From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH] make atomic_t volatile on all architectures Date: Wed, 8 Aug 2007 20:47:20 -0700 Message-ID: <20070809034720.GA12996@linux.vnet.ibm.com> References: <20070808230733.GA17270@shell.boston.redhat.com> <20070808.184824.133910636.davem@davemloft.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: herbert@gondor.apana.org.au, csnook@redhat.com, akpm@linux-foundation.org, torvalds@linux-foundation.org, ak@suse.de, heiko.carstens@de.ibm.com, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org To: David Miller Return-path: Received: from e6.ny.us.ibm.com ([32.97.182.146]:35746 "EHLO e6.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754477AbXHIDrY (ORCPT ); Wed, 8 Aug 2007 23:47:24 -0400 Content-Disposition: inline In-Reply-To: <20070808.184824.133910636.davem@davemloft.net> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Wed, Aug 08, 2007 at 06:48:24PM -0700, David Miller wrote: > From: Herbert Xu > 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