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 46204C3F2CD for ; Wed, 4 Mar 2020 09:39:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 262152073D for ; Wed, 4 Mar 2020 09:39:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387739AbgCDJjt (ORCPT ); Wed, 4 Mar 2020 04:39:49 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:46446 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729023AbgCDJjs (ORCPT ); Wed, 4 Mar 2020 04:39:48 -0500 Received: from p5de0bf0b.dip0.t-ipconnect.de ([93.224.191.11] helo=nanos.tec.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1j9QVC-0008B7-PP; Wed, 04 Mar 2020 10:39:42 +0100 Received: by nanos.tec.linutronix.de (Postfix, from userid 1000) id EC4DE101161; Wed, 4 Mar 2020 10:39:41 +0100 (CET) From: Thomas Gleixner To: Qian Cai , fweisbec@gmail.com, mingo@kernel.org Cc: elver@google.com, linux-kernel@vger.kernel.org, Qian Cai , Peter Zijlstra Subject: Re: [PATCH] tick/sched: fix data races at tick_do_timer_cpu In-Reply-To: <20200225030808.1207-1-cai@lca.pw> References: <20200225030808.1207-1-cai@lca.pw> Date: Wed, 04 Mar 2020 10:39:41 +0100 Message-ID: <87tv34laqq.fsf@nanos.tec.linutronix.de> MIME-Version: 1.0 Content-Type: text/plain X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Qian, Qian Cai writes: > tick_do_timer_cpu could be accessed concurrently where both plain writes > and plain reads are not protected by a lock. Thus, it could result in > data races. Fix them by adding pairs of READ|WRITE_ONCE(). The data > races were reported by KCSAN, They are reported, but are they actually a real problem? This completely lacks analysis why these 8 places need the READ/WRITE_ONCE() treatment at all and if so why the other 14 places accessing tick_do_timer_cpu are safe without it. I definitely appreciate the work done with KCSAN, but just making the tool shut up does not cut it. Thanks, tglx