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=-13.8 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,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 959BFC4338F for ; Wed, 28 Jul 2021 19:34:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 732E16023D for ; Wed, 28 Jul 2021 19:34:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231146AbhG1Te2 (ORCPT ); Wed, 28 Jul 2021 15:34:28 -0400 Received: from foss.arm.com ([217.140.110.172]:35262 "EHLO foss.arm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229986AbhG1TeX (ORCPT ); Wed, 28 Jul 2021 15:34:23 -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 3B3401FB; Wed, 28 Jul 2021 12:34:21 -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 BFA463F66F; Wed, 28 Jul 2021 12:34:18 -0700 (PDT) From: Valentin Schneider To: Frederic Weisbecker 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 , "Paul E. McKenney" , 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 In-Reply-To: <20210727230814.GC283787@lothringen> References: <20210721115118.729943-1-valentin.schneider@arm.com> <20210721115118.729943-3-valentin.schneider@arm.com> <20210727230814.GC283787@lothringen> Date: Wed, 28 Jul 2021 20:34:14 +0100 Message-ID: <87pmv2kzbd.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 28/07/21 01:08, Frederic Weisbecker wrote: > On Wed, Jul 21, 2021 at 12:51:17PM +0100, Valentin Schneider wrote: >> Signed-off-by: Valentin Schneider >> --- >> 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()) || > > I fear that won't work. We really need any caller of rcu_rdp_is_offloaded() > on the local rdp to have preemption disabled and not just migration disabled, > because we must protect against concurrent offloaded state changes. > > The offloaded state is changed by a workqueue that executes on the target rdp. > > Here is a practical example where it matters: > > CPU 0 > ----- > // =======> task rcuc running > rcu_core { > rcu_nocb_lock_irqsave(rdp, flags) { > if (!rcu_segcblist_is_offloaded(rdp->cblist)) { > // is not offloaded right now, so it's going > // to just disable IRQs. Oh no wait: > // preemption > // ========> workqueue running > rcu_nocb_rdp_offload(); > // ========> task rcuc resume > local_irq_disable(); > } > } > .... > rcu_nocb_unlock_irqrestore(rdp, flags) { > if (rcu_segcblist_is_offloaded(rdp->cblist)) { > // is offloaded right now so: > raw_spin_unlock_irqrestore(rdp, flags); > > And that will explode because that's an impaired unlock on nocb_lock. Harumph, that doesn't look good, thanks for pointing this out. AFAICT PREEMPT_RT doesn't actually require to disable softirqs here (since it forces RCU callbacks on the RCU kthreads), but disabled softirqs seem to be a requirement for much of the underlying functions and even some of the callbacks (delayed_put_task_struct() ~> vfree() pays close attention to in_interrupt() for instance). Now, if the offloaded state was (properly) protected by a local_lock, do you reckon we could then keep preemption enabled? >From a naive outsider PoV, rdp->nocb_lock looks like a decent candidate, but it's a *raw* spinlock (I can't tell right now whether changing this is a horrible idea or not), and then there's 81c0b3d724f4 ("rcu/nocb: Avoid ->nocb_lock capture by corresponding CPU") on top...