Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S938083AbXHHVhD (ORCPT ); Wed, 8 Aug 2007 17:37:03 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S934722AbXHHVgr (ORCPT ); Wed, 8 Aug 2007 17:36:47 -0400 Received: from smtp2.linux-foundation.org ([207.189.120.14]:43019 "EHLO smtp2.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933046AbXHHVgp (ORCPT ); Wed, 8 Aug 2007 17:36:45 -0400 Date: Wed, 8 Aug 2007 14:31:15 -0700 From: Andrew Morton To: Chris Snook Cc: Heiko Carstens , andi@firstfloor.org, David Miller , 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 Message-Id: <20070808143115.de539511.akpm@linux-foundation.org> In-Reply-To: <46BA30DC.20207@redhat.com> References: <20070808093300.GA14530@osiris.boeblingen.de.ibm.com> <20070808.032131.35507346.davem@davemloft.net> <20070808102835.GC14530@osiris.boeblingen.de.ibm.com> <46BA30DC.20207@redhat.com> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.6; i686-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1806 Lines: 40 On Wed, 08 Aug 2007 17:08:44 -0400 Chris Snook wrote: > 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? I'm not Andi, but this not-Andi thinks that permitting the compiler to cache the results of atomic_read() is dumb. - 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/