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=-16.7 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=unavailable 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 A5386C432BE for ; Tue, 27 Jul 2021 16:24:05 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8DC2E61BA1 for ; Tue, 27 Jul 2021 16:24:05 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229553AbhG0QYE (ORCPT ); Tue, 27 Jul 2021 12:24:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:32918 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229831AbhG0QYC (ORCPT ); Tue, 27 Jul 2021 12:24:02 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id A869461B96; Tue, 27 Jul 2021 16:24:02 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627403042; bh=Ffx/4O59u5b+9VZUiul6boaYE12c631vF/S8JIot1Go=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=U7j/ZhdydU58Pwnn/TKcb9cHWrS+jc5uVJs6/vqwSuGt/O41ikF5w3hmdeq9z/8fS FI3q+on0XddcEkd+XxTcWoQIUQANj4+1h5e2Pw9ndog22uE7KbucKB9cJrCKY1YVbf vA154cuthbtkRpKOGuNan3pnzpXdedLqA5I2zFYpervejvw5eYjTIv4F7fucexGzy8 C0aenzO0jdLInebFA1mL0eCWaKI7/eeoZaoO9H+LJHdQR1d/4M+W1zr1eaaMYGrEgK XGztI3Pu2U/Pu5EQe6m1iypsrFsBD5QGoZmIoZgjJEGwT+GqUdW3DX0LL3OelznBcv TQ596hw1VPhTA== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 83BDB5C099F; Tue, 27 Jul 2021 09:24:02 -0700 (PDT) Date: Tue, 27 Jul 2021 09:24:02 -0700 From: "Paul E. McKenney" To: Valentin Schneider Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-rt-users@vger.kernel.org, Catalin Marinas , Will Deacon , Ingo Molnar , Peter Zijlstra , Thomas Gleixner , Steven Rostedt , Daniel Bristot de Oliveira , Josh Triplett , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , Anshuman Khandual , Vincenzo Frascino , Steven Price , Ard Biesheuvel Subject: Re: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability Message-ID: <20210727162402.GB4397@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20210721115118.729943-1-valentin.schneider@arm.com> <20210721115118.729943-3-valentin.schneider@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210721115118.729943-3-valentin.schneider@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote: > Running v5.13-rt1 on my arm64 Juno board triggers: > > [ 0.156302] ============================= > [ 0.160416] WARNING: suspicious RCU usage > [ 0.164529] 5.13.0-rt1 #20 Not tainted > [ 0.168300] ----------------------------- > [ 0.172409] kernel/rcu/tree_plugin.h:69 Unsafe read of RCU_NOCB offloaded state! > [ 0.179920] > [ 0.179920] other info that might help us debug this: > [ 0.179920] > [ 0.188037] > [ 0.188037] rcu_scheduler_active = 1, debug_locks = 1 > [ 0.194677] 3 locks held by rcuc/0/11: > [ 0.198448] #0: ffff00097ef10cf8 ((softirq_ctrl.lock).lock){+.+.}-{2:2}, at: __local_bh_disable_ip (./include/linux/rcupdate.h:662 kernel/softirq.c:171) > [ 0.208709] #1: ffff80001205e5f0 (rcu_read_lock){....}-{1:2}, at: rt_spin_lock (kernel/locking/spinlock_rt.c:43 (discriminator 4)) > [ 0.217134] #2: ffff80001205e5f0 (rcu_read_lock){....}-{1:2}, at: __local_bh_disable_ip (kernel/softirq.c:169) > [ 0.226428] > [ 0.226428] stack backtrace: > [ 0.230889] CPU: 0 PID: 11 Comm: rcuc/0 Not tainted 5.13.0-rt1 #20 > [ 0.237100] Hardware name: ARM Juno development board (r0) (DT) > [ 0.243041] Call trace: > [ 0.245497] dump_backtrace (arch/arm64/kernel/stacktrace.c:163) > [ 0.249185] show_stack (arch/arm64/kernel/stacktrace.c:219) > [ 0.252522] dump_stack (lib/dump_stack.c:122) > [ 0.255947] lockdep_rcu_suspicious (kernel/locking/lockdep.c:6439) > [ 0.260328] rcu_rdp_is_offloaded (kernel/rcu/tree_plugin.h:69 kernel/rcu/tree_plugin.h:58) > [ 0.264537] rcu_core (kernel/rcu/tree.c:2332 kernel/rcu/tree.c:2398 kernel/rcu/tree.c:2777) > [ 0.267786] rcu_cpu_kthread (./include/linux/bottom_half.h:32 kernel/rcu/tree.c:2876) > [ 0.271644] smpboot_thread_fn (kernel/smpboot.c:165 (discriminator 3)) > [ 0.275767] kthread (kernel/kthread.c:321) > [ 0.279013] ret_from_fork (arch/arm64/kernel/entry.S:1005) > > In this case, this is the RCU core kthread accessing the local CPU's > rdp. Before that, rcu_cpu_kthread() invokes local_bh_disable(). > > Under !CONFIG_PREEMPT_RT (and rcutree.use_softirq=0), this ends up > incrementing the preempt_count, which satisfies the "local non-preemptible > read" of rcu_rdp_is_offloaded(). > > Under CONFIG_PREEMPT_RT however, this becomes > > local_lock(&softirq_ctrl.lock) > > which, under the same config, is migrate_disable() + rt_spin_lock(). > This *does* prevent the task from migrating away, but not in a way > rcu_rdp_is_offloaded() can notice. Note that the invoking task is an > smpboot thread, and thus cannot be migrated away in the first place. > > Check is_pcpu_safe() here rather than preemptible(). > > Signed-off-by: Valentin Schneider Acked-by: Paul E. McKenney > --- > kernel/rcu/tree_plugin.h | 3 +-- > 1 file changed, 1 insertion(+), 2 deletions(-) > > diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h > index ad0156b86937..6c3c4100da83 100644 > --- a/kernel/rcu/tree_plugin.h > +++ b/kernel/rcu/tree_plugin.h > @@ -70,8 +70,7 @@ static bool rcu_rdp_is_offloaded(struct rcu_data *rdp) > !(lockdep_is_held(&rcu_state.barrier_mutex) || > (IS_ENABLED(CONFIG_HOTPLUG_CPU) && lockdep_is_cpus_held()) || > rcu_lockdep_is_held_nocb(rdp) || > - (rdp == this_cpu_ptr(&rcu_data) && > - !(IS_ENABLED(CONFIG_PREEMPT_COUNT) && preemptible())) || > + (rdp == this_cpu_ptr(&rcu_data) && is_pcpu_safe()) || > rcu_current_is_nocb_kthread(rdp) || > rcu_running_nocb_timer(rdp)), > "Unsafe read of RCU_NOCB offloaded state" > -- > 2.25.1 >