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, URIBL_BLOCKED,USER_AGENT_SANE_1 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 D0595C742A7 for ; Sat, 13 Jul 2019 08:23:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id A75A120850 for ; Sat, 13 Jul 2019 08:23:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727655AbfGMIXp (ORCPT ); Sat, 13 Jul 2019 04:23:45 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:32096 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726334AbfGMIXp (ORCPT ); Sat, 13 Jul 2019 04:23:45 -0400 Received: from pps.filterd (m0098404.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.27/8.16.0.27) with SMTP id x6D8GuqC140611; Sat, 13 Jul 2019 04:21:16 -0400 Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com with ESMTP id 2tqbcpghfy-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sat, 13 Jul 2019 04:21:16 -0400 Received: from m0098404.ppops.net (m0098404.ppops.net [127.0.0.1]) by pps.reinject (8.16.0.27/8.16.0.27) with SMTP id x6D8Imex011412; Sat, 13 Jul 2019 04:21:15 -0400 Received: from ppma03dal.us.ibm.com (b.bd.3ea9.ip4.static.sl-reverse.com [169.62.189.11]) by mx0a-001b2d01.pphosted.com with ESMTP id 2tqbcpghfn-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sat, 13 Jul 2019 04:21:15 -0400 Received: from pps.filterd (ppma03dal.us.ibm.com [127.0.0.1]) by ppma03dal.us.ibm.com (8.16.0.27/8.16.0.27) with SMTP id x6D8JSWE020328; Sat, 13 Jul 2019 08:21:14 GMT Received: from b01cxnp22035.gho.pok.ibm.com (b01cxnp22035.gho.pok.ibm.com [9.57.198.25]) by ppma03dal.us.ibm.com with ESMTP id 2tq6x6hea4-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sat, 13 Jul 2019 08:21:14 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22035.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x6D8LDlZ41746710 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sat, 13 Jul 2019 08:21:13 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6DA5DB205F; Sat, 13 Jul 2019 08:21:13 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 11EA3B2064; Sat, 13 Jul 2019 08:21:13 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.195.235]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sat, 13 Jul 2019 08:21:13 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 1013216C6BA7; Sat, 13 Jul 2019 01:21:14 -0700 (PDT) Date: Sat, 13 Jul 2019 01:21:14 -0700 From: "Paul E. McKenney" To: Joel Fernandes Cc: linux-kernel@vger.kernel.org, Oleg Nesterov , Alexey Kuznetsov , Bjorn Helgaas , Borislav Petkov , c0d1n61at3@gmail.com, "David S. Miller" , edumazet@google.com, Greg Kroah-Hartman , Hideaki YOSHIFUJI , "H. Peter Anvin" , Ingo Molnar , Jonathan Corbet , Josh Triplett , keescook@chromium.org, kernel-hardening@lists.openwall.com, kernel-team@android.com, Lai Jiangshan , Len Brown , linux-acpi@vger.kernel.org, linux-doc@vger.kernel.org, linux-pci@vger.kernel.org, linux-pm@vger.kernel.org, Mathieu Desnoyers , neilb@suse.com, netdev@vger.kernel.org, Pavel Machek , peterz@infradead.org, "Rafael J. Wysocki" , Rasmus Villemoes , rcu@vger.kernel.org, Steven Rostedt , Tejun Heo , Thomas Gleixner , will@kernel.org, "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" Subject: Re: [PATCH v2 3/9] rcu/sync: Remove custom check for reader-section Message-ID: <20190713082114.GA26519@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190712170024.111093-1-joel@joelfernandes.org> <20190712170024.111093-4-joel@joelfernandes.org> <20190712213559.GA175138@google.com> <20190712233206.GZ26519@linux.ibm.com> <20190713030150.GA246587@google.com> <20190713031008.GA248225@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190713031008.GA248225@google.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=2019-07-13_02:,, 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-1907130098 Sender: linux-pm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pm@vger.kernel.org On Fri, Jul 12, 2019 at 11:10:08PM -0400, Joel Fernandes wrote: > On Fri, Jul 12, 2019 at 11:01:50PM -0400, Joel Fernandes wrote: > > On Fri, Jul 12, 2019 at 04:32:06PM -0700, Paul E. McKenney wrote: > > > On Fri, Jul 12, 2019 at 05:35:59PM -0400, Joel Fernandes wrote: > > > > On Fri, Jul 12, 2019 at 01:00:18PM -0400, Joel Fernandes (Google) wrote: > > > > > The rcu/sync code was doing its own check whether we are in a reader > > > > > section. With RCU consolidating flavors and the generic helper added in > > > > > this series, this is no longer need. We can just use the generic helper > > > > > and it results in a nice cleanup. > > > > > > > > > > Cc: Oleg Nesterov > > > > > Signed-off-by: Joel Fernandes (Google) > > > > > > > > Hi Oleg, > > > > Slightly unrelated to the patch, > > > > I tried hard to understand this comment below in percpu_down_read() but no dice. > > > > > > > > I do understand how rcu sync and percpu rwsem works, however the comment > > > > below didn't make much sense to me. For one, there's no readers_fast anymore > > > > so I did not follow what readers_fast means. Could the comment be updated to > > > > reflect latest changes? > > > > Also could you help understand how is a writer not able to change > > > > sem->state and count the per-cpu read counters at the same time as the > > > > comment tries to say? > > > > > > > > /* > > > > * We are in an RCU-sched read-side critical section, so the writer > > > > * cannot both change sem->state from readers_fast and start checking > > > > * counters while we are here. So if we see !sem->state, we know that > > > > * the writer won't be checking until we're past the preempt_enable() > > > > * and that once the synchronize_rcu() is done, the writer will see > > > > * anything we did within this RCU-sched read-size critical section. > > > > */ > > > > > > > > Also, > > > > I guess we could get rid of all of the gp_ops struct stuff now that since all > > > > the callbacks are the same now. I will post that as a follow-up patch to this > > > > series. > > > > > > Hello, Joel, > > > > > > Oleg has a set of patches updating this code that just hit mainline > > > this week. These patches get rid of the code that previously handled > > > RCU's multiple flavors. Or are you looking at current mainline and > > > me just missing your point? > > > > > > > Hi Paul, > > You are right on point. I have a bad habit of not rebasing my trees. In this > > case the feature branch of mine in concern was based on v5.1. Needless to > > say, I need to rebase my tree. > > > > Yes, this sync clean up patch does conflict when I rebase, but other patches > > rebase just fine. > > > > The 2 options I see are: > > 1. Let us drop this patch for now and I resend it later. > > 2. I resend all patches based on Linus's master branch. > > Below is the updated patch based on Linus master branch: > > ---8<----------------------- > > >From 5f40c9a07fcf3d6dafc2189599d0ba9443097d0f Mon Sep 17 00:00:00 2001 > From: "Joel Fernandes (Google)" > Date: Fri, 12 Jul 2019 12:13:27 -0400 > Subject: [PATCH v2.1 3/9] rcu/sync: Remove custom check for reader-section > > The rcu/sync code was doing its own check whether we are in a reader > section. With RCU consolidating flavors and the generic helper added in > this series, this is no longer need. We can just use the generic helper > and it results in a nice cleanup. > > Cc: Oleg Nesterov > Signed-off-by: Joel Fernandes (Google) > --- > include/linux/rcu_sync.h | 4 +--- > 1 file changed, 1 insertion(+), 3 deletions(-) > > diff --git a/include/linux/rcu_sync.h b/include/linux/rcu_sync.h > index 9b83865d24f9..0027d4c8087c 100644 > --- a/include/linux/rcu_sync.h > +++ b/include/linux/rcu_sync.h > @@ -31,9 +31,7 @@ struct rcu_sync { > */ > static inline bool rcu_sync_is_idle(struct rcu_sync *rsp) > { > - RCU_LOCKDEP_WARN(!rcu_read_lock_held() && > - !rcu_read_lock_bh_held() && > - !rcu_read_lock_sched_held(), > + RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(), I believe that replacing rcu_read_lock_sched_held() with preemptible() in a CONFIG_PREEMPT=n kernel will give you false-positive splats here. If you have not already done so, could you please give it a try? Thanx, Paul > "suspicious rcu_sync_is_idle() usage"); > return !READ_ONCE(rsp->gp_state); /* GP_IDLE */ > } > -- > 2.22.0.510.g264f2c817a-goog >