From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753459AbXCPKcE (ORCPT ); Fri, 16 Mar 2007 06:32:04 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753458AbXCPKcD (ORCPT ); Fri, 16 Mar 2007 06:32:03 -0400 Received: from 207.47.60.4.static.nextweb.net ([207.47.60.4]:5834 "EHLO rpc.xensource.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1753465AbXCPKcC (ORCPT ); Fri, 16 Mar 2007 06:32:02 -0400 User-Agent: Microsoft-Entourage/11.2.5.060620 Date: Fri, 16 Mar 2007 10:31:49 +0000 Subject: Re: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console From: Keir Fraser To: Ingo Molnar , Keir Fraser CC: , , Chris Wright , Andi Kleen , Andrew Morton , Message-ID: Thread-Topic: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console Thread-Index: Acdntk3HjC0cjtOpEduhtwAX8io7RQ== In-Reply-To: <20070316095851.GB301@elte.hu> Mime-version: 1.0 Content-type: text/plain; charset="US-ASCII" Content-transfer-encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On 16/3/07 09:58, "Ingo Molnar" wrote: >> These mb()'s are pretty standard for lock-free producer/consumer >> rings. Write descriptor /then/ write the updated producer. Read the >> producer /then/ read any descriptors revealed by this new producer >> value. > > then use rmb()/wmb(). Rarely does a ring protocol truly need mb(). It's needed for writing data /after/ reading the consumer index that shows you have space to write. Looking through xenbus_comms.c I think all the barriers are correct except there is a spurious extra mb() in xb_read(), where there is a later rmb() which is sufficient by itself. All the others have a purpose. -- Keir