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=-10.1 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS 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 899B4C433E1 for ; Fri, 31 Jul 2020 01:08:20 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 630A120829 for ; Fri, 31 Jul 2020 01:08:20 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="DCUEZCah" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731066AbgGaBIT (ORCPT ); Thu, 30 Jul 2020 21:08:19 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:40772 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728080AbgGaBIS (ORCPT ); Thu, 30 Jul 2020 21:08:18 -0400 Received: from mail-qk1-x744.google.com (mail-qk1-x744.google.com [IPv6:2607:f8b0:4864:20::744]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F2BD4C061574 for ; Thu, 30 Jul 2020 18:08:17 -0700 (PDT) Received: by mail-qk1-x744.google.com with SMTP id g26so27499243qka.3 for ; Thu, 30 Jul 2020 18:08:17 -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; bh=3NSWs9fH3ytbI2URqAjwjgMkPSY0dsXW9CcGF1uVSTI=; b=DCUEZCahWBQ5se4Wb6FtpTcpX+5HPU2uiktMcETLlzHbgFhchKEKxGoLDkatpBUQLA XtBq8yUxSEf71uywH1azt0L+QvPF9+WfyY+Ja/MCLUrvVXeCoaOu90v0vfWzLGrprQYi pLXeamdNC/tLaB7Bx+uSqdMCrWIdTAO7aXBMQ= 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; bh=3NSWs9fH3ytbI2URqAjwjgMkPSY0dsXW9CcGF1uVSTI=; b=lLVCFsCYlmIw/Xyn1liVyxGnCVd92uG/9QjqBHmWoxuyNC9gmtiBrviGYIDuPUPap1 ROi0KF9t3XUlOgSa3ECo3TAjAJc23tkgl6Qeol9Ly2wBz3ByB3QMTNJHKF2YBSH8iIgJ xiaRkHSLDIusXVnnbN7o/AkPUDVRyKbgFUJ7yNIcMi0Bj5TFZQ3gWAzWtyUxOl4oZ6Ex VPloWZ298D2pp9wLJUKVn5cenIyvG/O+fGqAvXxbcvFCccGJVwb7Bp4JMAt9G4/L9zGq s+ZyzmsphzleoKonufDGqJo6T/azMlOrZXqVhNBQ9w0QrF94tHNJQKcc9LlJ/Yun/0aT rh/Q== X-Gm-Message-State: AOAM531ZKbNAsy2nXIDDALNl3mPZJAkrp09omXq1vlA91jJVVjTGN2F9 1QWRU2wsdARNXJlEblFdTyyoYg== X-Google-Smtp-Source: ABdhPJyI0U9a+U+KQLY9JC7zCCP5+BKjTz7MSwV6GQuNCL70d4kspvRYMyOcL0oSfnx2+5yiLh5iCw== X-Received: by 2002:a37:8905:: with SMTP id l5mr1817785qkd.302.1596157697129; Thu, 30 Jul 2020 18:08:17 -0700 (PDT) Received: from localhost ([2620:15c:6:12:cad3:ffff:feb3:bd59]) by smtp.gmail.com with ESMTPSA id n68sm6179466qkd.89.2020.07.30.18.08.16 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jul 2020 18:08:16 -0700 (PDT) Date: Thu, 30 Jul 2020 21:08:16 -0400 From: Joel Fernandes To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, Neeraj Upadhyay , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , rcu@vger.kernel.org, Steven Rostedt Subject: Re: [PATCH 1/2] rcu/tree: Add a warning if CPU being onlined did not report QS already Message-ID: <20200731010816.GB2336096@google.com> References: <20200730030221.705255-1-joel@joelfernandes.org> <20200730162159.GZ9247@paulmck-ThinkPad-P72> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200730162159.GZ9247@paulmck-ThinkPad-P72> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jul 30, 2020 at 09:21:59AM -0700, Paul E. McKenney wrote: > On Wed, Jul 29, 2020 at 11:02:20PM -0400, Joel Fernandes (Google) wrote: > > Add a warning if CPU being onlined did not report QS already. This is to > > simplify the code in the CPU onlining path and also to make clear about > > where QS is reported. The act of QS reporting in CPU onlining path is > > is likely unnecessary as shown by code reading and testing with > > rcutorture's TREE03 and hotplug parameters. > > How about something like this for the commit log? > > ------------------------------------------------------------------------ > > Currently, rcu_cpu_starting() checks to see if the RCU core expects a > quiescent state from the incoming CPU. However, the current interaction > between RCU quiescent-state reporting and CPU-hotplug operations should > mean that the incoming CPU never needs to report a quiescent state. > First, the outgoing CPU reports a quiescent state if needed. Second, > the race where the CPU is leaving just as RCU is initializing a new > grace period is handled by an explicit check for this condition. Third, > the CPU's leaf rcu_node structure's ->lock serializes these checks. > > This means that if rcu_cpu_starting() ever feels the need to report > a quiescent state, then there is a bug somewhere in the CPU hotplug > code or the RCU grace-period handling code. This commit therefore > adds a WARN_ON_ONCE() to bring that bug to everyone's attention. Wow, this is so well written. I certainly need to up my writing game a bitt. But yes, this is flawless explanation and really will help people reading it in the future. I will make the change to my tree for the next revision. thanks, - Joel > ------------------------------------------------------------------------ > > > Cc: Paul E. McKenney > > Cc: Neeraj Upadhyay > > Suggested-by: Paul E. McKenney > > Signed-off-by: Joel Fernandes (Google) > > > > --- > > kernel/rcu/tree.c | 14 +++++++++++++- > > 1 file changed, 13 insertions(+), 1 deletion(-) > > > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > > index 65e1b5e92319..1e51962b565b 100644 > > --- a/kernel/rcu/tree.c > > +++ b/kernel/rcu/tree.c > > @@ -3996,7 +3996,19 @@ void rcu_cpu_starting(unsigned int cpu) > > rcu_gpnum_ovf(rnp, rdp); /* Offline-induced counter wrap? */ > > rdp->rcu_onl_gp_seq = READ_ONCE(rcu_state.gp_seq); > > rdp->rcu_onl_gp_flags = READ_ONCE(rcu_state.gp_flags); > > - if (rnp->qsmask & mask) { /* RCU waiting on incoming CPU? */ > > + > > + /* > > + * Delete QS reporting from here, by June 2021, if warning does not > > + * fire. Let us make the rules for reporting QS for an offline CPUs > > + * more explicit. The CPU onlining path does not need to report QS for > > + * an offline CPU. Either the QS should have reported during CPU > > + * offlining, or during rcu_gp_init() if it detected a race with either > > + * CPU offlining or task unblocking on previously offlined CPUs. Note > > + * that the FQS loop also does not report QS for an offline CPU any > > + * longer (unless it splats due to an offline CPU blocking the GP for > > + * too long). > > + */ > > Let's leave at least the WARN_ON_ONCE() indefinitely. If you don't > believe me, remove this code in your local tree, have someone give you > several branches, some with bugs injected, and then try to figure out > which have the bugs and then try to find those bugs. > > This is not a fastpath, so the overhead of the check is not a concern. > Believe me, the difficulty of bug location without this check is a very > real concern! ;-) > > On the other hand, I fully agree with the benefits of documenting the > design rules. But is this really the best place to do that from the > viewpoint of someone who is trying to figure out how RCU works? > > Thanx, Paul > > > + if (WARN_ON_ONCE(rnp->qsmask & mask)) { /* RCU waiting on incoming CPU? */ > > rcu_disable_urgency_upon_qs(rdp); > > /* Report QS -after- changing ->qsmaskinitnext! */ > > rcu_report_qs_rnp(mask, rnp, rnp->gp_seq, flags); > > -- > > 2.28.0.rc0.142.g3c755180ce-goog > >