From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-7.5 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0F9E6C433E0 for ; Mon, 8 Feb 2021 12:13:56 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BF06264E29 for ; Mon, 8 Feb 2021 12:13:55 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231822AbhBHMNg (ORCPT ); Mon, 8 Feb 2021 07:13:36 -0500 Received: from mail.xenproject.org ([104.130.215.37]:50508 "EHLO mail.xenproject.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233490AbhBHLvv (ORCPT ); Mon, 8 Feb 2021 06:51:51 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=xen.org; s=20200302mail; h=Content-Transfer-Encoding:Content-Type:In-Reply-To: MIME-Version:Date:Message-ID:From:References:Cc:To:Subject; bh=Yb8hwLCySjNi2+iqUZJlgDYQEPN0SidS4NbLYbSWASA=; b=0EP9GAFGUUaHqcqmkHxhIkNS/2 q+12rHMTFSmo7vkP8d46/QLtWVzWHba1fqhtQm1Y2z2JOKDtsVDeBpzFAfxoAa3D6PPfX4bAD4ndV 7rpEDV3zOiQ3K+NGx7pWQziyno6q4hapeUafIHQy//JxAUSFNREMC7hY5efEE6w/wgmo=; Received: from xenbits.xenproject.org ([104.239.192.120]) by mail.xenproject.org with esmtp (Exim 4.92) (envelope-from ) id 1l954G-0002lN-Lq; Mon, 08 Feb 2021 11:51:00 +0000 Received: from [54.239.6.177] (helo=a483e7b01a66.ant.amazon.com) by xenbits.xenproject.org with esmtpsa (TLS1.3:ECDHE_RSA_AES_128_GCM_SHA256:128) (Exim 4.92) (envelope-from ) id 1l954G-0001gh-EF; Mon, 08 Feb 2021 11:51:00 +0000 Subject: Re: [PATCH 7/7] xen/evtchn: read producer index only once To: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= , Jan Beulich Cc: Boris Ostrovsky , Stefano Stabellini , linux-kernel@vger.kernel.org, xen-devel@lists.xenproject.org References: <20210206104932.29064-1-jgross@suse.com> <20210206104932.29064-8-jgross@suse.com> <72334160-cffe-2d8a-23b7-2ea9ab1d803a@suse.com> <626f500a-494a-0141-7bf3-94fb86b47ed4@suse.com> From: Julien Grall Message-ID: <42e15cc4-56d1-b34b-d97e-d579e771788a@xen.org> Date: Mon, 8 Feb 2021 11:50:58 +0000 User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:78.0) Gecko/20100101 Thunderbird/78.7.1 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-GB Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/02/2021 10:59, Jürgen Groß wrote: > On 08.02.21 11:51, Jan Beulich wrote: > Yes, but I don't see an urgent need to fix that, as there would > be thousands of accesses in the kernel needing a fix. A compiler > tearing a naturally aligned access into multiple memory accesses > would be rejected as buggy from the kernel community IMO. I would not be so sure. From lwn [1]: "In the Linux kernel, tearing of plain C-language loads has been observed even given properly aligned and machine-word-sized loads.)" And for store tearing: "Note that this tearing can happen even on properly aligned and machine-word-sized accesses, and in this particular case, even for volatile stores. Some might argue that this behavior constitutes a bug in the compiler, but either way it illustrates the perceived value of store tearing from a compiler-writer viewpoint. [...] But for properly aligned machine-sized stores, WRITE_ONCE() will prevent store tearing." Cheers, [1] https://lwn.net/Articles/793253/#Load%20Tearing > > > Juergen -- Julien Grall