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=-2.6 required=3.0 tests=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 06AC0C3F2D7 for ; Thu, 5 Mar 2020 14:22:49 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D319120828 for ; Thu, 5 Mar 2020 14:22:48 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583418168; bh=TJrx6Vvx7IBPDbBVQ+OIz/BKikJsmMyfPgbEMTzoUJ8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=NKFTK1PRZ/85m8z8FM6LD+JU9CUvlp+LglbaF1XbFYBjSJbgrikAktzhAynizZI6U SHt7nu3xRjFYjYYugt0KLLSNpHuyXja7/mTeNjnhj2SLk0rAOmXVfgqrsPKoBH42qg Ly1duDfUV5/h/IfPnPkxErcvn9tMsdaq1HtEs9gw= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726317AbgCEOWr (ORCPT ); Thu, 5 Mar 2020 09:22:47 -0500 Received: from mail.kernel.org ([198.145.29.99]:57816 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726211AbgCEOWq (ORCPT ); Thu, 5 Mar 2020 09:22:46 -0500 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 ACFAB2073D; Thu, 5 Mar 2020 14:22:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583418165; bh=TJrx6Vvx7IBPDbBVQ+OIz/BKikJsmMyfPgbEMTzoUJ8=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=u7vjh0iKHd4Ot1+Kf4/5E1OvS+n+5cebbcrUIk2mcJNUkYzcCZljz23uOmj7LFN9U aw4mICHUMS87+HRBERcKpZBDlwRbKbctUZqI8V/x6kD6Z5rvdLw0qWlQiHGWoGX/M2 jsM1qtZdlpJnYrWhdBeqnRv3vtS2KAxZzuynIxWg= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 7BBD435226D4; Thu, 5 Mar 2020 06:22:45 -0800 (PST) Date: Thu, 5 Mar 2020 06:22:45 -0800 From: "Paul E. McKenney" To: Peter Zijlstra Cc: mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, linux-kernel@vger.kernel.org Subject: Re: Pinning down a blocked task to extract diagnostics Message-ID: <20200305142245.GB2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200305005049.GA21120@paulmck-ThinkPad-P72> <20200305080755.GS2596@hirez.programming.kicks-ass.net> <20200305081337.GA2619@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200305081337.GA2619@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 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) { 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); } else { /* Woo-hoo! It started running again!!! */ } Is there a better way to approach this? Thanx, Paul