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=-3.7 required=3.0 tests=BAYES_00, 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 39B13C4361B for ; Wed, 9 Dec 2020 22:06:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 0656423D1F for ; Wed, 9 Dec 2020 22:06:45 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2388537AbgLIWG2 (ORCPT ); Wed, 9 Dec 2020 17:06:28 -0500 Received: from out01.mta.xmission.com ([166.70.13.231]:55764 "EHLO out01.mta.xmission.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729913AbgLIWGG (ORCPT ); Wed, 9 Dec 2020 17:06:06 -0500 Received: from in01.mta.xmission.com ([166.70.13.51]) by out01.mta.xmission.com with esmtps (TLS1.2) tls TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256 (Exim 4.93) (envelope-from ) id 1kn7aO-005WVg-0Y; Wed, 09 Dec 2020 15:05:24 -0700 Received: from ip68-227-160-95.om.om.cox.net ([68.227.160.95] helo=x220.xmission.com) by in01.mta.xmission.com with esmtpsa (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.87) (envelope-from ) id 1kn7aN-000401-6k; Wed, 09 Dec 2020 15:05:23 -0700 From: ebiederm@xmission.com (Eric W. Biederman) To: Linus Torvalds Cc: Al Viro , Linux Kernel Mailing List , linux-fsdevel , Christian Brauner , Oleg Nesterov , Jann Horn References: <87r1on1v62.fsf@x220.int.ebiederm.org> <20201120231441.29911-15-ebiederm@xmission.com> <20201207232900.GD4115853@ZenIV.linux.org.uk> <877dprvs8e.fsf@x220.int.ebiederm.org> <20201209040731.GK3579531@ZenIV.linux.org.uk> <877dprtxly.fsf@x220.int.ebiederm.org> <20201209142359.GN3579531@ZenIV.linux.org.uk> <87o8j2svnt.fsf_-_@x220.int.ebiederm.org> Date: Wed, 09 Dec 2020 16:04:44 -0600 In-Reply-To: (Linus Torvalds's message of "Wed, 9 Dec 2020 11:13:38 -0800") Message-ID: <87eejyprer.fsf@x220.int.ebiederm.org> User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) MIME-Version: 1.0 Content-Type: text/plain X-XM-SPF: eid=1kn7aN-000401-6k;;;mid=<87eejyprer.fsf@x220.int.ebiederm.org>;;;hst=in01.mta.xmission.com;;;ip=68.227.160.95;;;frm=ebiederm@xmission.com;;;spf=neutral X-XM-AID: U2FsdGVkX1/+PqrkfZvGQ+wGIDdWEx2/OaAfcYGrrPY= X-SA-Exim-Connect-IP: 68.227.160.95 X-SA-Exim-Mail-From: ebiederm@xmission.com Subject: Re: [PATCH] files: rcu free files_struct X-SA-Exim-Version: 4.2.1 (built Thu, 05 May 2016 13:38:54 -0600) X-SA-Exim-Scanned: Yes (on in01.mta.xmission.com) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Linus Torvalds writes: > On Wed, Dec 9, 2020 at 10:05 AM Eric W. Biederman wrote: >> >> - struct file * file = xchg(&fdt->fd[i], NULL); >> + struct file * file = fdt->fd[i]; >> if (file) { >> + rcu_assign_pointer(fdt->fd[i], NULL); > > This makes me nervous. Why did we use to do that xchg() there? That > has atomicity guarantees that now are gone. > > Now, this whole thing should be called for just the last ref of the fd > table, so presumably that atomicity was never needed in the first > place. But the fact that we did that very expensive xchg() then makes > me go "there's some reason for it". > > Is this xchg() just bogus historical leftover? It kind of looks that > way. But maybe that change should be done separately? Removing the xchg in a separate patch seems reasonable. Just to make the review easier. I traced the xchg back to 7cf4dc3c8dbf ("move files_struct-related bits from kernel/exit.c to fs/file.c") when put_files_struct was introduced. The xchg did not exist before that change. There were many other xchgs in the code back then so I suspect was left over from some way an earlier version of the change worked and simply was not removed when the patch was updated. Eric