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=-8.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, USER_AGENT_MUTT 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 25868C43613 for ; Thu, 20 Jun 2019 21:10:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 0308D2070B for ; Thu, 20 Jun 2019 21:10:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726402AbfFTVKx (ORCPT ); Thu, 20 Jun 2019 17:10:53 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:8142 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726256AbfFTVKw (ORCPT ); Thu, 20 Jun 2019 17:10:52 -0400 Received: from pps.filterd (m0098419.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x5KL8FFR088897; Thu, 20 Jun 2019 17:10:05 -0400 Received: from ppma01wdc.us.ibm.com (fd.55.37a9.ip4.static.sl-reverse.com [169.55.85.253]) by mx0b-001b2d01.pphosted.com with ESMTP id 2t8hsyg2a1-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 20 Jun 2019 17:10:04 -0400 Received: from pps.filterd (ppma01wdc.us.ibm.com [127.0.0.1]) by ppma01wdc.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x5KL5Utw001317; Thu, 20 Jun 2019 21:10:04 GMT Received: from b01cxnp22034.gho.pok.ibm.com (b01cxnp22034.gho.pok.ibm.com [9.57.198.24]) by ppma01wdc.us.ibm.com with ESMTP id 2t8hrp00gy-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Thu, 20 Jun 2019 21:10:04 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22034.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x5KLA3n130474734 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Thu, 20 Jun 2019 21:10:04 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id DF168B2064; Thu, 20 Jun 2019 21:10:03 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id B181BB205F; Thu, 20 Jun 2019 21:10:03 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.26]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Thu, 20 Jun 2019 21:10:03 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id AA5FD16C6B85; Thu, 20 Jun 2019 14:10:05 -0700 (PDT) Date: Thu, 20 Jun 2019 14:10:05 -0700 From: "Paul E. McKenney" To: Scott Wood Cc: Sebastian Andrzej Siewior , Peter Zijlstra , Steven Rostedt , Thomas Gleixner , Juri Lelli , Clark Williams , linux-rt-users@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [RFC PATCH RT 3/4] rcu: unlock special: Treat irq and preempt disabled the same Message-ID: <20190620211005.GW26519@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190619011908.25026-1-swood@redhat.com> <20190619011908.25026-4-swood@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190619011908.25026-4-swood@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-06-20_14:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1906200152 Sender: linux-rt-users-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-rt-users@vger.kernel.org On Tue, Jun 18, 2019 at 08:19:07PM -0500, Scott Wood wrote: > [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. Far easier to deal with it than to debug the lack of it. ;-) > 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. > ] Yes, invoke_rcu_core() can in some cases invoke the scheduler sooner. Setting the CPU-local bits might not have effect until the next interrupt. So if -rt wants the simpler and slower approach, the change needs to use IS_ENABLED(CONFIG_PREEMPT_RT_FULL) or similar. Not that this is an issue until CONFIG_PREEMPT_RT_FULL hits mainline. Thanx, Paul > 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 >