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=-8.7 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 79F82C5519F for ; Tue, 17 Nov 2020 15:37:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 1A6E52463C for ; Tue, 17 Nov 2020 15:37:54 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726298AbgKQPhe (ORCPT ); Tue, 17 Nov 2020 10:37:34 -0500 Received: from foss.arm.com ([217.140.110.172]:60078 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725790AbgKQPhd (ORCPT ); Tue, 17 Nov 2020 10:37:33 -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 B5F0BD6E; Tue, 17 Nov 2020 07:37:32 -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 B948E3F718; Tue, 17 Nov 2020 07:37:31 -0800 (PST) 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> 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 Message-ID: In-reply-to: Date: Tue, 17 Nov 2020 15:37:24 +0000 MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 17/11/20 12:52, Valentin Schneider wrote: > On 17/11/20 09:46, Peter Zijlstra wrote: >> 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. >> + */ > > 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 > That should be giving the right > ordering for the rest of ttwu() wrt. those 'current' bits, considering they > are written before that smp_mb__after_spinlock(). > > In any case, consider me convinced: > > Reviewed-by: Valentin Schneider > >> + unsigned sched_remote_wakeup:1; >> + >> /* Bit to tell LSMs we're in execve(): */ >> unsigned in_execve:1; >> unsigned in_iowait:1;