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,URIBL_BLOCKED,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 7FECDC2BA19 for ; Wed, 15 Apr 2020 21:49:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5C2BA20784 for ; Wed, 15 Apr 2020 21:49:29 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731237AbgDOVt2 (ORCPT ); Wed, 15 Apr 2020 17:49:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:43080 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730954AbgDOVtW (ORCPT ); Wed, 15 Apr 2020 17:49:22 -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 A854020768; Wed, 15 Apr 2020 21:49:20 +0000 (UTC) Date: Wed, 15 Apr 2020 17:49:18 -0400 From: Steven Rostedt To: John Stultz Cc: paulmck@kernel.org, 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: <20200415174918.154a86d0@gandalf.local.home> In-Reply-To: References: <20200415085348.5511a5fe@gandalf.local.home> <20200415161424.584d07d3@gandalf.local.home> <20200415164116.40564f2c@gandalf.local.home> 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 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? -- Steve