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=-3.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS 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 0239AC07E96 for ; Thu, 15 Jul 2021 14:08:45 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id D0E04613C3 for ; Thu, 15 Jul 2021 14:08:44 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238613AbhGOOLg (ORCPT ); Thu, 15 Jul 2021 10:11:36 -0400 Received: from foss.arm.com ([217.140.110.172]:53014 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229620AbhGOOLg (ORCPT ); Thu, 15 Jul 2021 10:11:36 -0400 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C45666D; Thu, 15 Jul 2021 07:08:42 -0700 (PDT) Received: from e113632-lin (usa-sjc-imap-foss1.foss.arm.com [10.121.207.14]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 3BF953F694; Thu, 15 Jul 2021 07:08:41 -0700 (PDT) From: Valentin Schneider To: Peter Zijlstra Cc: Thomas Gleixner , LKML , Ingo Molnar , Juri Lelli , Steven Rostedt , Daniel Bristot de Oliveira , Will Deacon , Waiman Long , Boqun Feng , Sebastian Andrzej Siewior , Davidlohr Bueso Subject: Re: [patch 03/50] sched: Prepare for RT sleeping spin/rwlocks In-Reply-To: <20210715092703.GI2725@worktop.programming.kicks-ass.net> References: <20210713151054.700719949@linutronix.de> <20210713160746.207208684@linutronix.de> <87r1g0mqir.mognet@arm.com> <20210715092703.GI2725@worktop.programming.kicks-ass.net> Date: Thu, 15 Jul 2021 15:08:36 +0100 Message-ID: <87im1bmzyz.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 15/07/21 11:27, Peter Zijlstra wrote: > On Thu, Jul 15, 2021 at 12:20:28AM +0100, Valentin Schneider wrote: >> Hi, >> >> On 13/07/21 17:10, Thomas Gleixner wrote: >> > From: Thomas Gleixner >> > >> > Waiting for spinlocks and rwlocks on non RT enabled kernels is task::state >> > preserving. Any wakeup which matches the state is valid. >> > >> > RT enabled kernels substitutes them with 'sleeping' spinlocks. This creates >> > an issue vs. task::state. >> > >> > In order to block on the lock the task has to overwrite task::state and a >> > consecutive wakeup issued by the unlocker sets the state back to >> > TASK_RUNNING. As a consequence the task loses the state which was set >> > before the lock acquire and also any regular wakeup targeted at the task >> > while it is blocked on the lock. >> > >> >> I'm not sure I get this for spinlocks - p->__state != TASK_RUNNING means >> task is stopped (or about to be), IMO that doesn't go with spinning. I was >> thinking perhaps ptrace could be an issue, but I don't have a clear picture >> on that either. What am I missing? > > spinlocks will become rtmutex. They're going to clobber __state by > virtue of a nested block. I wasn't expecting there to be any task taking spinlocks with state != TASK_RUNNING, but I just didn't know where to look. For instance do_wait() sets current to TASK_INTERRUPTIBLE and can then faff around with some sighand locks before eventually calling into schedule(), so clearly that one would be affected by the clobbering. The more you know...