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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 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 1B3BFC2BA19 for ; Wed, 15 Apr 2020 22:05:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id E839220784 for ; Wed, 15 Apr 2020 22:05:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1586988310; bh=LcmVk+AMRQ3I615QuE9bZzomRfxqsLPUOgb5U3DaVgs=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=BZk3E3Bc/rNmg1JRnjTfzqK/7e3MwPKe686gCCTt2QSEwPb7RL5nnsdnPJXxJGmg0 DJCzjXOSrgCAk6dL9T+Bmv+N0fefD0rZbSgm/CHFVcaALMI1oOVP37wIJKd5XrZrAA lCctS3kH1FTArnkz+8BTqC2ULco5Y5iezZ5oFgeM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730072AbgDOWFE (ORCPT ); Wed, 15 Apr 2020 18:05:04 -0400 Received: from mail.kernel.org ([198.145.29.99]:45674 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727839AbgDOWFA (ORCPT ); Wed, 15 Apr 2020 18:05:00 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (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 E234C2076D; Wed, 15 Apr 2020 22:04:59 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1586988299; bh=LcmVk+AMRQ3I615QuE9bZzomRfxqsLPUOgb5U3DaVgs=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=TyTN6XRS19bqHCa2oJwjj/9dUpcAIHXqxkUXY/MydHuKsrpWcd1URLwQ+9Rb6Du1x bv7FKcYyXo8ni7d66CDtmw009qFCygHv+gXEDGdpsLbqGRMnjOyXwL2o1F4WfDvXRU +D+jhl2fzbppuITiTnMA9gcco1sulGduam7so11M= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id AE8F53522AD1; Wed, 15 Apr 2020 15:04:59 -0700 (PDT) Date: Wed, 15 Apr 2020 15:04:59 -0700 From: "Paul E. McKenney" To: Steven Rostedt 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: <20200415220459.GE17661@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <20200415085348.5511a5fe@gandalf.local.home> <20200415161424.584d07d3@gandalf.local.home> <20200415164116.40564f2c@gandalf.local.home> <20200415174918.154a86d0@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200415174918.154a86d0@gandalf.local.home> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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. Thanx, Paul