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 7582FC432BE for ; Wed, 28 Jul 2021 22:01:42 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 58E6C60FED for ; Wed, 28 Jul 2021 22:01:42 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232078AbhG1WBm (ORCPT ); Wed, 28 Jul 2021 18:01:42 -0400 Received: from mail.kernel.org ([198.145.29.99]:46206 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231668AbhG1WBl (ORCPT ); Wed, 28 Jul 2021 18:01:41 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id 0BF6E60F5E; Wed, 28 Jul 2021 22:01:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627509699; bh=eT1ZYBz1kwcqDTxbLvzyK/4f/qPZ0QCHOcZMPByW5y0=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=abA7RTUyN/hXppfCfRb6ywSxGoiOkGSVe/n0+PWvd8OLQkmiM691COMzZYC/qO9cU z5VtYefEq9b8ZJpuZ4yKPSuvr5k/B0PK8KLFlGF3hiRH4o8e/qR0p2TgUixgromYnF RYgM9pbxUFXo4d8Rn2Acw/W1emcZyOFThFMzVufoQyyuddm/3IrGj6evxNQ+NU19Ek iXdgSSyYvpaNzTZshWBz8KyZpFn6IpF23c07BS5MHlfEwiknl0msaTRww6zDSFTJ5U dpldAlQo9OkIBsw0pObRlvcv4Wxjj8aLEobuVyLfczrKfUIP/uhPNMPgkw/YTgUqMT 5NJdXY7/Rx8cw== Date: Thu, 29 Jul 2021 00:01:37 +0200 From: Frederic Weisbecker 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 , "Paul E. McKenney" , Josh Triplett , Mathieu Desnoyers , Lai Jiangshan , Joel Fernandes , Anshuman Khandual , Vincenzo Frascino , Steven Price , Ard Biesheuvel , Sebastian Andrzej Siewior Subject: Re: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability Message-ID: <20210728220137.GD293265@lothringen> References: <20210721115118.729943-1-valentin.schneider@arm.com> <20210721115118.729943-3-valentin.schneider@arm.com> <20210727230814.GC283787@lothringen> <87pmv2kzbd.mognet@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87pmv2kzbd.mognet@arm.com> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 28, 2021 at 08:34:14PM +0100, Valentin Schneider wrote: > 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? I guess we could take such a local lock on the update side (rcu_nocb_rdp_offload) and then take it on rcuc kthread/softirqs and maybe other places. But we must make sure that rcu_core() is preempt-safe from a general perspective in the first place. From a quick glance I can't find obvious issues...yet. Paul maybe you can see something? > > 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 Yeah that's not possible, nocb_lock is too low level and has to be called with IRQs disabled. So if we take that local_lock solution, we need a new lock. Thanks.