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=-5.6 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,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 36E44C2D0E4 for ; Tue, 17 Nov 2020 10:36:31 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B5B6F22447 for ; Tue, 17 Nov 2020 10:36:30 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="HoVhIyYm" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726310AbgKQKg2 (ORCPT ); Tue, 17 Nov 2020 05:36:28 -0500 Received: from mail.kernel.org ([198.145.29.99]:53810 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725770AbgKQKg2 (ORCPT ); Tue, 17 Nov 2020 05:36:28 -0500 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id A00F4223C7; Tue, 17 Nov 2020 10:36:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605609387; bh=gZaiJIRNLJTPM7wL507aXVpZaly9y7V7PkaeLC69J6k=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=HoVhIyYm4EnEMnk5qXn6L0Rw/qRCDYcrO4inuFB8vpHDuDtD32HzE4xLprJiX38ZX eTgEbktWBRW8UHk0lKtUX3meCoQXqACQSpPlt7k1lnMZDZAjHcTqYliy6uaZKhvdBX EWcRfzv5zikih5dCRAmowXJYbK6A+sRr/dY3G8RE= Date: Tue, 17 Nov 2020 10:36:23 +0000 From: Will Deacon To: Peter Zijlstra 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: <20201117103622.GA32035@willie-the-truck> 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> <20201117094621.GE3121429@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201117094621.GE3121429@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Nov 17, 2020 at 10:46:21AM +0100, Peter Zijlstra wrote: > 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; Much better, thanks! Will 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=-5.5 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,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 58080C2D0E4 for ; Tue, 17 Nov 2020 10:37:48 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id C74E022447 for ; Tue, 17 Nov 2020 10:37:47 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="y8l44LMF"; dkim=fail reason="signature verification failed" (1024-bit key) header.d=kernel.org header.i=@kernel.org header.b="HoVhIyYm" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C74E022447 Authentication-Results: mail.kernel.org; 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Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1keyLh-0000dV-DG; Tue, 17 Nov 2020 10:36:33 +0000 Received: from mail.kernel.org ([198.145.29.99]) by merlin.infradead.org with esmtps (Exim 4.92.3 #3 (Red Hat Linux)) id 1keyLd-0000cM-9Y for linux-arm-kernel@lists.infradead.org; Tue, 17 Nov 2020 10:36:30 +0000 Received: from willie-the-truck (236.31.169.217.in-addr.arpa [217.169.31.236]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id A00F4223C7; Tue, 17 Nov 2020 10:36:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1605609387; bh=gZaiJIRNLJTPM7wL507aXVpZaly9y7V7PkaeLC69J6k=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=HoVhIyYm4EnEMnk5qXn6L0Rw/qRCDYcrO4inuFB8vpHDuDtD32HzE4xLprJiX38ZX eTgEbktWBRW8UHk0lKtUX3meCoQXqACQSpPlt7k1lnMZDZAjHcTqYliy6uaZKhvdBX EWcRfzv5zikih5dCRAmowXJYbK6A+sRr/dY3G8RE= Date: Tue, 17 Nov 2020 10:36:23 +0000 From: Will Deacon To: Peter Zijlstra Subject: Re: [PATCH] sched: Fix data-race in wakeup Message-ID: <20201117103622.GA32035@willie-the-truck> 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> <20201117094621.GE3121429@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201117094621.GE3121429@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201117_053629_436063_8D9163C0 X-CRM114-Status: GOOD ( 21.83 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Davidlohr Bueso , Mel Gorman , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue, Nov 17, 2020 at 10:46:21AM +0100, Peter Zijlstra wrote: > 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; Much better, thanks! Will _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel