Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S939567AbXHIMgY (ORCPT ); Thu, 9 Aug 2007 08:36:24 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S936551AbXHIMf7 (ORCPT ); Thu, 9 Aug 2007 08:35:59 -0400 Received: from static-ip-62-75-166-246.inaddr.intergenia.de ([62.75.166.246]:38254 "EHLO vs166246.vserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933912AbXHIMf6 (ORCPT ); Thu, 9 Aug 2007 08:35:58 -0400 From: Michael Buesch To: Andi Kleen Subject: Re: [patch] ipvs: force read of atomic_t in while loop Date: Thu, 9 Aug 2007 14:35:18 +0200 User-Agent: KMail/1.9.6 Cc: Chris Snook , Heiko Carstens , 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, torvalds@osdl.org References: <20070808093300.GA14530@osiris.boeblingen.de.ibm.com> <46BA30DC.20207@redhat.com> <20070809001533.GA17798@one.firstfloor.org> In-Reply-To: <20070809001533.GA17798@one.firstfloor.org> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200708091435.18595.mb@bu3sch.de> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2524 Lines: 55 On Thursday 09 August 2007 02:15:33 Andi Kleen wrote: > On Wed, Aug 08, 2007 at 05:08:44PM -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 also think readding volatile makes sense. An alternative would be > to stick an rmb() into atomic_read() -- that would also stop speculative reads. > Disadvantage is that it clobbers all memory, not just the specific value. > > But you really have to complain to Linus (cc'ed). He came up > with the volatile removale change iirc. Isn't it possible through some inline assembly trick that only a certain variable has to be reloaded? So we could define something like that: #define reload_var(x) __asm__ __volatile__ (whatever, x) I don't know inline assembly that much, but isn't it possible with that to kind of "fake-touch" the variable, so the compiler must reload it (and only it) to make sure it's up to date? -- Greetings Michael. - 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/