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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 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 8D65BC3F2D1 for ; Thu, 5 Mar 2020 14:28:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6A1482073D for ; Thu, 5 Mar 2020 14:28:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726233AbgCEO2t (ORCPT ); Thu, 5 Mar 2020 09:28:49 -0500 Received: from mail.kernel.org ([198.145.29.99]:59362 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726087AbgCEO2t (ORCPT ); Thu, 5 Mar 2020 09:28:49 -0500 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (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 0173420732; Thu, 5 Mar 2020 14:28:46 +0000 (UTC) Date: Thu, 5 Mar 2020 09:28:45 -0500 From: Steven Rostedt To: "Paul E. McKenney" Cc: Peter Zijlstra , mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, bsegall@google.com, mgorman@suse.de, linux-kernel@vger.kernel.org Subject: Re: Pinning down a blocked task to extract diagnostics Message-ID: <20200305092845.4296c35e@gandalf.local.home> In-Reply-To: <20200305142245.GB2935@paulmck-ThinkPad-P72> References: <20200305005049.GA21120@paulmck-ThinkPad-P72> <20200305080755.GS2596@hirez.programming.kicks-ass.net> <20200305081337.GA2619@hirez.programming.kicks-ass.net> <20200305142245.GB2935@paulmck-ThinkPad-P72> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 5 Mar 2020 06:22:45 -0800 "Paul E. McKenney" wrote: > On Thu, Mar 05, 2020 at 09:13:37AM +0100, Peter Zijlstra wrote: > > On Thu, Mar 05, 2020 at 09:07:55AM +0100, Peter Zijlstra wrote: > > > On Wed, Mar 04, 2020 at 04:50:49PM -0800, Paul E. McKenney wrote: > > > > Hello! > > > > > > > > Suppose that I need to extract diagnostics information from a blocked > > > > task, but that I absolutely cannot tolerate this task awakening in the > > > > midst of this extraction process. Is the following code the right way > > > > to make this work given a task "t"? > > > > > > > > raw_spin_lock_irq(&t->pi_lock); > > > > if (t->on_rq) { > > > > /* Task no longer blocked, so ignore it. */ > > > > } else { > > > > /* Extract consistent diagnostic information. */ > > > > } > > > > raw_spin_unlock_irq(&t->pi_lock); > > > > > > > > It looks like all the wakeup paths acquire ->pi_lock, but I figured I > > > > should actually ask... > > > > > > Close, the thing pi_lock actually guards is the t->state transition *to* > > > TASK_WAKING/TASK_RUNNING, so something like this: > > > > Almost, we must indeed also check ->on_rq, otherwise it might change the > > state back itself. > > > > > > > > raw_spin_lock_irq(&t->pi_lock); > > > switch (t->state) { > > > case TASK_RUNNING: > > > case TASK_WAKING: > > > /* ignore */ > > > break; > > > > > > default: > > if (t->on_rq) > > break; > > > > > /* Extract consistent diagnostic information. */ > > > break; > > > } > > > raw_spin_unlock_irq(&t->pi_lock); > > > > > > ought to work. But if you're going to do this, please add a reference to > > > that code in a comment on top of try_to_wake_up(), such that we can > > > later find all the code that relies on this. > > How about if I add something like this, located right by try_to_wake_up()? > > bool try_to_keep_sleeping(struct task_struct *t) > { > raw_spin_lock_irq(&t->pi_lock); > switch (t->state) { > case TASK_RUNNING: > case TASK_WAKING: > raw_spin_unlock_irq(&t->pi_lock); > return false; > > default: > if (t->on_rq) { Somehow I think there still needs to be a read barrier before the test to on_rq. > raw_spin_unlock_irq(&t->pi_lock); > return false; > } > > /* OK to extract consistent diagnostic information. */ > return true; > } > /* NOTREACHED */ > } > > Then a use might look like this: > > if (try_to_keep_sleeping(t)) > /* Extract consistent diagnostic information. */ > raw_spin_unlock_irq(&t->pi_lock); Perhaps we should have a allow_awake(t) to match it? allow_awake(t); Where we have: static inline allow_awake(struct task_struct *t) { raw_spin_unlock_irq(&t->pi_lock); } -- Steve? > } else { > /* Woo-hoo! It started running again!!! */ > } > > Is there a better way to approach this? > > Thanx, Paul