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 1CB9CC2BA19 for ; Wed, 15 Apr 2020 22:55:01 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id EAA9F20784 for ; Wed, 15 Apr 2020 22:55:00 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1586991301; bh=bXxQU6IuZNZL59Z8TCcquyoG8MiLRK3KYG2b8JN4hjc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=2PDvkIHF0pO4YnEBolZ3laXLcVHAxfr3mNpai0ga+O72wm75mcBIbZwzzV97q8mW3 FXj91qBjlyeYNYl5f3IypLOQY4uZKr7gz+zi7E4N12sdeufEfKwjuZcK/NRaLFX64n cpNRjPEgt78DmR8gS9NXcfAKGPhy69dpPlitNwWI= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2387595AbgDOWy5 (ORCPT ); Wed, 15 Apr 2020 18:54:57 -0400 Received: from mail.kernel.org ([198.145.29.99]:37416 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2387549AbgDOWyv (ORCPT ); Wed, 15 Apr 2020 18:54:51 -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 6C87C2076D; Wed, 15 Apr 2020 22:54:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1586991290; bh=bXxQU6IuZNZL59Z8TCcquyoG8MiLRK3KYG2b8JN4hjc=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=dIJcSHLmvB5yKUo0B5lQeYGT61/ZN9ijsAB+Y/UZD3/M+KrXIKKjSKpia9vh87kFF MHO7bJ5CIPlytUu9XCHi0WKx0c2VxYl4JCn+ySQx3RqpecZaOhmrpWXqt2fq9jrzhR X04+S4VYiffWR04aLvOWq+UXKeQwRDL31oO0SvQ0= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 46A893522AD1; Wed, 15 Apr 2020 15:54:50 -0700 (PDT) Date: Wed, 15 Apr 2020 15:54:50 -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: <20200415225450.GG17661@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> <20200415220459.GE17661@paulmck-ThinkPad-P72> <20200415185121.381a4bc3@gandalf.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200415185121.381a4bc3@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 06:51:21PM -0400, Steven Rostedt wrote: > 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. Whew!!! ;-) Thanx, Paul