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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,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 9156BC3A59E for ; Wed, 21 Aug 2019 13:59:39 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 70E09233A0 for ; Wed, 21 Aug 2019 13:59:39 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728822AbfHUN7f (ORCPT ); Wed, 21 Aug 2019 09:59:35 -0400 Received: from Galois.linutronix.de ([193.142.43.55]:55791 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726371AbfHUN7f (ORCPT ); Wed, 21 Aug 2019 09:59:35 -0400 Received: from bigeasy by Galois.linutronix.de with local (Exim 4.80) (envelope-from ) id 1i0R99-0005uA-PW; Wed, 21 Aug 2019 15:59:31 +0200 Date: Wed, 21 Aug 2019 15:59:31 +0200 From: Sebastian Andrzej Siewior To: Thomas Gleixner Cc: Julien Grall , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, maz@kernel.org, rostedt@goodmis.org Subject: Re: [RT PATCH 1/3] hrtimer: Use READ_ONCE to access timer->base in hrimer_grab_expiry_lock() Message-ID: <20190821135931.x6s2b2cwvrxgvoyi@linutronix.de> References: <20190821092409.13225-1-julien.grall@arm.com> <20190821092409.13225-2-julien.grall@arm.com> <20190821134437.efc3cs55o7uatrpj@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On 2019-08-21 15:50:33 [+0200], Thomas Gleixner wrote: > On Wed, 21 Aug 2019, Sebastian Andrzej Siewior wrote: > > > On 2019-08-21 10:24:07 [+0100], Julien Grall wrote: > > > The update to timer->base is protected by the base->cpu_base->lock(). > > > However, hrtimer_grab_expirty_lock() does not access it with the lock. > > > > > > So it would theorically be possible to have timer->base changed under > > > our feet. We need to prevent the compiler to refetch timer->base so the > > > check and the access is performed on the same base. > > > > It is not a problem if the timer's bases changes. We get here because we > > want to help the timer to complete its callback. > > The base can only change if the timer gets re-armed on another CPU which > > means is completed callback. In every case we can cancel the timer on > > the next iteration. > > It _IS_ a problem when the base changes and the compiler reloads > > CPU0 CPU1 > base = timer->base; > > lock(base->....); > switch base > > reload > base = timer->base; > > unlock(base->....); > > See? so read_once() it is then. Sebastian