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Violators will be prosecuted; (version=TLSv1/SSLv3 cipher=AES256-GCM-SHA384 bits=256/256) Sat, 13 Jul 2019 16:50:12 +0100 Received: from b01ledav003.gho.pok.ibm.com (b01ledav003.gho.pok.ibm.com [9.57.199.108]) by b01cxnp22036.gho.pok.ibm.com (8.14.9/8.14.9/NCO v10.0) with ESMTP id x6DFoBAX12125004 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-GCM-SHA384 bits=256 verify=OK); Sat, 13 Jul 2019 15:50:11 GMT Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 6BAFFB2064; Sat, 13 Jul 2019 15:50:11 +0000 (GMT) Received: from b01ledav003.gho.pok.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 0ABFEB205F; Sat, 13 Jul 2019 15:50:11 +0000 (GMT) Received: from paulmck-ThinkPad-W541 (unknown [9.85.158.189]) by b01ledav003.gho.pok.ibm.com (Postfix) with ESMTP; Sat, 13 Jul 2019 15:50:10 +0000 (GMT) Received: by paulmck-ThinkPad-W541 (Postfix, from userid 1000) id A684116C5DA0; Sat, 13 Jul 2019 08:50:10 -0700 (PDT) Date: Sat, 13 Jul 2019 08:50:10 -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 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> <20190713082114.GA26519@linux.ibm.com> <20190713133049.GA133650@google.com> <20190713144108.GD26519@linux.ibm.com> <20190713153606.GD133650@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190713153606.GD133650@google.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 19071315-0072-0000-0000-00000448A0C2 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00011421; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000286; SDB=6.01231630; UDB=6.00648823; IPR=6.01012922; MB=3.00027705; MTD=3.00000008; XFM=3.00000015; UTC=2019-07-13 15:50:21 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 19071315-0073-0000-0000-00004CB8E70B Message-Id: <20190713155010.GF26519@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-07-13_04:,, 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-1907130192 Sender: linux-acpi-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-acpi@vger.kernel.org On Sat, Jul 13, 2019 at 11:36:06AM -0400, Joel Fernandes wrote: > On Sat, Jul 13, 2019 at 07:41:08AM -0700, Paul E. McKenney wrote: > > On Sat, Jul 13, 2019 at 09:30:49AM -0400, Joel Fernandes wrote: > > > On Sat, Jul 13, 2019 at 01:21:14AM -0700, Paul E. McKenney wrote: > > > > 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? > > > > > > 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. ;-) Thanx, Paul