Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S937393AbXHMGDY (ORCPT ); Mon, 13 Aug 2007 02:03:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754907AbXHMGDK (ORCPT ); Mon, 13 Aug 2007 02:03:10 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:42823 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754712AbXHMGDI (ORCPT ); Mon, 13 Aug 2007 02:03:08 -0400 Date: Sun, 12 Aug 2007 23:03:02 -0700 From: "Paul E. McKenney" To: Herbert Xu Cc: csnook@redhat.com, dhowells@redhat.com, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, torvalds@linux-foundation.org, netdev@vger.kernel.org, akpm@linux-foundation.org, ak@suse.de, heiko.carstens@de.ibm.com, davem@davemloft.net, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au, wjiang@resilience.com, cfriesen@nortel.com, zlynx@acm.org, rpjday@mindspring.com, jesper.juhl@gmail.com Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv Message-ID: <20070813060302.GF13410@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20070811042943.GA13410@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1619 Lines: 34 On Mon, Aug 13, 2007 at 01:15:52PM +0800, Herbert Xu wrote: > Paul E. McKenney wrote: > > On Sat, Aug 11, 2007 at 08:54:46AM +0800, Herbert Xu wrote: > >> Chris Snook 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 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/