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=-0.7 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED 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 B6021C34022 for ; Wed, 19 Feb 2020 05:23:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 95766208E4 for ; Wed, 19 Feb 2020 05:23:35 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726096AbgBSFXf (ORCPT ); Wed, 19 Feb 2020 00:23:35 -0500 Received: from zeniv.linux.org.uk ([195.92.253.2]:36886 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725819AbgBSFXf (ORCPT ); Wed, 19 Feb 2020 00:23:35 -0500 Received: from viro by ZenIV.linux.org.uk with local (Exim 4.92.3 #3 (Red Hat Linux)) id 1j4HpZ-00F4vo-LP; Wed, 19 Feb 2020 05:23:29 +0000 Date: Wed, 19 Feb 2020 05:23:29 +0000 From: Al Viro To: Qian Cai Cc: hch@infradead.org, darrick.wong@oracle.com, elver@google.com, linux-xfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] fs: fix a data race in i_size_write/i_size_read Message-ID: <20200219052329.GP23230@ZenIV.linux.org.uk> References: <20200219045228.GO23230@ZenIV.linux.org.uk> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-xfs-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-xfs@vger.kernel.org On Wed, Feb 19, 2020 at 12:08:40AM -0500, Qian Cai wrote: > > > > On Feb 18, 2020, at 11:52 PM, Al Viro wrote: > > > > If aligned 64bit stores on 64bit host (note the BITS_PER_LONG ifdefs) end up > > being split, the kernel is FUBAR anyway. Details, please - how could that > > end up happening? > > My understanding is the compiler might decide to split the load into saying two 4-byte loads. Then, we might have, > > Load1 > Store > Load2 > > where the load value could be a garbage. Also, Marco (the KCSAN maintainer) who knew more of compiler than me mentioned that there is no guarantee that the store will not be split either. Thus, the WRITE_ONCE(). > I would suggest * if some compiler does that, ask the persons responsible for that "optimization" which flags should be used to disable it. * if they fail to provide such, educate them regarding the usefulness of their idea * if that does not help, don't use the bloody piece of garbage. Again, is that pure theory (because I can't come up with any reason why splitting a 32bit load would be any less legitimate than doing the same to a 64bit one on a 64bit architecture), or is there anything that really would pull that off?