From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754713Ab2ASO7o (ORCPT ); Thu, 19 Jan 2012 09:59:44 -0500 Received: from terminus.zytor.com ([198.137.202.10]:36585 "EHLO mail.zytor.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752517Ab2ASO7n (ORCPT ); Thu, 19 Jan 2012 09:59:43 -0500 Message-ID: <4F182FCB.2090300@zytor.com> Date: Thu, 19 Jan 2012 06:59:23 -0800 From: "H. Peter Anvin" User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:8.0) Gecko/20111115 Thunderbird/8.0 MIME-Version: 1.0 To: Jan Beulich CC: mingo@elte.hu, "eric.dumazet@gmail.com" , tglx@linutronix.de, luca@luca-barbieri.com, linux-kernel@vger.kernel.org Subject: Re: [PATCH 2/2] ix86: atomic64 assembly improvements References: <4F16E41B020000780006D7A5@nat28.tlf.novell.com> <4F16F505.8040809@zytor.com> <4F170679020000780006D85B@nat28.tlf.novell.com> <4F1705A8.301@zytor.com> <4F17EDE9020000780006D97A@nat28.tlf.novell.com> <4F182C55.7040305@zytor.com> <4F183BCA020000780006DB68@nat28.tlf.novell.com> In-Reply-To: <4F183BCA020000780006DB68@nat28.tlf.novell.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 01/19/2012 06:50 AM, Jan Beulich wrote: >> >> It doesn't have anything to do with caches on or off. > > How does it not? If any part of the bus topology is only 32 bits wide, > a 64-bit read or write simply can't be executed atomically without > asserting LOCK#. > Furthermore, if you're going down that rathole then we can't even trust MOV (nor, for that matter, can you trust LOCK in a lot of circumstances.) In short, you need to be extremely careful about what you do to uncached memory *at all* (and you need to know exactly what is behind this bus) but pessimizing these kinds of construct for that reason is wrong in the extreme. -hpa