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

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Message-ID: <20151001151310.GE4043@linux.vnet.ibm.com>
Date:	Thu, 1 Oct 2015 08:13:10 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Boqun Feng <boqun.feng@...il.com>, 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>,
	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 02:36:26PM +0200, Peter Zijlstra wrote:
> 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.

Agreed, no need for ordering on failed cmpxchg.

							Thanx, Paul

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