Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S965213AbXHHVOs (ORCPT ); Wed, 8 Aug 2007 17:14:48 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S934074AbXHHVOd (ORCPT ); Wed, 8 Aug 2007 17:14:33 -0400 Received: from mx1.redhat.com ([66.187.233.31]:38711 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1763241AbXHHVOc (ORCPT ); Wed, 8 Aug 2007 17:14:32 -0400 Message-ID: <46BA30DC.20207@redhat.com> Date: Wed, 08 Aug 2007 17:08:44 -0400 From: Chris Snook User-Agent: Thunderbird 1.5.0.12 (Macintosh/20070509) MIME-Version: 1.0 To: Heiko Carstens , andi@firstfloor.org CC: David Miller , akpm@linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, schwidefsky@de.ibm.com, wensong@linux-vs.org, horms@verge.net.au Subject: Re: [patch] ipvs: force read of atomic_t in while loop References: <20070808093300.GA14530@osiris.boeblingen.de.ibm.com> <20070808.032131.35507346.davem@davemloft.net> <20070808102835.GC14530@osiris.boeblingen.de.ibm.com> In-Reply-To: <20070808102835.GC14530@osiris.boeblingen.de.ibm.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1558 Lines: 33 Heiko Carstens wrote: > On Wed, Aug 08, 2007 at 03:21:31AM -0700, David Miller wrote: >> From: Heiko Carstens >> Date: Wed, 8 Aug 2007 11:33:00 +0200 >> >>> Just saw this while grepping for atomic_reads in a while loops. >>> Maybe we should re-add the volatile to atomic_t. Not sure. >> I think whatever the choice, it should be done consistently >> on every architecture. >> >> It's just asking for trouble if your arch does it differently from >> every other. > > Well..currently it's i386/x86_64 and s390 which have no volatile > in atomic_t. And yes, of course I agree it should be consistent > across all architectures. But it isn't. Based on recent discussion, it's pretty clear that there's a lot of confusion about this. A lot of people (myself included, until I thought about it long and hard) will reasonably assume that calling atomic_read() will actually read the value from memory. Leaving out the volatile declaration seems like a pessimization to me. If you force people to use barrier() everywhere they're working with atomic_t, it will force re-reads of all the non-atomic data in use as well, which will cause more memory fetches of things that generally don't need barrier(). That and it's a bug waiting to happen. Andi -- your thoughts on the matter? - 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/