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,URIBL_BLOCKED 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 C4A21C433E1 for ; Fri, 31 Jul 2020 01:42:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9EEDB207F5 for ; Fri, 31 Jul 2020 01:42:28 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="KhBQky18" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731058AbgGaBm1 (ORCPT ); Thu, 30 Jul 2020 21:42:27 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:45976 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730904AbgGaBmZ (ORCPT ); Thu, 30 Jul 2020 21:42:25 -0400 Received: from mail-qv1-xf42.google.com (mail-qv1-xf42.google.com [IPv6:2607:f8b0:4864:20::f42]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 393BDC061575 for ; Thu, 30 Jul 2020 18:42:24 -0700 (PDT) Received: by mail-qv1-xf42.google.com with SMTP id s15so9035486qvv.7 for ; Thu, 30 Jul 2020 18:42:24 -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=tG7wGILsQIxgOHJ5P5VpUtUWNoX8RG8/YlzKPvWH/qk=; b=KhBQky188hI0RZrecoXzncHCyiN2s1DRjMhqfuy4W3CKUegnZv6mRRmKh2Hh3W3Y+J VByu+Uyl5M9GOkZoXbz/nZZXoeHZRYx6CpxseUimv/cWdo0dPbzI+Gj0h+H6I0zvw4VI iWM9krgGMAWu7PZDiPS7fMWdIQXbwT4Z6QdHI= 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=tG7wGILsQIxgOHJ5P5VpUtUWNoX8RG8/YlzKPvWH/qk=; b=tCsQCJSXvM7AuoFnunwbdg5Vr3ei8pRerIsE9YBBESvVbB2vwNnjbuQOZs36PXvDcq hP5Xtv6jUOro+PMp7VBKB4GpiJxmE/k2J2hbTM8LZk5gNtAVB5nB6WjP27y2KENop8xq S8WRPBSGINYwkk9K+/Nov7QATFjiPJvu1FLwkiUX9L6az9M9p9EgNubFU/p+K913C4tS SI7hW/Ia0qHSoIJXJI7QdqjVTxr06U9Vom7y5LIvzQ4wzG6mSlMQyJDCwkoTzeYBsHqi JEgtFPw1VfNzl5GxdS847bN7z64NeJUo8u+af+HH6+FJUtBIgUH4MHGnpZ6guDJ/6sOI TNuA== X-Gm-Message-State: AOAM5337Sl60Mg0UcbE0rhBWnxTC3fkdSAVAjC/UBh4gLdd5UY6iQ+v4 dNatqjPYNd0pB7gX21ZYmWFNMxYH3pg= X-Google-Smtp-Source: ABdhPJxx7zgrqF32V8N7VHL+U3yCb6dF7acF0NAD2l9JTDPJmL1FzlkVQxj8PbomlgrKwQqutZiA0A== X-Received: by 2002:ad4:414b:: with SMTP id z11mr1868063qvp.116.1596159743300; Thu, 30 Jul 2020 18:42:23 -0700 (PDT) Received: from localhost ([2620:15c:6:12:cad3:ffff:feb3:bd59]) by smtp.gmail.com with ESMTPSA id h23sm5932224qkl.38.2020.07.30.18.42.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jul 2020 18:42:22 -0700 (PDT) Date: Thu, 30 Jul 2020 21:42:22 -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: <20200731014222.GA2349603@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. > > ------------------------------------------------------------------------ > > > 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? I can move this comment to: "Hotplug CPU" section in Documentation/RCU/Design/Requirements/Requirements.rst And I could make the comment here as: /* * Delete QS reporting from here, by June 2021, if the warning does not * fire. Leave the warning indefinitely. Check RCU design requirements * in Documentation/RCU/ about CPU hotplug requirements. */ I will post my v3 with changes to the requirements document. Let me know any other comments, thanks, - Joel