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=-8.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, 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 65532C74A44 for ; Sun, 14 Jul 2019 18:53:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 41F162089C for ; Sun, 14 Jul 2019 18:53:09 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728164AbfGNSxC (ORCPT ); Sun, 14 Jul 2019 14:53:02 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:6002 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728125AbfGNSxC (ORCPT ); Sun, 14 Jul 2019 14:53:02 -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 x6EIlCEJ106075; Sun, 14 Jul 2019 14:50:30 -0400 Received: from pps.reinject (localhost [127.0.0.1]) by mx0a-001b2d01.pphosted.com with ESMTP id 2tqvuf5gts-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 14 Jul 2019 14:50:30 -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 x6EIoUZH135521; Sun, 14 Jul 2019 14:50:30 -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 2tqvuf5gta-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 14 Jul 2019 14:50:29 -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 x6EInRHA026321; Sun, 14 Jul 2019 18:50:28 GMT Received: from b01cxnp22033.gho.pok.ibm.com (b01cxnp22033.gho.pok.ibm.com [9.57.198.23]) by ppma03dal.us.ibm.com with ESMTP id 2tq6x6tdsf-1 (version=TLSv1.2 cipher=ECDHE-RSA-AES256-GCM-SHA384 bits=256 verify=NOT); Sun, 14 Jul 2019 18:50:28 +0000 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22033.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x6EIoSV749414412 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sun, 14 Jul 2019 18:50:28 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id F3E19B205F; Sun, 14 Jul 2019 18:50:27 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 88499B2065; Sun, 14 Jul 2019 18:50:27 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.203.247]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sun, 14 Jul 2019 18:50:27 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id 7A5C116C8FBA; Sun, 14 Jul 2019 11:50:27 -0700 (PDT) Date: Sun, 14 Jul 2019 11:50:27 -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: <20190714185027.GL26519@linux.ibm.com> Reply-To: paulmck@linux.ibm.com References: <20190713031008.GA248225@google.com> <20190713082114.GA26519@linux.ibm.com> <20190713133049.GA133650@google.com> <20190713144108.GD26519@linux.ibm.com> <20190713153606.GD133650@google.com> <20190713155010.GF26519@linux.ibm.com> <20190713161316.GA39321@google.com> <20190713212812.GH26519@linux.ibm.com> <20190714181053.GB34501@google.com> <20190714183820.GD34501@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190714183820.GD34501@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=2019-07-14_06:,, 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-1907140234 Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Sun, Jul 14, 2019 at 02:38:20PM -0400, Joel Fernandes wrote: > On Sun, Jul 14, 2019 at 02:10:53PM -0400, Joel Fernandes wrote: > > On Sat, Jul 13, 2019 at 02:28:12PM -0700, Paul E. McKenney wrote: > [snip] > > > > > > > > > > > > > > > > > > > > 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? > > > > > > > > > > > > > > > > Hi Paul, > > > > > > > > I don't think it will cause splats for !CONFIG_PREEMPT. > > > > > > > > > > > > > > > > Currently, rcu_read_lock_any_held() introduced in this patch returns true if > > > > > > > > !preemptible(). This means that: > > > > > > > > > > > > > > > > The following expression above: > > > > > > > > RCU_LOCKDEP_WARN(!rcu_read_lock_any_held(),...) > > > > > > > > > > > > > > > > Becomes: > > > > > > > > RCU_LOCKDEP_WARN(preemptible(), ...) > > > > > > > > > > > > > > > > For, CONFIG_PREEMPT=n kernels, this means: > > > > > > > > RCU_LOCKDEP_WARN(0, ...) > > > > > > > > > > > > > > > > Which would mean no splats. Or, did I miss the point? > > > > > > > > > > > > > > I suggest trying it out on a CONFIG_PREEMPT=n kernel. > > > > > > > > > > > > Sure, will do, sorry did not try it out yet because was busy with weekend > > > > > > chores but will do soon, thanks! > > > > > > > > > > I am not faulting you for taking the weekend off, actually. ;-) > > > > > > > > ;-) > > > > > > > > I tried doing RCU_LOCKDEP_WARN(preemptible(), ...) in this code path and I > > > > don't get any splats. I also disassembled the code and it seems to me > > > > RCU_LOCKDEP_WARN() becomes a NOOP which also the above reasoning confirms. > > > > > > OK, very good. Could you do the same thing for the RCU_LOCKDEP_WARN() > > > in synchronize_rcu()? Why or why not? > > > > > > > Hi Paul, > > > > Yes synchronize_rcu() can also make use of this technique since it is > > strictly illegal to call synchronize_rcu() within a reader section. > > > > I will add this to the set of my patches as well and send them all out next > > week, along with the rcu-sync and bh clean ups we discussed. > > After sending this email, it occurs to me it wont work in synchronize_rcu() > for !CONFIG_PREEMPT kernels. This is because in a !CONFIG_PREEMPT kernel, > executing in kernel mode itself looks like being in an RCU reader. So we > should leave that as is. However it will work fine for rcu_sync_is_idle (for > CONFIG_PREEMPT=n kernels) as I mentioned earlier. > > Were trying to throw me a Quick-Quiz ? ;-) In that case, hope I passed! You did pass. This time. ;-) Thanx, Paul