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 498C5C3F2CD for ; Thu, 5 Mar 2020 04:27:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 13A7E208CD for ; Thu, 5 Mar 2020 04:27:08 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583382429; bh=WZjQCTkf8uRUWaIOBbpLwjySe/6G1sEj2Z+vxOKRr9A=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=uMXzauP8Cehk5X9aSG5jqqBkxBEbdNwEhsvuANYw9PfanMFxtwiEBQn5wBtZo00aV eSh9buYasmIygJjbTWv2ji2B2aH25gSvuzTHvxepM7l36ow6peClF8xgpZ7/19mfcK Q4HXJwbmKqW1I5R05zrv4ePKWsi83k8bKyCmPaOc= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725893AbgCEEZl (ORCPT ); Wed, 4 Mar 2020 23:25:41 -0500 Received: from mail.kernel.org ([198.145.29.99]:41134 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725818AbgCEEZl (ORCPT ); Wed, 4 Mar 2020 23:25:41 -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 BBF702073B; Thu, 5 Mar 2020 04:25:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1583382340; bh=WZjQCTkf8uRUWaIOBbpLwjySe/6G1sEj2Z+vxOKRr9A=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=tW3MUexwEc0MlHeHCOIaECvT8QRiUww+6GXde+GuzAtu0PuJm4awXkFvUog+4IOl+ slVeG/r4uhIs6UHPL8B8eByG56KVfMITBGmaP9fO7kesTFM4PpSiyypU6plYUYo5Yj 9kR4o8qaFACqkYjf02LWhP9h/Aoe2YK3QLnrvAAY= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 92E8735227C6; Wed, 4 Mar 2020 20:25:40 -0800 (PST) Date: Wed, 4 Mar 2020 20:25:40 -0800 From: "Paul E. McKenney" To: Steven Rostedt Cc: mingo@redhat.com, peterz@infradead.org, 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: <20200305042540.GZ2935@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200305005049.GA21120@paulmck-ThinkPad-P72> <20200304225226.6d631baa@oasis.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200304225226.6d631baa@oasis.local.home> 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 Wed, Mar 04, 2020 at 10:52:26PM -0500, Steven Rostedt wrote: > On Wed, 4 Mar 2020 16:50:49 -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... > > IIUC, the rtmutex code uses pi_lock to tinker with the task while it is > blocked (not woken). But the on_rq test may not be correct. It appears > that can change without holding the task's pi_lock. I'm looking at the > ttwu_do_activate() code which modifies the t->on_rq without holding the > pi_lock. Seems you may need to check the p->state as well. See the > comment in try_to_wake_up() about testing on_rq vs the state. Thank you, Steve! Thanx, Paul