From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933603AbXCPLmI (ORCPT ); Fri, 16 Mar 2007 07:42:08 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S933602AbXCPLmI (ORCPT ); Fri, 16 Mar 2007 07:42:08 -0400 Received: from smtp.osdl.org ([65.172.181.24]:42274 "EHLO smtp.osdl.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933603AbXCPLmG (ORCPT ); Fri, 16 Mar 2007 07:42:06 -0400 Date: Fri, 16 Mar 2007 03:41:23 -0800 From: Andrew Morton To: Keir Fraser Cc: Ingo Molnar , Keir Fraser , , , Chris Wright , Andi Kleen , Subject: Re: [Xen-devel] Re: [patch 21/26] Xen-paravirt_ops: Use the hvc console infrastructure for Xen console Message-Id: <20070316034123.9de92632.akpm@linux-foundation.org> In-Reply-To: References: <20070316095851.GB301@elte.hu> X-Mailer: Sylpheed version 2.2.7 (GTK+ 2.8.17; x86_64-unknown-linux-gnu) 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 Fri, 16 Mar 2007 10:31:49 +0000 Keir Fraser wrote: > 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. > If Ingo couldn't work this out from reading the code then nobody else can, and we have a maintainability problem which can only be solved with adequate commenting.