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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 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 B948AC43603 for ; Tue, 10 Dec 2019 22:56:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 791052073B for ; Tue, 10 Dec 2019 22:56:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576018608; bh=XWDREuj9dAdtDalevfFif+p6LIl9S2O4mIRyDr996/4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=pnd7gwF4FFxKrxNyO6gn975LHINJ4isfyzSgooMehho8L1TDHjKavaVVigoss4Ryo Qdm6FFuPgwgvNByebOKjMYu3CQ7RkiNmufBgOecu25jGtwnwVauRbmh7gkvUz9JmOZ 6nMSI3GphazmkTdtOr+/9xsfieUSHgRp66oChlhQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727257AbfLJW4r (ORCPT ); Tue, 10 Dec 2019 17:56:47 -0500 Received: from mail.kernel.org ([198.145.29.99]:41650 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726892AbfLJW4r (ORCPT ); Tue, 10 Dec 2019 17:56:47 -0500 Received: from paulmck-ThinkPad-P72.home (unknown [199.201.64.130]) (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 994B92053B; Tue, 10 Dec 2019 22:56:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1576018605; bh=XWDREuj9dAdtDalevfFif+p6LIl9S2O4mIRyDr996/4=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=SP4ifQLvAuOKWVhyW64pmq5mrOWwJOo/YYVIh5Bul9lwGAl/Y51PLhuzJm+uNok70 PHrhLHfHvp2cT6k9SFs2hUM7kTGbt+ly0Z99oT74r0JkEo6EG/ImO2NzR3Suow7ROL rrHYMqQbm9y5He1w7pDuYDGPu0kCc2q4cxtDvO+w= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 3A68F352276D; Tue, 10 Dec 2019 14:56:45 -0800 (PST) Date: Tue, 10 Dec 2019 14:56:45 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: Tejun Heo , jiangshanlai@gmail.com, linux-kernel@vger.kernel.org, Ingo Molnar , Thomas Gleixner Subject: Re: Workqueues splat due to ending up on wrong CPU Message-ID: <20191210225645.GW2889@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20191202201338.GH16681@devbig004.ftw2.facebook.com> <20191203095521.GH2827@hirez.programming.kicks-ass.net> <20191204201150.GA14040@paulmck-ThinkPad-P72> <20191205102928.GG2810@hirez.programming.kicks-ass.net> <20191205103213.GB2871@hirez.programming.kicks-ass.net> <20191205144805.GR2889@paulmck-ThinkPad-P72> <20191206185208.GA25636@paulmck-ThinkPad-P72> <20191206220020.GA27511@paulmck-ThinkPad-P72> <20191209185908.GA8470@paulmck-ThinkPad-P72> <20191210090839.GJ2844@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20191210090839.GJ2844@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Dec 10, 2019 at 10:08:39AM +0100, Peter Zijlstra wrote: > On Mon, Dec 09, 2019 at 10:59:08AM -0800, Paul E. McKenney wrote: > > And it survived! ;-) > > > > Peter, could I please have your Signed-off-by? Or take my Tested-by if > > you would prefer to send it up some other way. > > How's this? Very good, thank you! I have queued it on -rcu, but please let me know if you would rather that it go in via some other path. Thanx, Paul > --- > Subject: cpu/hotplug, stop_machine: Fix stop_machine vs hotplug order > From: Peter Zijlstra > Date: Tue Dec 10 09:34:54 CET 2019 > > Paul reported a very sporadic, rcutorture induced, workqueue failure. > When the planets align, the workqueue rescuer's self-migrate fails and > then triggers a WARN for running a work on the wrong CPU. > > Tejun then figured that set_cpus_allowed_ptr()'s stop_one_cpu() call > could be ignored! When stopper->enabled is false, stop_machine will > insta complete the work, without actually doing the work. Worse, it > will not WARN about this (we really should fix this). > > It turns out there is a small window where a freshly online'ed CPU is > marked 'online' but doesn't yet have the stopper task running: > > BP AP > > bringup_cpu() > __cpu_up(cpu, idle) --> start_secondary() > ... > cpu_startup_entry() > bringup_wait_for_ap() > wait_for_ap_thread() <-- cpuhp_online_idle() > while (1) > do_idle() > > ... available to run kthreads ... > > stop_machine_unpark() > stopper->enable = true; > > Close this by moving the stop_machine_unpark() into > cpuhp_online_idle(), such that the stopper thread is ready before we > start the idle loop and schedule. > > Reported-by: "Paul E. McKenney" > Debugged-by: Tejun Heo > Signed-off-by: Peter Zijlstra (Intel) > --- > --- a/kernel/cpu.c > +++ b/kernel/cpu.c > @@ -525,8 +525,7 @@ static int bringup_wait_for_ap(unsigned > if (WARN_ON_ONCE((!cpu_online(cpu)))) > return -ECANCELED; > > - /* Unpark the stopper thread and the hotplug thread of the target cpu */ > - stop_machine_unpark(cpu); > + /* Unpark the hotplug thread of the target cpu */ > kthread_unpark(st->thread); > > /* > @@ -1089,8 +1088,8 @@ void notify_cpu_starting(unsigned int cp > > /* > * Called from the idle task. Wake up the controlling task which brings the > - * stopper and the hotplug thread of the upcoming CPU up and then delegates > - * the rest of the online bringup to the hotplug thread. > + * hotplug thread of the upcoming CPU up and then delegates the rest of the > + * online bringup to the hotplug thread. > */ > void cpuhp_online_idle(enum cpuhp_state state) > { > @@ -1100,6 +1099,12 @@ void cpuhp_online_idle(enum cpuhp_state > if (state != CPUHP_AP_ONLINE_IDLE) > return; > > + /* > + * Unpart the stopper thread before we start the idle loop (and start > + * scheduling); this ensures the stopper task is always available. > + */ > + stop_machine_unpark(smp_processor_id()); > + > st->state = CPUHP_AP_ONLINE_IDLE; > complete_ap_thread(st, true); > }