linux-kernel - Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail

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Date:	Fri, 27 Feb 2015 13:44:19 -0500
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Catalin Marinas <catalin.marinas@....com>
Cc:	Will Deacon <Will.Deacon@....com>,
	Steve Capper <steve.capper@...aro.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] ARM64: cmpxchg.h: Clear the exclusive access bit on fail

On Fri, Feb 27, 2015 at 1:33 PM, Catalin Marinas
<catalin.marinas@....com> wrote:
> It's either badly formatted or I don't get it. Are the "stxr x1" and
> "stxr x7" happening on the same CPU (P0)? If yes, that's badly written
> code, not even architecturally compliant (you are not allowed other
> memory accesses between ldxr and stxr).

OK. Is that the same case with ldaxr (acquire) and stlxr (release)?
AFAIK, memory accesses between acquire and release exclusive
operations are allowed.

>
>>     The last store exclusive succeeds since the exclusive bit is set which
>> should not happen. Clearing the exclusive bit before returning from cmpxchg
>> prevents this happening.
>>
>> Now I am not sure how likely this will happen. One can argue that a cmpxchg()
>> will not happen between an external ldxr/stxr. But isn't clearing the exclusive
>> bit better?
>
> The only way cmpxchg() could happen between a different ldxr/stxr is
> during an interrupt. But ERET automatically clears the exclusive
> monitor, so the "stxr x7" would not succeed.

That makes sense. But please consider the ldaxr/stlxr case and let me know.

Thanks!
-- 
Pranith
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