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 5208CC4363A for ; Thu, 29 Oct 2020 18:10:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id F3B41207DE for ; Thu, 29 Oct 2020 18:10:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725984AbgJ2SKD (ORCPT ); Thu, 29 Oct 2020 14:10:03 -0400 Received: from foss.arm.com ([217.140.110.172]:42026 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725807AbgJ2SJ7 (ORCPT ); Thu, 29 Oct 2020 14:09:59 -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 17352139F; Thu, 29 Oct 2020 11:09:59 -0700 (PDT) Received: from e113632-lin (e113632-lin.cambridge.arm.com [10.1.194.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id ED5733F66E; Thu, 29 Oct 2020 11:09:56 -0700 (PDT) References: <20201023101158.088940906@infradead.org> <20201023102347.406912197@infradead.org> <20201029173832.GU2628@hirez.programming.kicks-ass.net> User-agent: mu4e 0.9.17; emacs 26.3 From: Valentin Schneider To: Peter Zijlstra Cc: tglx@linutronix.de, mingo@kernel.org, linux-kernel@vger.kernel.org, bigeasy@linutronix.de, qais.yousef@arm.com, swood@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org, dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de, bristot@redhat.com, vincent.donnefort@arm.com, tj@kernel.org, ouwen210@hotmail.com Subject: Re: [PATCH v4 14/19] sched, lockdep: Annotate ->pi_lock recursion In-reply-to: <20201029173832.GU2628@hirez.programming.kicks-ass.net> Date: Thu, 29 Oct 2020 18:09:54 +0000 Message-ID: MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 29/10/20 17:38, Peter Zijlstra wrote: > On Thu, Oct 29, 2020 at 04:27:16PM +0000, Valentin Schneider wrote: >> >> On 23/10/20 11:12, Peter Zijlstra wrote: >> > @@ -2617,6 +2618,20 @@ void sched_set_stop_task(int cpu, struct >> > sched_setscheduler_nocheck(stop, SCHED_FIFO, ¶m); >> > >> > stop->sched_class = &stop_sched_class; >> > + >> > + /* >> > + * The PI code calls rt_mutex_setprio() with ->pi_lock held to >> > + * adjust the effective priority of a task. As a result, >> > + * rt_mutex_setprio() can trigger (RT) balancing operations, >> > + * which can then trigger wakeups of the stop thread to push >> > + * around the current task. >> > + * >> > + * The stop task itself will never be part of the PI-chain, it >> > + * never blocks, therefore that ->pi_lock recursion is safe. >> >> Isn't it that the stopper task can only run when preemption is re-enabled, >> and the ->pi_lock is dropped before then? >> >> If we were to have an SCA-like function that would kick the stopper but >> "forget" to release the pi_lock, then we would very much like lockdep to >> complain, right? Or is that something else entirely? > > You've forgotten the other, and original, purpose of ->pi_lock, guarding > the actual PI chain. Please have a look at rt_mutex_adjust_prio_chain() > and its comment. > > But no, this isn't about running, this is about doing an actual wakeup > (of the stopper task) while holding an ->pi_lock instance (guaranteed > not the stopper task's). And since wakeup will take ->pi_lock, lockdep > will get all whiny about ->pi_lock self recursion. > Gotcha. Thanks, and apologies for the noise. >> > + * Tell lockdep about this by placing the stop->pi_lock in its >> > + * own class. >> > + */ >> > + lockdep_set_class(&stop->pi_lock, &stop_pi_lock); >> > } >> > >> > cpu_rq(cpu)->stop = stop;