Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754883Ab0DSQL4 (ORCPT ); Mon, 19 Apr 2010 12:11:56 -0400 Received: from claw.goop.org ([74.207.240.146]:50058 "EHLO claw.goop.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754829Ab0DSQLz (ORCPT ); Mon, 19 Apr 2010 12:11:55 -0400 Message-ID: <4BCC80CA.90902@goop.org> Date: Mon, 19 Apr 2010 09:11:54 -0700 From: Jeremy Fitzhardinge User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.1.9) Gecko/20100330 Fedora/3.0.4-1.fc12 Lightning/1.0b2pre Thunderbird/3.0.4 MIME-Version: 1.0 To: Avi Kivity CC: Glauber Costa , Peter Zijlstra , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Marcelo Tosatti , Zachary Amsden Subject: Re: [PATCH 1/5] Add a global synchronization point for pvclock References: <1271356648-5108-1-git-send-email-glommer@redhat.com> <1271356648-5108-2-git-send-email-glommer@redhat.com> <4BC8CA52.4090703@goop.org> <1271673545.1674.743.camel@laptop> <4BCC3584.1050501@redhat.com> <1271675100.1674.818.camel@laptop> <4BCC3A3E.9070909@redhat.com> <20100419142158.GD14158@mothafucka.localdomain> <4BCC69D5.3050209@redhat.com> In-Reply-To: <4BCC69D5.3050209@redhat.com> X-Enigmail-Version: 1.0.1 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 887 Lines: 24 On 04/19/2010 07:33 AM, Avi Kivity wrote: > On 04/19/2010 05:21 PM, Glauber Costa wrote: >> >>> Oh yes, just trying to avoid a patch with both atomic64_read() and >>> ACCESS_ONCE(). >>> >> you're mixing the private version of the patch you saw with this one. >> there isn't any atomic reads in here. I'll use a barrier then >> > > This patch writes last_value atomically, but reads it non-atomically. > A barrier is insufficient. Well, on a 32b system, you can explicitly order the updates of low and high, then do a high-low-checkhigh read. That would be much more efficient than atomic64. If we really care about 32b. J -- 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/