > fails, so we need to implement these using assembly.
> 
> I think that is actually expected and documented. That is, a cmpxchg
> only implies barriers on success. See:
> 
>   ed2de9f74ecb ("locking/Documentation: Clarify failed cmpxchg() memory ordering semantics")

Also:

654672d4ba1a6 (Will Deacon     2015-08-06 17:54:37 +0100  28)  * store portion of the operation. Note that a failed cmpxchg_acquire
654672d4ba1a6 (Will Deacon     2015-08-06 17:54:37 +0100  29)  * does -not- imply any memory ordering constraints.
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