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=-15.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=ham 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 43827C4338F for ; Wed, 28 Jul 2021 20:03:04 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 28AAC60F5E for ; Wed, 28 Jul 2021 20:03:04 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230289AbhG1UDF (ORCPT ); Wed, 28 Jul 2021 16:03:05 -0400 Received: from mail.efficios.com ([167.114.26.124]:45560 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230125AbhG1UDF (ORCPT ); Wed, 28 Jul 2021 16:03:05 -0400 Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id C649235B4A9; Wed, 28 Jul 2021 16:03:02 -0400 (EDT) Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10032) with ESMTP id 1neallEIlD3z; Wed, 28 Jul 2021 16:03:02 -0400 (EDT) Received: from localhost (localhost [127.0.0.1]) by mail.efficios.com (Postfix) with ESMTP id 3570635B4A6; Wed, 28 Jul 2021 16:03:02 -0400 (EDT) DKIM-Filter: OpenDKIM Filter v2.10.3 mail.efficios.com 3570635B4A6 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=efficios.com; s=default; t=1627502582; bh=b//lJLcPByuvZRIn7/xeEND5gAZOf4XdRcGyQEVeavU=; h=Date:From:To:Message-ID:MIME-Version; b=ZYOrhnNqKYJK7ZSr8cZjqOBTtE8PKtnP9KuIAzfUe5Rzqg+lLL5NCJbb433RuP1Pp RCkSUZ62764URo+zbIUzJgRthtunezURj1eO4Ww7tIJGGebAUNlnaDcCNjLthDGN6A A+pg9aVUqlYAqYl+E46qhMz7Zo94Xo/B+6pQ49oPyJ3GRJMD4VA4p9VRGHRY5oEJ8x Ie/sBmTh2VzppVfuPba1QPzLJOZLxSrgLrEyVLPOFwGr5tCnElnKkkLLUef6xr7NxP 5WMEjsJN6ZGgJA8gh8b7QGD+w3JE/YJ0I/K2L2YhgfwbVxRst5Ne9BVh3Vwr9t9bxI lX3O+6wIQAfuw== X-Virus-Scanned: amavisd-new at efficios.com Received: from mail.efficios.com ([127.0.0.1]) by localhost (mail03.efficios.com [127.0.0.1]) (amavisd-new, port 10026) with ESMTP id X339T_vesXcS; Wed, 28 Jul 2021 16:03:02 -0400 (EDT) Received: from mail03.efficios.com (mail03.efficios.com [167.114.26.124]) by mail.efficios.com (Postfix) with ESMTP id 1EEDE35B435; Wed, 28 Jul 2021 16:03:02 -0400 (EDT) Date: Wed, 28 Jul 2021 16:03:02 -0400 (EDT) From: Mathieu Desnoyers To: paulmck Cc: rcu , linux-kernel , kernel-team , Ingo Molnar , Lai Jiangshan , Andrew Morton , Josh Triplett , Thomas Gleixner , Peter Zijlstra , rostedt , David Howells , Eric Dumazet , fweisbec , Oleg Nesterov , "Joel Fernandes, Google" , Linus Torvalds Message-ID: <874308613.9545.1627502582005.JavaMail.zimbra@efficios.com> In-Reply-To: <20210728194505.GA1500024@paulmck-ThinkPad-P17-Gen-1> References: <20210721202042.GA1472052@paulmck-ThinkPad-P17-Gen-1> <20210721202127.2129660-4-paulmck@kernel.org> <20210728173715.GA9416@paulmck-ThinkPad-P17-Gen-1> <2135064974.9081.1627496585724.JavaMail.zimbra@efficios.com> <20210728185854.GK4397@paulmck-ThinkPad-P17-Gen-1> <20210728194505.GA1500024@paulmck-ThinkPad-P17-Gen-1> Subject: Re: [PATCH v2 rcu 04/18] rcu: Weaken ->dynticks accesses and updates MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit X-Originating-IP: [167.114.26.124] X-Mailer: Zimbra 8.8.15_GA_4059 (ZimbraWebClient - FF90 (Linux)/8.8.15_GA_4059) Thread-Topic: Weaken ->dynticks accesses and updates Thread-Index: 568K9ZSxkflip1GHOVAM8nm+9R9/hw== Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org ----- On Jul 28, 2021, at 3:45 PM, paulmck paulmck@kernel.org wrote: [...] > > And how about like this? > > Thanx, Paul > > ------------------------------------------------------------------------ > > commit cb8914dcc6443cca15ce48d937a93c0dfdb114d3 > Author: Paul E. McKenney > Date: Wed Jul 28 12:38:42 2021 -0700 > > rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting() > > The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks > counter of an incoming CPU if required. It is currently is invoked "is currently is" -> "is currently" > from rcutree_prepare_cpu(), which runs before the incoming CPU is > running, and thus on some other CPU. This makes the per-CPU accesses in > rcu_dynticks_eqs_online() iffy at best, and it all "works" only because > the running CPU cannot possibly be in dyntick-idle mode, which means > that rcu_dynticks_eqs_online() never has any effect. One could argue > that this means that rcu_dynticks_eqs_online() is unnecessary, however, > removing it makes the CPU-online process vulnerable to slight changes > in the CPU-offline process. Why favor moving this from the prepare_cpu to the cpu_starting hotplug step, rather than using the target cpu's rdp from rcutree_prepare_cpu ? Maybe there was a good reason for having this very early in the prepare_cpu step ? Also, the commit message refers to this bug as having no effect because the running CPU cannot possibly be in dyntick-idle mode. I understand that calling this function was indeed effect-less, but then why is it OK for the CPU coming online to skip this call in the first place ? This commit message hints at "slight changes in the CPU-offline process" which could break it, but therer is no explanation of what makes this not an actual bug fix. Thanks, Mathieu > > This commit therefore moves the call to rcu_dynticks_eqs_online() from > rcutree_prepare_cpu() to rcu_cpu_starting(), this latter being guaranteed > to be running on the incoming CPU. The call to this function must of > course be placed before this rcu_cpu_starting() announces this CPU's > presence to RCU. > > Reported-by: Mathieu Desnoyers > Signed-off-by: Paul E. McKenney > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 0172a5fd6d8de..aa00babdaf544 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -4129,7 +4129,6 @@ int rcutree_prepare_cpu(unsigned int cpu) > rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs); > rdp->blimit = blimit; > rdp->dynticks_nesting = 1; /* CPU not up, no tearing. */ > - rcu_dynticks_eqs_online(); > raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */ > > /* > @@ -4249,6 +4248,7 @@ void rcu_cpu_starting(unsigned int cpu) > mask = rdp->grpmask; > WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1); > WARN_ON_ONCE(!(rnp->ofl_seq & 0x1)); > + rcu_dynticks_eqs_online(); > smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier(). > raw_spin_lock_irqsave_rcu_node(rnp, flags); > WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask); -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com