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, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 50093C6379D for ; Wed, 18 Nov 2020 09:51:53 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F31E122202 for ; Wed, 18 Nov 2020 09:51:52 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727519AbgKRJvl (ORCPT ); Wed, 18 Nov 2020 04:51:41 -0500 Received: from foss.arm.com ([217.140.110.172]:49980 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725774AbgKRJvl (ORCPT ); Wed, 18 Nov 2020 04:51:41 -0500 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id B255111D4; Wed, 18 Nov 2020 01:51:40 -0800 (PST) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id C91653F719; Wed, 18 Nov 2020 01:51:39 -0800 (PST) References: <20201116142005.GE3121392@hirez.programming.kicks-ass.net> <20201116193149.GW3371@techsingularity.net> <20201117083016.GK3121392@hirez.programming.kicks-ass.net> <20201117091545.GA31837@willie-the-truck> <20201117092936.GA3121406@hirez.programming.kicks-ass.net> <20201117094621.GE3121429@hirez.programming.kicks-ass.net> <20201117161318.GP3121392@hirez.programming.kicks-ass.net> <20201118080515.GR3121392@hirez.programming.kicks-ass.net> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Peter Zijlstra Cc: Will Deacon , Mel Gorman , Davidlohr Bueso , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: Fix data-race in wakeup In-reply-to: <20201118080515.GR3121392@hirez.programming.kicks-ass.net> Date: Wed, 18 Nov 2020 09:51:33 +0000 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 18/11/20 08:05, Peter Zijlstra wrote: > On Tue, Nov 17, 2020 at 07:32:16PM +0000, Valentin Schneider wrote: >> >> On 17/11/20 16:13, Peter Zijlstra wrote: >> > On Tue, Nov 17, 2020 at 03:37:24PM +0000, Valentin Schneider wrote: >> > >> >> >> + /* >> >> >> + * This field must not be in the scheduler word above due to wakelist >> >> >> + * queueing no longer being serialized by p->on_cpu. However: >> >> >> + * >> >> >> + * p->XXX = X; ttwu() >> >> >> + * schedule() if (p->on_rq && ..) // false >> >> >> + * smp_mb__after_spinlock(); if (smp_load_acquire(&p->on_cpu) && //true >> >> >> + * deactivate_task() ttwu_queue_wakelist()) >> >> >> + * p->on_rq = 0; p->sched_remote_wakeup = Y; >> >> >> + * >> >> >> + * guarantees all stores of 'current' are visible before >> >> >> + * ->sched_remote_wakeup gets used, so it can be in this word. >> >> >> + */ >> >> > >> >> > Isn't the control dep between that ttwu() p->on_rq read and >> >> > p->sched_remote_wakeup write "sufficient"? >> >> >> >> smp_acquire__after_ctrl_dep() that is, since we need >> >> ->on_rq load => 'current' bits load + store >> > >> > I don't think we need that extra barrier; after all, there will be a >> > complete schedule() between waking the task and it actually becoming >> > current. >> >> Apologies for the messy train of thought; what I was trying to say is that >> we have already the following, which AIUI is sufficient: >> >> * p->XXX = X; ttwu() >> * schedule() if (p->on_rq && ..) // false >> * smp_mb__after_spinlock(); smp_acquire__after_ctrl_dep(); >> * deactivate_task() ttwu_queue_wakelist() >> * p->on_rq = 0; p->sched_remote_wakeup = Y; >> > > Ah, you meant the existing smp_acquire__after_ctrl_dep(). Yeah, that's > not required here either ;-) > > The reason I had the ->on_cpu thing in there is because it shows we > violate the regular ->on_cpu handoff rules, not for the acquire. > Gotcha > The only ordering that matters on the RHS of that thing is the ->on_rq > load to p->sched_remote_wakeup store ctrl dep. That, combined with the > LHS, guarantees there is a strict order on the stores. > > Makes sense? Yep, thanks!