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=-2.1 required=3.0 tests=DKIM_INVALID,DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,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 B6214C3F2CD for ; Thu, 5 Mar 2020 08:13:55 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 867002146E for ; Thu, 5 Mar 2020 08:13:55 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="L51CopMg" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726059AbgCEINy (ORCPT ); Thu, 5 Mar 2020 03:13:54 -0500 Received: from merlin.infradead.org ([205.233.59.134]:58598 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725816AbgCEINy (ORCPT ); Thu, 5 Mar 2020 03:13:54 -0500 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=merlin.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=YdEB9LdscQVobrvA17oqJJGGJE1kljWNj63rZ1r26J0=; b=L51CopMgQxp/klLcoYR/ooVg5o Tv8EWhZDfxO77RCfawXJEIDVXEkSOYwMmFDrn4C6nE92puh4AlfRiYdmeMAKJZAS1Qp5IH6oxmQoo FOXw4Ed6zSqxRZBNx2k5lOZLHn4ISCOYf/DVN7KpxO22fsVbYAHgGh7HC3QKl12vjJUgNoD6lXUJ7 PnHE20l37riUnFiwQz2xK+W5iJS6QT2u5cWix9agnpqTG6mF5ZFDL20MJlvx07OysbY06ERHjGPQD kp417MhOMqrD9kRUQR82EFFI1e4yC/9k9ZbJL9c5f37Ajvjy1Rf9CDC4n8NQtiBngP2LiDin2Y69g Vxof6irA==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=noisy.programming.kicks-ass.net) by merlin.infradead.org with esmtpsa (Exim 4.92.3 #3 (Red Hat Linux)) id 1j9ldT-0007Uy-Si; Thu, 05 Mar 2020 08:13:40 +0000 Received: from hirez.programming.kicks-ass.net (hirez.programming.kicks-ass.net [192.168.1.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (Client did not present a certificate) by noisy.programming.kicks-ass.net (Postfix) with ESMTPS id 6A2083013A4; Thu, 5 Mar 2020 09:11:38 +0100 (CET) Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id 4F6E52599AFBF; Thu, 5 Mar 2020 09:13:37 +0100 (CET) Date: Thu, 5 Mar 2020 09:13:37 +0100 From: Peter Zijlstra To: "Paul E. McKenney" 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: <20200305081337.GA2619@hirez.programming.kicks-ass.net> References: <20200305005049.GA21120@paulmck-ThinkPad-P72> <20200305080755.GS2596@hirez.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200305080755.GS2596@hirez.programming.kicks-ass.net> User-Agent: Mutt/1.10.1 (2018-07-13) 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 09:07:55AM +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.