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=-6.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham 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 14C67C31E5E for ; Wed, 19 Jun 2019 01:19:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E85722085A for ; Wed, 19 Jun 2019 01:19:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729965AbfFSBTR (ORCPT ); Tue, 18 Jun 2019 21:19:17 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49658 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729721AbfFSBTQ (ORCPT ); Tue, 18 Jun 2019 21:19:16 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.phx2.redhat.com [10.5.11.15]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id E200D308A951; Wed, 19 Jun 2019 01:19:15 +0000 (UTC) Received: from t460p.redhat.com (ovpn-117-83.phx2.redhat.com [10.3.117.83]) by smtp.corp.redhat.com (Postfix) with ESMTP id 0517B17C41; Wed, 19 Jun 2019 01:19:14 +0000 (UTC) From: Scott Wood To: Sebastian Andrzej Siewior Cc: "Paul E. McKenney" , Peter Zijlstra , Steven Rostedt , Thomas Gleixner , Juri Lelli , Clark Williams , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org, Scott Wood Subject: [RFC PATCH RT 3/4] rcu: unlock special: Treat irq and preempt disabled the same Date: Tue, 18 Jun 2019 20:19:07 -0500 Message-Id: <20190619011908.25026-4-swood@redhat.com> In-Reply-To: <20190619011908.25026-1-swood@redhat.com> References: <20190619011908.25026-1-swood@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.79 on 10.5.11.15 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.41]); Wed, 19 Jun 2019 01:19:16 +0000 (UTC) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org [Note: Just before posting this I noticed that the invoke_rcu_core stuff is part of the latest RCU pull request, and it has a patch that addresses this in a more complicated way that appears to deal with the bare irq-disabled sequence as well. Assuming we need/want to support such sequences, is the invoke_rcu_core() call actually going to result in scheduling any sooner? resched_curr() just does the same setting of need_resched when it's the same cpu. ] Since special should never be getting set inside an irqs-disabled critical section, this is safe as long as there are no sequences of rcu_read_lock()/local_irq_disable()/rcu_read_unlock()/local_irq_enable() (without preempt_disable() wrapped around the IRQ disabling, as spinlocks do). If there are such sequences, then the grace period may be delayed until the next time need_resched is checked. This is needed because otherwise, in a sequence such as: 1. rcu_read_lock() 2. *preempt*, set rcu_read_unlock_special.b.blocked 3. preempt_disable() 4. rcu_read_unlock() 5. preempt_enable() ...rcu_read_unlock_special.b.blocked will not be cleared during step 4, because of the disabled preemption. If an interrupt is then taken between steps 4 and 5, and that interrupt enters scheduler code that takes pi/rq locks, and an rcu read lock inside that, then when dropping that rcu read lock we will end up in rcu_read_unlock_special() again -- but this time, since irqs are disabled, it will call invoke_rcu_core() in the RT tree (regardless of PREEMPT_RT_FULL), which calls wake_up_process(). This can cause a pi/rq lock deadlock. An example of interrupt code that does this is scheduler_tick(). The above sequence can be found in (at least) __lock_task_sighand() (for !PREEMPT_RT_FULL) and d_alloc_parallel(). It's potentially an issue on non-RT as well. While raise_softirq_irqoff() doesn't call wake_up_process() when in_interrupt() is true, if code between steps 4 and 5 directly calls into scheduler code, and that code uses RCU with pi/rq lock held, wake_up_process() can still be called. On RT, migrate_enable() is such a codepath, so an in_interrupt() check alone would not work on RT. Instead, keep track of whether we've already had an rcu_read_unlock_special() with preemption disabled but haven't yet scheduled, and rely on the preempt_enable() yet to come instead of calling invoke_rcu_core(). Signed-off-by: Scott Wood --- kernel/rcu/tree_plugin.h | 10 ++-------- 1 file changed, 2 insertions(+), 8 deletions(-) diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 5d63914b3687..d7ddbcc7231c 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -630,14 +630,8 @@ static void rcu_read_unlock_special(struct task_struct *t) if (preempt_bh_were_disabled || irqs_were_disabled) { WRITE_ONCE(t->rcu_read_unlock_special.b.exp_hint, false); /* Need to defer quiescent state until everything is enabled. */ - if (irqs_were_disabled) { - /* Enabling irqs does not reschedule, so... */ - invoke_rcu_core(); - } else { - /* Enabling BH or preempt does reschedule, so... */ - set_tsk_need_resched(current); - set_preempt_need_resched(); - } + set_tsk_need_resched(current); + set_preempt_need_resched(); local_irq_restore(flags); return; } -- 1.8.3.1