Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756042Ab3CPSdy (ORCPT ); Sat, 16 Mar 2013 14:33:54 -0400 Received: from mx1.redhat.com ([209.132.183.28]:63392 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752154Ab3CPSdx (ORCPT ); Sat, 16 Mar 2013 14:33:53 -0400 Date: Sat, 16 Mar 2013 19:30:22 +0100 From: Oleg Nesterov To: "Paul E. McKenney" Cc: Frederic Weisbecker , Ming Lei , Shaohua Li , Al Viro , Andrew Morton , linux-kernel@vger.kernel.org Subject: Re: + atomic-improve-atomic_inc_unless_negative-atomic_dec_unless_positive .patch added to -mm tree Message-ID: <20130316183022.GB7560@redhat.com> References: <20130314162413.GA21344@redhat.com> <20130315134632.GA18335@redhat.com> <20130315165131.GA32065@redhat.com> <20130315175117.GA2462@redhat.com> <20130315201739.GK3656@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130315201739.GK3656@linux.vnet.ibm.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1387 Lines: 41 On 03/15, Paul E. McKenney wrote: > > On Fri, Mar 15, 2013 at 07:34:32PM +0100, Frederic Weisbecker wrote: > > 2013/3/15 Oleg Nesterov : > > > > > > My point was: should we fix atomic_add_unless() then? If not, why > > > should atomic_add_unless_negative() differ? > > > > They shouldn't differ I guess. > > Completely agreed. It is not like memory ordering is simple, so we should > keep the rules simple. It is hardly possible to argue with this ;) > The rule is that if an atomic primitive returns non-void, then there is > a full memory barrier before and after. This case is documented... > This applies to primitives > returning boolean as well, with atomic_dec_and_test() setting this > precedent from what I can see. I don't think this is the "fair" comparison. Unlike atomic_add_unless(), atomic_dec_and_test() always changes the memory even if it "fails". If atomic_add_unless() returns 0, nothing was changed and if we add the barrier it is not clear what it should be paired with. But OK. I have to agree that "keep the rules simple" makes sense, so we should change atomic_add_unless() as well. Oleg. -- 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/