Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933145Ab3CLR5Z (ORCPT ); Tue, 12 Mar 2013 13:57:25 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:39405 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932470Ab3CLR5X (ORCPT ); Tue, 12 Mar 2013 13:57:23 -0400 Date: Tue, 12 Mar 2013 10:55:57 -0700 From: "Paul E. McKenney" To: Frederic Weisbecker Cc: Ming Lei , Andrew Morton , linux-kernel@vger.kernel.org, Shaohua Li , Al Viro Subject: Re: [PATCH] atomic: improve atomic_inc_unless_negative/atomic_dec_unless_positive Message-ID: <20130312175557.GF3725@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1362843501-31159-1-git-send-email-tom.leiming@gmail.com> <20130312033906.GA3725@linux.vnet.ibm.com> <20130312143853.GD3725@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: No X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13031217-5806-0000-0000-0000204E8908 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3215 Lines: 78 On Tue, Mar 12, 2013 at 04:02:47PM +0100, Frederic Weisbecker wrote: > 2013/3/12 Paul E. McKenney : > > On Tue, Mar 12, 2013 at 12:03:23PM +0800, Ming Lei wrote: > >> On Tue, Mar 12, 2013 at 11:39 AM, Paul E. McKenney > >> wrote: > >> > > >> > Atomic operations that return a value are required to act as full memory > >> > barriers. This means that code relying on ordering provided by these > >> > atomic operations must also do ordering, either by using an explicit > >> > memory barrier or by relying on guarantees from atomic operations. > >> > > >> > For example: > >> > > >> > CPU 0 CPU 1 > >> > > >> > X = 1; r1 = Z; > >> > if (atomic_inc_unless_negative(&Y) smp_mb(); > >> > do_something(); > >> > Z = 1; r2 = X; > >> > > >> > Assuming X and Z are initially zero, if r1==1, we are guaranteed > >> > that r2==1. However, CPU 1 needs its smp_mb() in order to pair with > >> > the barrier implicit in atomic_inc_unless_negative(). > >> > > >> > Make sense? > >> > >> Yes, it does, and thanks for the explanation. > >> > >> But looks the above example is not what Frederic described: > >> > >> "the above atomic_read() might return -1 because there is no > >> guarantee it's seeing the recent update on the remote CPU." > >> > >> Even I am not sure if adding one smp_mb() around atomic_read() > >> can guarantee that too. > > > > Frederic was likely thinking of some other scenario that would be > > broken by atomic_inc_unless_negative() failing to act as a full > > memory barrier. Here is another example: > > > > > > CPU 0 CPU 1 > > > > X = 1; > > if (atomic_inc_unless_negative(&Y) r1 = atomic_xchg(&Y, -1); > > r2 = X; > > > > If atomic_inc_unless_negative() acts as a full memory barrier, then > > if CPU 0 reaches the assignment from X, the results will be guaranteed > > to be 1. Otherwise, there is no guarantee. > > Your scenarios show an interesting guarantee I did not think about. > But my concern was on such a situation: > > CPU 0 CPU 1 > > atomic_set(&X, -1) > atomic_inc(&X) > atomic_add_unless_negative(&X, 5) > > On the above situation, CPU 0 may still see X == -1 and thus not add > the 5. Of course all that only make sense with datas coming along. That could happen, but you would need CPU 1 to carry out some other reference for it to be a bug. Otherwise, CPU 1's atomic_inc() just happened after all of CPU 0's code. But yes, it would be possible to misorder with some larger scenario starting with this example. Especially given that atomic_inc() does not make any ordering guarantees. Thanx, Paul Thanx, Paul -- 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/