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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6DED9C43334 for ; Thu, 7 Jul 2022 16:20:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S235843AbiGGQUF (ORCPT ); Thu, 7 Jul 2022 12:20:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:59834 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230209AbiGGQUD (ORCPT ); Thu, 7 Jul 2022 12:20:03 -0400 Received: from ams.source.kernel.org (ams.source.kernel.org [IPv6:2604:1380:4601:e00::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 8B0C725EAF; Thu, 7 Jul 2022 09:20:02 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by ams.source.kernel.org (Postfix) with ESMTPS id 46F2FB82124; Thu, 7 Jul 2022 16:20:01 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 9377DC3411E; Thu, 7 Jul 2022 16:19:58 +0000 (UTC) Date: Thu, 7 Jul 2022 12:19:56 -0400 From: Steven Rostedt To: John Keeping Cc: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org, Sebastian Andrzej Siewior , Peter Zijlstra , Thomas Gleixner , Ingo Molnar , Juri Lelli , Vincent Guittot , Dietmar Eggemann , Ben Segall , Mel Gorman , Daniel Bristot de Oliveira , Valentin Schneider Subject: Re: [PATCH] sched/core: Always flush pending blk_plug Message-ID: <20220707121956.0ab19fd8@gandalf.local.home> In-Reply-To: <20220707143902.529938-1-john@metanate.com> References: <20220707143902.529938-1-john@metanate.com> X-Mailer: Claws Mail 3.17.8 (GTK+ 2.24.33; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 7 Jul 2022 15:39:02 +0100 John Keeping wrote: > Here the kworker is waiting on msdos_sb_info::s_lock which is held by > tar which is in turn waiting for a buffer which is locked waiting to be > flushed, but this operation is plugged in the kworker. > > The lock is a normal struct mutex, so tsk_is_pi_blocked() will always > return false on !RT and thus the behaviour changes for RT. > > It seems that the intent here is to skip blk_flush_plug() in the case > where a non-preemptible lock (such as a spinlock) has been converted to > a rtmutex on RT, which is the case covered by the SM_RTLOCK_WAIT > schedule flag. But sched_submit_work() is only called from schedule() > which is never called in this scenario, so the check can simply be > deleted. > > Looking at the history of the -rt patchset, in fact this change was > present from v5.9.1-rt20 until being dropped in v5.13-rt1 as it was part > of a larger patch [1] most of which was replaced by commit b4bfa3fcfe3b > ("sched/core: Rework the __schedule() preempt argument"). > Nice investigation. So basically what you are saying is that commit b4bfa3fcfe3b was the implementation of [1], but left out the removal of the tsk_is_pi_blocked(), and that what you are seeing is the problem that is described in [1]. Can you add this in the change log: "As described in [1]: The schedule process must distinguish between blocking on a regular sleeping lock (rwsem and mutex) and a RT-only sleeping lock (spinlock and rwlock): - rwsem and mutex must flush block requests (blk_schedule_flush_plug()) even if blocked on a lock. This can not deadlock because this also happens for non-RT. There should be a warning if the scheduling point is within a RCU read section. - spinlock and rwlock must not flush block requests. This will deadlock if the callback attempts to acquire a lock which is already acquired. Similarly to being preempted, there should be no warning if the scheduling point is within a RCU read section. and with the tsk_is_pi_blocked() in the scheduler path, we hit the first issue." Reviewed-by: Steven Rostedt (Google) -- Steve