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=-3.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 02849C5519F for ; Tue, 17 Nov 2020 14:29:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7B12D2222E for ; Tue, 17 Nov 2020 14:29:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=efficios.com header.i=@efficios.com header.b="WB/3nIfk" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728063AbgKQO2n (ORCPT ); Tue, 17 Nov 2020 09:28:43 -0500 Received: from mail.efficios.com ([167.114.26.124]:57560 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727473AbgKQO2n (ORCPT ); Tue, 17 Nov 2020 09:28:43 -0500 Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id CF3F72E06C7; Tue, 17 Nov 2020 09:28:41 -0500 (EST) Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id O_VS5QOc43TJ; Tue, 17 Nov 2020 09:28:41 -0500 (EST) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id 952402E06C6; Tue, 17 Nov 2020 09:28:41 -0500 (EST) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com 952402E06C6 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1605623321; bh=0FQwR6rczTZMYoYMOVLsPCxXwfIFesuddkR+IgOj37Y=; h=Date:From:To:Message-ID:MIME-Version; b=WB/3nIfkR50y6k3W9pkDUx6XbqK7IeHE2tuxlDNq1m8dhUXZUMR1+dMZgsAeiKkjv nKcJXEgtWKlubfAW7mvRs2hUWBBLytsmgP0YkN6g8r7SP6muLcfhJoSBSqpVP/k0aU eSKaTa9dkGlnm78BJAUBKylQAWyEkDQzblN2kvUGtwqKaPSBcYVjawRUbR4bd1QBwM fhk9R3jfx3r0sRBwmRtGVuBaNR4IL8l2WUxbDRY89SBBg6PoqZZtDCg8TDZceYeZP7 RZn8bmMXNnc6T5f/hLzLEzYI3THJbraUYI1ezv89CU0hLaBtFtn3MRQgM/CIEA/Iei HN53YM4urphUg== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id 0RT43ZNkUwWJ; Tue, 17 Nov 2020 09:28:41 -0500 (EST) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id 86C102E06C5; Tue, 17 Nov 2020 09:28:41 -0500 (EST) Date: Tue, 17 Nov 2020 09:28:41 -0500 (EST) From: Mathieu Desnoyers To: Andy Lutomirski Cc: linux-api , Peter Oskolkov , Peter Zijlstra Message-ID: <621527792.47384.1605623321460.JavaMail.zimbra@efficios.com> In-Reply-To: References: Subject: Re: Is adding an argument to an existing syscall okay? MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_3975 (ZimbraWebClient - FF82 (Linux)/8.8.15_GA_3975) Thread-Topic: Is adding an argument to an existing syscall okay? Thread-Index: rSZkK7YOdWhdpW695WZvp+si/GxLxQ== Precedence: bulk List-ID: X-Mailing-List: linux-api@vger.kernel.org ----- On Nov 16, 2020, at 6:57 PM, Andy Lutomirski luto@kernel.org wrote: > Linux 5.10 contains this patch: > > commit 2a36ab717e8fe678d98f81c14a0b124712719840 > Author: Peter Oskolkov > Date: Wed Sep 23 16:36:16 2020 -0700 > > rseq/membarrier: Add MEMBARRIER_CMD_PRIVATE_EXPEDITED_RSEQ > > This adds an argument to an existing syscall. Before the patch, > membarrier had 2 parameters; now it has 3. Is this really okay? At > least the patch is careful and ignores the third parameter unless a > previously unused flag bit is set. Hi Andy, I wondered about exactly this on August 12 2020: https://lore.kernel.org/r/1477195446.6156.1597261492255.JavaMail.zimbra@efficios.com And then on August 25, after receiving no feedback, I told Peter to try this approach: https://lore.kernel.org/r/1336467655.17779.1598374701401.JavaMail.zimbra@efficios.com and nobody complained until now. As you note, the extra argument is only used when previously unused flag bits are set. So your question is very relevant, and I still look forward to receiving feedback on this matter. Thanks, Mathieu > > --Andy -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com