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=-3.9 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 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 C2C75C5519F for ; Tue, 17 Nov 2020 16:13:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5716524655 for ; Tue, 17 Nov 2020 16:13:50 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="ERIGSMjo" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727571AbgKQQNa (ORCPT ); Tue, 17 Nov 2020 11:13:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57888 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727524AbgKQQN3 (ORCPT ); Tue, 17 Nov 2020 11:13:29 -0500 Received: from merlin.infradead.org (merlin.infradead.org [IPv6:2001:8b0:10b:1231::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id E0078C0613CF for ; Tue, 17 Nov 2020 08:13:28 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=noYJcmkZ1pKuEG0/DtLEB1n7xvrgps4QtlKybXX4u7Q=; b=ERIGSMjoZeskjSWCb0nMDzylOJ BllmA8djxY7fO25/odBSbGFx5iv/7Mnt5PXHKGuFrKJxfKYvnmhSLppaxZQVSj5BSS5Mr/GB6npi/ y4bfS9lOJ0QAAPqXcQtuqgaq/KYk7xPOLYSZ4/kk+yI6PU6njfFe6jRwK/K8Hpk4PfF1zt4EHE5gW h2leHUQJWYSEFcfBx9b4x7Lla1iSsWGXSbTmr6F7r1lu8wu8qyRV1iywrZyLnMrie8wwcGWFxNKD6 evhN8EM7wwFC4IefdNs9h0g5vJM8gQJ5TaKEGns4Ywh52vRTXB8ve4bfFppowRkSAjIjyLKJQ5pIo 2mxo2IKA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kf3be-0008So-JC; Tue, 17 Nov 2020 16:13:22 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 654883019CE; Tue, 17 Nov 2020 17:13:18 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 4F6C0203DEE9D; Tue, 17 Nov 2020 17:13:18 +0100 (CET) Date: Tue, 17 Nov 2020 17:13:18 +0100 From: Peter Zijlstra To: Valentin Schneider 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 Message-ID: <20201117161318.GP3121392@hirez.programming.kicks-ass.net> References: <20201116131102.GA29992@willie-the-truck> <20201116133721.GQ3371@techsingularity.net> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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.