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=-0.8 required=3.0 tests=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 98FB9C4CECD for ; Mon, 16 Sep 2019 16:56:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 789BC20678 for ; Mon, 16 Sep 2019 16:56:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730002AbfIPQ4F (ORCPT ); Mon, 16 Sep 2019 12:56:05 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49752 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727821AbfIPQ4F (ORCPT ); Mon, 16 Sep 2019 12:56:05 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.phx2.redhat.com [10.5.11.14]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 078A318CB8E2; Mon, 16 Sep 2019 16:56:05 +0000 (UTC) Received: from ovpn-117-172.phx2.redhat.com (ovpn-117-172.phx2.redhat.com [10.3.117.172]) by smtp.corp.redhat.com (Postfix) with ESMTP id 6C9E75D9DC; Mon, 16 Sep 2019 16:55:58 +0000 (UTC) Message-ID: <500cabaa80f250b974409ee4a4fca59bf2e24564.camel@redhat.com> Subject: Re: [PATCH RT v3 5/5] rcutorture: Avoid problematic critical section nesting on RT From: Scott Wood To: Joel Fernandes Cc: Sebastian Andrzej Siewior , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, "Paul E . McKenney" , Thomas Gleixner , Steven Rostedt , Peter Zijlstra , Juri Lelli , Clark Williams Date: Mon, 16 Sep 2019 11:55:57 -0500 In-Reply-To: <20190912221706.GC150506@google.com> References: <20190911165729.11178-1-swood@redhat.com> <20190911165729.11178-6-swood@redhat.com> <20190912221706.GC150506@google.com> Organization: Red Hat Content-Type: text/plain; charset="UTF-8" User-Agent: Evolution 3.30.5 (3.30.5-1.fc29) MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.14 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.6.2 (mx1.redhat.com [10.5.110.63]); Mon, 16 Sep 2019 16:56:05 +0000 (UTC) Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On Thu, 2019-09-12 at 18:17 -0400, Joel Fernandes wrote: > On Wed, Sep 11, 2019 at 05:57:29PM +0100, Scott Wood wrote: > > rcutorture was generating some nesting scenarios that are not > > reasonable. Constrain the state selection to avoid them. > > > > Example #1: > > > > 1. preempt_disable() > > 2. local_bh_disable() > > 3. preempt_enable() > > 4. local_bh_enable() > > > > On PREEMPT_RT, BH disabling takes a local lock only when called in > > non-atomic context. Thus, atomic context must be retained until after > > BH > > is re-enabled. Likewise, if BH is initially disabled in non-atomic > > context, it cannot be re-enabled in atomic context. > > > > Example #2: > > > > 1. rcu_read_lock() > > 2. local_irq_disable() > > 3. rcu_read_unlock() > > 4. local_irq_enable() > > If I understand correctly, these examples are not unrealistic in the real > world unless RCU is used in the scheduler. I hope you mean "not realistic", at least when it comes to explicit preempt/irq disabling rather than spinlock variants that don't disable preempt/irqs on PREEMPT_RT. > > If the thread is preempted between steps 1 and 2, > > rcu_read_unlock_special.b.blocked will be set, but it won't be > > acted on in step 3 because IRQs are disabled. Thus, reporting of the > > quiescent state will be delayed beyond the local_irq_enable(). > > Yes, with consolidated RCU this can happen but AFAIK it has not seen to be > a > problem since deferred QS reporting will happen take care of it, which can > also happen from subsequent rcu_read_unlock_special(). The defer_qs_iw_pending stuff isn't in 5.2-rt. Still, given patch 4/5 (and special.b.deferred_qs on mainline) this shouldn't present a deadlock concern (letting the test run a bit now to double check) so this patch could probably be limited to the "example #1" sequence. > > For now, these scenarios will continue to be tested on non-PREEMPT_RT > > kernels, until debug checks are added to ensure that they are not > > happening elsewhere. > > Are you seeing real issues that need this patch? It would be good to not > complicate rcutorture if not needed. rcutorture crashes on RT without this patch (in particular due to the local_bh_disable misordering). -Scott