linux-kernel - Re: [PATCH] make atomic_t volatile on all architectures

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Message-ID: <20070809034720.GA12996@linux.vnet.ibm.com>
Date:	Wed, 8 Aug 2007 20:47:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	David Miller <davem@...emloft.net>
Cc:	herbert@...dor.apana.org.au, csnook@...hat.com,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	ak@...e.de, heiko.carstens@...ibm.com,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	schwidefsky@...ibm.com, wensong@...ux-vs.org, horms@...ge.net.au,
	wjiang@...ilience.com, cfriesen@...tel.com, zlynx@....org
Subject: Re: [PATCH] make atomic_t volatile on all architectures

On Wed, Aug 08, 2007 at 06:48:24PM -0700, David Miller wrote:
> From: Herbert Xu <herbert@...dor.apana.org.au>
> Date: Thu, 09 Aug 2007 09:03:27 +0800
> 
> > Such loops should always use something like cpu_relax() which comes
> > with a barrier.
> 
> This is an excellent point.
> 
> And it needs to be weighed with the error prone'ness Andrew mentioned.
> There probably is a middle ground somewhere.

OK...  I'll bite.  ACCESS_ONCE(), see http://lkml.org/lkml/2007/7/11/664.

This would allow ACCESS_ONCE(atomic_read(&x)) to be used where refetching
would be problematic, but allow the compiler free rein in cases where
refetching is OK.

The ACCESS_ONCE() primitive of course has its limitations as well, but
you did ask for a middle ground.  ;-)

							Thanx, Paul
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