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 0E263C2D0E4 for ; Tue, 17 Nov 2020 09:46:52 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B2979222EC for ; Tue, 17 Nov 2020 09:46:51 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="IQZoff32" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727350AbgKQJq3 (ORCPT ); Tue, 17 Nov 2020 04:46:29 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:54520 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726203AbgKQJq3 (ORCPT ); Tue, 17 Nov 2020 04:46:29 -0500 Received: from casper.infradead.org (casper.infradead.org [IPv6:2001:8b0:10b:1236::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 38FC1C0613CF for ; Tue, 17 Nov 2020 01:46:29 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.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=Jet1I61eSfgzRK7zbEllS/kKGywrT/l/67+QQnNONvM=; b=IQZoff32/nmRD+w0cicNPPNHB6 ZAjzmVeojiVehfQ6QgEq8JeFXnGc0POsvvL2rznJA+XABVtidjsl+DeT3Qri4ArCT3UOMoqxuG58I 1JvRqEPsmrmTxV6ONqBt/F4w38ByUuDkmCUUgy7G9bV/kaL2alGktaT0X725qN3G4Zr64kL6yUy7h dbTczWB9SLvRel6uMZY7hY7yPzJ79IOR6EeyGG1ntD/Kp/XEiqE27Mo9XY858v/7vqoSHuvOX1LE0 v+e7lrJlRU4Ri8znl/vIr3RscbybxKJjxUhfN9Vijsr95WQQJ57xawuyRalT7eSpAvhGFBlkvavzu ENZ4iKZw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1kexZ8-0006bc-Er; Tue, 17 Nov 2020 09:46:23 +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 0EC02303DA0; Tue, 17 Nov 2020 10:46:22 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id E8DA620116725; Tue, 17 Nov 2020 10:46:21 +0100 (CET) Date: Tue, 17 Nov 2020 10:46:21 +0100 From: Peter Zijlstra To: Will Deacon Cc: 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: <20201117094621.GE3121429@hirez.programming.kicks-ass.net> References: <20201116091054.GL3371@techsingularity.net> <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> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201117092936.GA3121406@hirez.programming.kicks-ass.net> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 17, 2020 at 10:29:36AM +0100, Peter Zijlstra wrote: > On Tue, Nov 17, 2020 at 09:15:46AM +0000, Will Deacon wrote: > > On Tue, Nov 17, 2020 at 09:30:16AM +0100, Peter Zijlstra wrote: > > > /* Unserialized, strictly 'current' */ > > > > > > + /* > > > + * p->in_iowait = 1; 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 = X; > > > + * > > > + * Guarantees all stores of 'current' are visible before > > > + * ->sched_remote_wakeup gets used. > > > > I'm still not sure this is particularly clear -- don't we want to highlight > > that the store of p->on_rq is unordered wrt the update to > > p->sched_contributes_to_load() in deactivate_task()? How's this then? It still doesn't explicitly call out the specific race, but does mention the more fundamental issue that wakelist queueing doesn't respect the regular rules anymore. --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -775,7 +775,6 @@ struct task_struct { unsigned sched_reset_on_fork:1; unsigned sched_contributes_to_load:1; unsigned sched_migrated:1; - unsigned sched_remote_wakeup:1; #ifdef CONFIG_PSI unsigned sched_psi_wake_requeue:1; #endif @@ -785,6 +784,21 @@ struct task_struct { /* Unserialized, strictly 'current' */ + /* + * 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. + */ + unsigned sched_remote_wakeup:1; + /* Bit to tell LSMs we're in execve(): */ unsigned in_execve:1; unsigned in_iowait:1;