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=-2.2 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_2 autolearn=no 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 D3F06C2BA19 for ; Wed, 15 Apr 2020 22:51:38 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B56DA2076D for ; Wed, 15 Apr 2020 22:51:38 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732931AbgDOWvg (ORCPT ); Wed, 15 Apr 2020 18:51:36 -0400 Received: from mail.kernel.org ([198.145.29.99]:34244 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725800AbgDOWvY (ORCPT ); Wed, 15 Apr 2020 18:51:24 -0400 Received: from gandalf.local.home (cpe-66-24-58-225.stny.res.rr.com [66.24.58.225]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 8B2642076D; Wed, 15 Apr 2020 22:51:23 +0000 (UTC) Date: Wed, 15 Apr 2020 18:51:21 -0400 From: Steven Rostedt To: "Paul E. McKenney" Cc: John Stultz , Josh Triplett , lkml , Bjorn Andersson , Saravana Kannan , Todd Kjos , Stephen Boyd , Peter Zijlstra , Thomas Gleixner Subject: Re: On trace_*_rcuidle functions in modules Message-ID: <20200415185121.381a4bc3@gandalf.local.home> In-Reply-To: <20200415220459.GE17661@paulmck-ThinkPad-P72> References: <20200415085348.5511a5fe@gandalf.local.home> <20200415161424.584d07d3@gandalf.local.home> <20200415164116.40564f2c@gandalf.local.home> <20200415174918.154a86d0@gandalf.local.home> <20200415220459.GE17661@paulmck-ThinkPad-P72> X-Mailer: Claws Mail 3.17.3 (GTK+ 2.24.32; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 15 Apr 2020 15:04:59 -0700 "Paul E. McKenney" wrote: > On Wed, Apr 15, 2020 at 05:49:18PM -0400, Steven Rostedt wrote: > > On Wed, 15 Apr 2020 14:02:04 -0700 > > John Stultz wrote: > > > > > > > > So in my case your concerns may not be a problem, but I guess > > > generally it might. Though I'd hope the callback would be unregistered > > > (and whatever waiting for the grace period to complete be done) before > > > the module removal is complete. But maybe I'm still missing your > > > point? > > > > Hmm, you may have just brought up a problem here... > > > > You're saying that cpu_pm_register_notifier() callers are called from non > > RCU watching context? If that's the case, we have this: > > > > int cpu_pm_unregister_notifier(struct notifier_block *nb) > > { > > return atomic_notifier_chain_unregister(&cpu_pm_notifier_chain, nb); > > } > > > > And this: > > > > int atomic_notifier_chain_unregister(struct atomic_notifier_head *nh, > > struct notifier_block *n) > > { > > unsigned long flags; > > int ret; > > > > spin_lock_irqsave(&nh->lock, flags); > > ret = notifier_chain_unregister(&nh->head, n); > > spin_unlock_irqrestore(&nh->lock, flags); > > synchronize_rcu(); > > return ret; > > } > > > > Which means that if something registered a cpu_pm notifier, then > > unregistered it, and freed whatever the notifier accesses, then there's a > > chance that the synchronize_rcu() can return before the called notifier > > finishes, and anything that notifier accesses could have been freed. > > > > I believe that module code should not be able to be run in RCU non watching > > context, and neither should notifiers. I think we just stumbled on a bug. > > > > Paul? > > Or we say that such modules cannot be unloaded. Or that such modules' > exit handlers, after disentangling themselves from the idle loop, must > invoke synchronize_rcu_rude() or similar, just as modules that use > call_rcu() are currently required to invoke rcu_barrier(). > > Or is it possible to upgrade the protection that modules use? > > My guess is that invoking rcu_irq_enter() and rcu_irq_exit() around every > potential call into module code out of the PM code is a non-starter, > but I cannot prove that either way. > No this has nothing to do with modules. This is a bug right now with the cpu_pm notifier (after looking at the code, it's not a bug right now, see below). Say you have something that allocates some data and registers a callback to the cpu_pm notifier that access that data. Then for some reason, you want to remove that notifier and free the data. Usually you would do: cpu_pm_unregister_notifier(my_notifier); kfree(my_data); But the problem is that the callback of that my_notifier could be executing in a RCU non-watching space, and the cpu_pm_unregister_notifier() can return before the my_notifier is done, and the my_data is freed. Then the callback for the my_notifier could still be accessing the my_data. /me goes and reads the code and sees this is not an issue, and you can ignore the above concern. I was about to suggest a patch, but that has already been written... 313c8c16ee62b ("PM / CPU: replace raw_notifier with atomic_notifier") Which surrounds the notifier callbacks with rcu_irq_enter_irqson() Which means that if John moves the code to use the notifier, then he could also remove the _rcuidle(), because RCU will be watching. -- Steve