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

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Message-ID: <20070811042943.GA13410@linux.vnet.ibm.com>
Date:	Fri, 10 Aug 2007 21:29:43 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	Chris Snook <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

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.

						Thanx, Paul
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