linux-kernel - Re: [PATCH] bitops: Use volatile in generic atomic bitops.

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Date:	Wed, 27 Jul 2011 14:09:38 +0100
From:	Will Newton <will.newton@...il.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Linux Kernel list <linux-kernel@...r.kernel.org>, stable@...nel.org
Subject: Re: [PATCH] bitops: Use volatile in generic atomic bitops.

On Wed, Jul 27, 2011 at 1:55 PM, Arnd Bergmann <arnd@...db.de> wrote:
> On Wednesday 27 July 2011, Will Newton wrote:
>> The generic atomic bitops currently explicitly cast away the
>> volatile from the pointer passed to them. This will allow the
>> access to the bitfield to happen outside of the critical section
>> thus making the bitops no longer interrupt-safe. Remove this cast
>> and add a volatile keyword to make sure all accesses to the
>> bitfield happen inside the critical section.
>>
>> Signed-off-by: Will Newton <will.newton@...tec.com>
>
> Have you observed this behavior? The interrupt disable/enable should
> always come with a barrier that should prevent the bitops from
> leaking out, so I don't see how this causes problems in practice.

Yes, although my arch does not have these barriers. Now I see from
memory-barriers.txt that lock/unlock are required to implement a
compiler barrier, sorry for the noise!
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