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

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Message-ID: <20070813060302.GF13410@linux.vnet.ibm.com>
Date:	Sun, 12 Aug 2007 23:03:02 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	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 Mon, Aug 13, 2007 at 01:15:52PM +0800, Herbert Xu wrote:
> 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.

If communicating among CPUs, sure.  However, when communicating between
mainline and interrupt/NMI handlers on the same CPU, the barrier() and
most expecially the *mb() macros are gross overkill.  So there really
truly is a place for volatile -- not a large place, to be sure, but a
place nonetheless.

						Thanx, Paul
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