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=-5.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 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 843D6C3A5A0 for ; Mon, 19 Aug 2019 15:42:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 57374206BB for ; Mon, 19 Aug 2019 15:42:20 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727653AbfHSPmU (ORCPT ); Mon, 19 Aug 2019 11:42:20 -0400 Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:60738 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1727639AbfHSPmT (ORCPT ); Mon, 19 Aug 2019 11:42:19 -0400 Received: from pps.filterd (m0098416.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x7JFRiLG141291; Mon, 19 Aug 2019 11:41:43 -0400 Received: from pps.reinject (localhost [127.0.0.1]) by mx0b-001b2d01.pphosted.com with ESMTP id 2ufwbmbkqq-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 19 Aug 2019 11:41:43 -0400 Received: from m0098416.ppops.net (m0098416.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.27/8.16.0.27) with SMTP id x7JFSJec143822; Mon, 19 Aug 2019 11:41:41 -0400 Received: from ppma01dal.us.ibm.com (83.d6.3fa9.ip4.static.sl-reverse.com [169.63.214.131]) by mx0b-001b2d01.pphosted.com with ESMTP id 2ufwbmbkq7-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 19 Aug 2019 11:41:41 -0400 Received: from pps.filterd (ppma01dal.us.ibm.com [127.0.0.1]) by ppma01dal.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x7JFPuOq005182; Mon, 19 Aug 2019 15:41:40 GMT Received: from b01cxnp23033.gho.pok.ibm.com (b01cxnp23033.gho.pok.ibm.com [9.57.198.28]) by ppma01dal.us.ibm.com with ESMTP id 2ue976euu5-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Mon, 19 Aug 2019 15:41:40 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp23033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x7JFfdug40567100 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Mon, 19 Aug 2019 15:41:39 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id D485FB2067; Mon, 19 Aug 2019 15:41:39 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id A7BC6B2064; Mon, 19 Aug 2019 15:41:39 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.70.82.154]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Mon, 19 Aug 2019 15:41:39 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id BD70716C124D; Mon, 19 Aug 2019 08:41:43 -0700 (PDT) Date: Mon, 19 Aug 2019 08:41:43 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [RFC v2] rcu/tree: Try to invoke_rcu_core() if in_irq() during unlock Message-ID: <20190819154143.GA18470@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190818223230.GA143857@google.com> <20190818223511.GB143857@google.com> <20190818233135.GQ28441@linux.ibm.com> <20190818233839.GA160903@google.com> <20190819012153.GR28441@linux.ibm.com> <20190819014143.GB160903@google.com> <20190819014623.GC160903@google.com> <20190819022927.GS28441@linux.ibm.com> <20190819125757.GA6946@linux.ibm.com> <20190819143314.GT28441@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190819143314.GT28441@linux.ibm.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=2019-08-19_03:,, 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=8.0.1-1906280000 definitions=main-1908190170 Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org On Mon, Aug 19, 2019 at 07:33:14AM -0700, Paul E. McKenney wrote: > On Mon, Aug 19, 2019 at 05:57:57AM -0700, Paul E. McKenney wrote: > > On Sun, Aug 18, 2019 at 07:29:27PM -0700, Paul E. McKenney wrote: > > > On Sun, Aug 18, 2019 at 09:46:23PM -0400, Joel Fernandes wrote: > > > > On Sun, Aug 18, 2019 at 09:41:43PM -0400, Joel Fernandes wrote: > > > > > On Sun, Aug 18, 2019 at 06:21:53PM -0700, Paul E. McKenney wrote: > > > > [snip] > > > > > > > > Also, your commit log's point #2 is "in_irq() implies in_interrupt() > > > > > > > > which implies raising softirq will not do any wake ups." This mention > > > > > > > > of softirq seems a bit odd, given that we are going to wake up a rcuc > > > > > > > > kthread. Of course, this did nothing to quell my suspicions. ;-) > > > > > > > > > > > > > > Yes, I should delete this #2 from the changelog since it is not very relevant > > > > > > > (I feel now). My point with #2 was that even if were to raise a softirq > > > > > > > (which we are not), a scheduler wakeup of ksoftirqd is impossible in this > > > > > > > path anyway since in_irq() implies in_interrupt(). > > > > > > > > > > > > Please! Could you also add a first-principles explanation of why > > > > > > the added condition is immune from scheduler deadlocks? > > > > > > > > > > Sure I can add an example in the change log, however I was thinking of this > > > > > example which you mentioned: > > > > > https://lore.kernel.org/lkml/20190627173831.GW26519@linux.ibm.com/ > > > > > > > > > > previous_reader() > > > > > { > > > > > rcu_read_lock(); > > > > > do_something(); /* Preemption happened here. */ > > > > > local_irq_disable(); /* Cannot be the scheduler! */ > > > > > do_something_else(); > > > > > rcu_read_unlock(); /* Must defer QS, task still queued. */ > > > > > do_some_other_thing(); > > > > > local_irq_enable(); > > > > > } > > > > > > > > > > current_reader() /* QS from previous_reader() is still deferred. */ > > > > > { > > > > > local_irq_disable(); /* Might be the scheduler. */ > > > > > do_whatever(); > > > > > rcu_read_lock(); > > > > > do_whatever_else(); > > > > > rcu_read_unlock(); /* Must still defer reporting QS. */ > > > > > do_whatever_comes_to_mind(); > > > > > local_irq_enable(); > > > > > } > > > > > > > > > > One modification of the example could be, previous_reader() could also do: > > > > > previous_reader() > > > > > { > > > > > rcu_read_lock(); > > > > > do_something_that_takes_really_long(); /* causes need_qs in > > > > > the unlock_special_union to be set */ > > > > > local_irq_disable(); /* Cannot be the scheduler! */ > > > > > do_something_else(); > > > > > rcu_read_unlock(); /* Must defer QS, task still queued. */ > > > > > do_some_other_thing(); > > > > > local_irq_enable(); > > > > > } > > > > > > > > The point you were making in that thread being, current_reader() -> > > > > rcu_read_unlock() -> rcu_read_unlock_special() would not do any wakeups > > > > because previous_reader() sets the deferred_qs bit. > > > > > > > > Anyway, I will add all of this into the changelog. > > > > > > Examples are good, but what makes it so that there are no examples of > > > its being unsafe? > > > > > > And a few questions along the way, some quick quiz, some more serious. > > > Would it be safe if it checked in_interrupt() instead of in_irq()? > > > If not, should the in_interrupt() in the "if" condition preceding the > > > added "else if" be changed to in_irq()? Would it make sense to add an > > > "|| !irqs_were_disabled" do your new "else if" condition? Would the > > > body of the "else if" actually be executed in current mainline? > > > > > > In an attempt to be at least a little constructive, I am doing some > > > testing of this patch overnight, along with a WARN_ON_ONCE() to see if > > > that invoke_rcu_core() is ever reached. > > > > And that WARN_ON_ONCE() never triggered in two-hour rcutorture runs of > > TREE01, TREE02, TREE03, and TREE09. (These are the TREE variants in > > CFLIST that have CONFIG_PREEMPT=y.) > > > > This of course raises other questions. But first, do you see that code > > executing in your testing? > > Never mind! Idiot here forgot the "--bootargs rcutree.use_softirq"... So this time I ran the test this way: tools/testing/selftests/rcutorture/bin/kvm.sh --cpus 8 --duration 10 --configs "TREE01 TREE02 TREE03 TREE09" --bootargs "rcutree.use_softirq=0" Still no splats. Though only 10-minute runs instead of the two-hour runs I did last night. (Got other stuff I need to do, sorry!) My test version of your patch is shown below. Please let me know if I messed something up. Thanx, Paul ------------------------------------------------------------------------ diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h index 2defc7fe74c3..abf2fbba2568 100644 --- a/kernel/rcu/tree_plugin.h +++ b/kernel/rcu/tree_plugin.h @@ -621,6 +621,10 @@ static void rcu_read_unlock_special(struct task_struct *t) // Using softirq, safe to awaken, and we get // no help from enabling irqs, unlike bh/preempt. raise_softirq_irqoff(RCU_SOFTIRQ); + } else if (exp && in_irq() && !use_softirq && + !t->rcu_read_unlock_special.b.deferred_qs) { + WARN_ON_ONCE(1); // Live code? + invoke_rcu_core(); } else { // Enabling BH or preempt does reschedule, so... // Also if no expediting or NO_HZ_FULL, slow is OK.