Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964925AbWIKQuJ (ORCPT ); Mon, 11 Sep 2006 12:50:09 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S964926AbWIKQuJ (ORCPT ); Mon, 11 Sep 2006 12:50:09 -0400 Received: from iolanthe.rowland.org ([192.131.102.54]:48135 "HELO iolanthe.rowland.org") by vger.kernel.org with SMTP id S964925AbWIKQuI (ORCPT ); Mon, 11 Sep 2006 12:50:08 -0400 Date: Mon, 11 Sep 2006 12:50:07 -0400 (EDT) From: Alan Stern X-X-Sender: stern@iolanthe.rowland.org To: "Paul E. McKenney" cc: Oliver Neukum , David Howells , Kernel development list Subject: Re: Uses for memory barriers In-Reply-To: <20060911162059.GA1496@us.ibm.com> Message-ID: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1106 Lines: 29 On Mon, 11 Sep 2006, Paul E. McKenney wrote: > This is a summary of the Linux memory-barrier semantics as I understand > them: > > 1. A given CPU will always perceive its own memory operations > as occuring in program order. > > 2. All stores to a given single memory location will be perceived > as having occurred in the same order by all CPUs. This is > "coherence". (And this is the property that I was forgetting > about when I first looked at your second example.) ... This can't be right. Together 1 and 2 would obviate the need for wmb(). The CPU doing "STORE A; STORE B" will always see the operations occuring in program order by 1, and hence every other CPU would always see them occurring in the same order by 2 -- even without wmb(). Either 2 is too strong, or else what you mean by "perceived" isn't sufficiently clear. Alan Stern - 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/