linux-kernel - Re: [PATCH v2 7/7] ARM: atomics: define our SMP atomics in terms of

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Message-ID: <20150717093555.GC18994@arm.com>
Date:	Fri, 17 Jul 2015 10:35:55 +0100
From:	Will Deacon <will.deacon@....com>
To:	Waiman Long <waiman.long@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v2 7/7] ARM: atomics: define our SMP atomics in terms of
 _relaxed operations

On Fri, Jul 17, 2015 at 01:00:34AM +0100, Waiman Long wrote:
> On 07/16/2015 05:08 PM, Peter Zijlstra wrote:
> > On Thu, Jul 16, 2015 at 04:40:03PM -0400, Waiman Long wrote:
> >> On 07/16/2015 11:32 AM, Will Deacon wrote:
> >>> @@ -117,6 +115,8 @@ static inline unsigned long __xchg(unsigned long x, volatile void *ptr, int size
> >>>   #error "SMP is not supported on this platform"
> >               ^^^^^^^^^^^^^
> 
> That #error is only for ARMv5 or below.
> 
> >>>   #endif
> >>>
> >>> +#define xchg xchg_relaxed
> >> Is that a typo? I think xchg() needs to be a full memory barrier.
> > Pointless on UP.

I don't see the problem here. As Peter pointed out, this code only gets
looked at if !SMP and structuring it this way means I can have one
definition of xchg_relaxed, regardless of architecture version.

Will
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