From mboxrd@z Thu Jan 1 00:00:00 1970 From: Chris Snook Subject: Re: [PATCH 1/24] make atomic_read() behave consistently on alpha Date: Thu, 09 Aug 2007 14:13:52 -0400 Message-ID: <46BB5960.9040009@redhat.com> References: <20070809132442.GA13042@shell.boston.redhat.com> <20070809143255.GA8424@linux.vnet.ibm.com> <46BB2A5A.5090006@redhat.com> <20070809150445.GB8424@linux.vnet.ibm.com> <46BB31A6.4080507@redhat.com> <20070809161024.GC8424@linux.vnet.ibm.com> <46BB4281.7010803@redhat.com> <20070809165853.GD8424@linux.vnet.ibm.com> <46BB4B7B.4070007@redhat.com> <20070809174150.GE8424@linux.vnet.ibm.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: paulmck@linux.vnet.ibm.com Return-path: Received: from mx1.redhat.com ([66.187.233.31]:43900 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757558AbXHISTx (ORCPT ); Thu, 9 Aug 2007 14:19:53 -0400 In-Reply-To: <20070809174150.GE8424@linux.vnet.ibm.com> Sender: netdev-owner@vger.kernel.org List-Id: netdev.vger.kernel.org Paul E. McKenney wrote: > On Thu, Aug 09, 2007 at 01:14:35PM -0400, Chris Snook wrote: >> If you're depending on volatile writes >> being visible to other CPUs, you're screwed either way, because the CPU can >> hold that data in cache as long as it wants before it writes it to memory. >> When this finally does happen, it will happen atomically, which is all that >> atomic_set guarantees. If you need to guarantee that the value is written >> to memory at a particular time in your execution sequence, you either have >> to read it from memory to force the compiler to store it first (and a >> volatile cast in atomic_read will suffice for this) or you have to use >> LOCK_PREFIX instructions which will invalidate remote cache lines >> containing the same variable. This patch doesn't change either of these >> cases. > > The case that it -can- change is interactions with interrupt handlers. > And NMI/SMI handlers, for that matter. You have a point here, but only if you can guarantee that the interrupt handler is running on a processor sharing the cache that has the not-yet-written volatile value. That implies a strictly non-SMP architecture. At the moment, none of those have volatile in their declaration of atomic_t, so this patch can't break any of them. -- Chris