From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752645Ab0DSOhx (ORCPT ); Mon, 19 Apr 2010 10:37:53 -0400 Received: from mx1.redhat.com ([209.132.183.28]:55401 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751824Ab0DSOhw (ORCPT ); Mon, 19 Apr 2010 10:37:52 -0400 Message-ID: <4BCC6ABA.9030006@redhat.com> Date: Mon, 19 Apr 2010 17:37:46 +0300 From: Avi Kivity 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 Thunderbird/3.0.4 MIME-Version: 1.0 To: Glauber Costa CC: Peter Zijlstra , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Jeremy Fitzhardinge , 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> <4BCA026D.3070309@redhat.com> <4BCA02D1.2020608@redhat.com> <1271673836.1674.757.camel@laptop> <4BCC34DF.6030702@redhat.com> <1271674575.1674.793.camel@laptop> <4BCC3AC2.8050601@redhat.com> <1271675983.1674.853.camel@laptop> <20100419143219.GF14158@mothafucka.localdomain> In-Reply-To: <20100419143219.GF14158@mothafucka.localdomain> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 04/19/2010 05:32 PM, Glauber Costa wrote: > >> Right, another option is to put the initial read outside of the loop, >> that way you'll have the best of all cases, a single LOCK'ed op in the >> loop, and only a single LOCK'ed op for the fast path on sensible >> architectures ;-) >> >> last = atomic64_read(&last_value); >> > isn't a barrier enough here? > > No. On i386, the statement last = last_value; will be split by the compiler into two 32-bit loads. If a write (atomic, using cmpxchg) on another cpu happens between those two loads, then the variable last will have a corrupted value. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic.