Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030225AbWILOyv (ORCPT ); Tue, 12 Sep 2006 10:54:51 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030227AbWILOyu (ORCPT ); Tue, 12 Sep 2006 10:54:50 -0400 Received: from e5.ny.us.ibm.com ([32.97.182.145]:44737 "EHLO e5.ny.us.ibm.com") by vger.kernel.org with ESMTP id S1030225AbWILOyu (ORCPT ); Tue, 12 Sep 2006 10:54:50 -0400 Date: Tue, 12 Sep 2006 07:55:09 -0700 From: "Paul E. McKenney" To: Oliver Neukum Cc: David Howells , Alan Stern , Kernel development list Subject: Re: Uses for memory barriers Message-ID: <20060912145509.GE1291@us.ibm.com> Reply-To: paulmck@us.ibm.com References: <20060911162059.GA1496@us.ibm.com> <32145.1158051703@warthog.cambridge.redhat.com> <200609121222.01260.oliver@neukum.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <200609121222.01260.oliver@neukum.org> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1473 Lines: 43 On Tue, Sep 12, 2006 at 12:22:00PM +0200, Oliver Neukum wrote: > Am Dienstag, 12. September 2006 11:01 schrieb David Howells: > > Paul E. McKenney wrote: > > > > > 2. All stores to a given single memory location will be perceived > > > as having occurred in the same order by all CPUs. > > > > Does that take into account a CPU combining or discarding coincident memory > > operations? > > > > For instance, a CPU asked to issue two writes to the same location may discard > > the first if it hasn't done it yet. > > Does it make sense? If you do: > mov #x, $a > wmb > mov #y, $b > wmb > mov #z, $a > > The CPU must not discard any write. If you do > > mov #x, $a > mov #y, $b > wmb > mov #z, $a > > The first store to $a is superfluous if you have only inter-CPU > issues in mind. In both cases, the CPU might "discard" the write, if there are no intervening reads or writes to the same location. The only difference between your two examples is the ordering of the first store to $a and the store to $b. In your first example, other CPUs must see the first store to $a as happening first, while in your second example, other CPUs might see the store to $b as happening first. 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/