From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Snook Subject: Re: [PATCH 6/24] make atomic_read() behave consistently on frv Date: Mon, 13 Aug 2007 02:44:13 -0400 Message-ID: <46BFFDBD.6080804@redhat.com> References: <46BCC26B.6080600@redhat.com> <46BB46B2.60808@redhat.com> <20070809134150.GA14890@shell.boston.redhat.com> <2708.1186737826@redhat.com> <7680.1186822071@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Cc: 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 To: David Howells Return-path: Received: from mx1.redhat.com ([66.187.233.31]:33608 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S967818AbXHMGuh (ORCPT ); Mon, 13 Aug 2007 02:50:37 -0400 In-Reply-To: <7680.1186822071@redhat.com> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org David Howells 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. > > I'm not sure that actually answers my question. Why not smp_rmb()? > > David I would assume because we want to waste time efficiently even on non-smp architectures, rather than frying the CPU or draining the battery. Certain looping execution patterns can cause the CPU to operate above thermal design power. I have fans on my workstation that only ever come on when running LINPACK, and that's generally memory bandwidth-bound. Just imagine what happens when you're executing the same few non-serializing instructions in a tight loop without ever stalling on memory fetches, or being scheduled out. If there's another reason, I'd like to hear it too, because I'm just guessing here. -- Chris