linux-kernel - Re: [PATCH 6/24] make atomic_read() behave consistently on frv

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Message-Id: <E1IKSHQ-0007hg-00@gondolin.me.apana.org.au>
Date:	Mon, 13 Aug 2007 13:15:52 +0800
From:	Herbert Xu <herbert@...dor.apana.org.au>
To:	paulmck@...ux.vnet.ibm.com
Cc:	herbert@...dor.apana.org.au, csnook@...hat.com,
	dhowells@...hat.com, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, torvalds@...ux-foundation.org,
	netdev@...r.kernel.org, akpm@...ux-foundation.org, ak@...e.de,
	heiko.carstens@...ibm.com, davem@...emloft.net,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org,
	rpjday@...dspring.com, jesper.juhl@...il.com
Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv

Paul E. McKenney <paulmck@...ux.vnet.ibm.com> wrote:
> On Sat, Aug 11, 2007 at 08:54:46AM +0800, Herbert Xu wrote:
>> Chris Snook <csnook@...hat.com> wrote:
>> > 
>> > cpu_relax() contains a barrier, so it should do the right thing.  For 
>> > non-smp architectures, I'm concerned about interacting with interrupt 
>> > handlers.  Some drivers do use atomic_* operations.
>> 
>> What problems with interrupt handlers? Access to int/long must
>> be atomic or we're in big trouble anyway.
> 
> Reordering due to compiler optimizations.  CPU reordering does not
> affect interactions with interrupt handlers on a given CPU, but
> reordering due to compiler code-movement optimization does.  Since
> volatile can in some cases suppress code-movement optimizations,
> it can affect interactions with interrupt handlers.

If such reordering matters, then you should use one of the
*mb macros or barrier() rather than relying on possibly
hidden volatile cast.

Cheers,
-- 
Visit Openswan at http://www.openswan.org/
Email: Herbert Xu ~{PmV>HI~} <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
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