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 9FEC8C4320A for ; Tue, 27 Jul 2021 23:08:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 80F5460F9B for ; Tue, 27 Jul 2021 23:08:19 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232903AbhG0XIT (ORCPT ); Tue, 27 Jul 2021 19:08:19 -0400 Received: from mail.kernel.org ([198.145.29.99]:55150 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232198AbhG0XIR (ORCPT ); Tue, 27 Jul 2021 19:08:17 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id AF79F60EB2; Tue, 27 Jul 2021 23:08:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1627427297; bh=fGzQ1yxz3VodJVrSOBzAcSVWcjc+avK52D1CDLlbVEA=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=XGjLo5NK3LAP03NixFWTa/BVZ0L+YJ8vcQXLdfXsz/VIGe2B0qk684NU5Q0AQJmSW JBUY/FQtGmx/2CsyIf21zVm0fhs8xBAw18Duo0myWqc+xGnINpjwsOLuhkVH6XdpsC k8lz89BOs5lR+0fmScyn4rAWRcTwQFWHgqknjVwfOOyjwVCZYcxUPuSjuBNDvADPbs uC5fh5f2eSmLo+VYMMn6Z6/N/CMgPIZxO27xBNBkIxUECfDdLV3Jj+K/zf+Vdt8B9m 8r7LA5Ek1jascxDILuJhUFvxttj2E+fdT+0NZbcX4cN7J84d/IGnGR9/qLZsDeVTbj 0a0HV9Vzw/dAA== Date: Wed, 28 Jul 2021 01:08:14 +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 Subject: Re: [PATCH 2/3] rcu/nocb: Check for migratability rather than pure preemptability Message-ID: <20210727230814.GC283787@lothringen> 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: > 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.