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=-8.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,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 EF71CC74A5B for ; Sat, 13 Jul 2019 16:13:24 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C9CBD20651 for ; Sat, 13 Jul 2019 16:13:24 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="IbD7TNzU" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727919AbfGMQNT (ORCPT ); Sat, 13 Jul 2019 12:13:19 -0400 Received: from mail-pl1-f195.google.com ([209.85.214.195]:42975 "EHLO mail-pl1-f195.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727768AbfGMQNT (ORCPT ); Sat, 13 Jul 2019 12:13:19 -0400 Received: by mail-pl1-f195.google.com with SMTP id ay6so6231592plb.9 for ; Sat, 13 Jul 2019 09:13:19 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=P5pOxQ/9cE4E0rEdYSNFXC/ccfck6yCVHmggZDiN1+4=; b=IbD7TNzULkRDZ4vtMV1A2y75Z9Px4ISXgzResob5aggpyTamvUzCbH4iSLQb1wwmOh OoXTyjSUraQlHRlg89uD2xOvXr3zH4xzsSV6GnqMKMc0mr8El6KM+ouahQt0bfXlhguU aMudMnehVdSsV+XHeqvRoPP7hgbcfILo9hfaY= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=P5pOxQ/9cE4E0rEdYSNFXC/ccfck6yCVHmggZDiN1+4=; b=ThQVamW1ir2DU6+dpxEz+G9R25ouR2adHlvMEEJGJQBc/rQ4zjRmGVi/VsmmiVqz4x 8yvU+VM9eONr4rCujnqc0FHH3qtPI/y+WMUtsI02bJiw9TNDVr1163qZzXUWHMsPJFEJ ySAOxS6hSI6+RHJ3E9X9uAlSgzIa9Q9LgoaduflGWrznXHvIwFB7dz2HmsKkQeCMcncp sN3A7/VUSWkkYOGWF7fEQoN1z1PvRXEUZh13IslVgSCE3i2QfJZWlG3/MldiliSJcXsc pLlaXVmQPRRkBnhW/0HwKa9jWmeUNzF4QhYJChZgPg6m9UM0JQj8PIAYc53B0Veccyze Z6+w== X-Gm-Message-State: APjAAAXR8N9VIiS+tud5K7Fe4bCOsphBYhHFdeQ53Rc1p7uv4urdqlzi esEJqpQU+OInEjmoixI3BIQ= X-Google-Smtp-Source: APXvYqwFMp6BrQzYRwJK/zz2CZpCvsW0ckooTHNgOUC4Xyk+0Y0gaziATIzVhXmqxPvOg6Gj2sbbFw== X-Received: by 2002:a17:902:7407:: with SMTP id g7mr18660945pll.214.1563034398495; Sat, 13 Jul 2019 09:13:18 -0700 (PDT) Received: from localhost ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id w1sm10171343pjt.30.2019.07.13.09.13.17 (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Sat, 13 Jul 2019 09:13:17 -0700 (PDT) Date: Sat, 13 Jul 2019 12:13:16 -0400 From: Joel Fernandes To: "Paul E. McKenney" 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: <20190713161316.GA39321@google.com> References: <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> <20190713155010.GF26519@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190713155010.GF26519@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-pci-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-pci@vger.kernel.org On Sat, Jul 13, 2019 at 08:50:10AM -0700, Paul E. McKenney wrote: > 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. ;-) ;-) 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. thanks, - Joel