Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756642AbXFYO47 (ORCPT ); Mon, 25 Jun 2007 10:56:59 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752781AbXFYO4v (ORCPT ); Mon, 25 Jun 2007 10:56:51 -0400 Received: from ogre.sisk.pl ([217.79.144.158]:40338 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754021AbXFYO4u (ORCPT ); Mon, 25 Jun 2007 10:56:50 -0400 From: "Rafael J. Wysocki" To: Oleg Nesterov Subject: Re: [PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks Date: Mon, 25 Jun 2007 17:03:42 +0200 User-Agent: KMail/1.9.5 Cc: Andrew Morton , Nigel Cunningham , "Paul E. McKenney" , Pavel Machek , Uli Luckas , linux-kernel@vger.kernel.org References: <20070625104332.GA147@tv-sign.ru> In-Reply-To: <20070625104332.GA147@tv-sign.ru> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200706251703.42979.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2050 Lines: 64 On Monday, 25 June 2007 12:43, Oleg Nesterov wrote: > Rafael J. Wysocki wrote: > > > > static int usermodehelper_pm_callback(struct notifier_block *nfb, > > unsigned long action, > > void *ignored) > > { > > + long retval; > > + > > switch (action) { > > case PM_HIBERNATION_PREPARE: > > case PM_SUSPEND_PREPARE: > > usermodehelper_disabled = 1; > > - return NOTIFY_OK; > > + /* > > + * From now on call_usermodehelper_exec() won't start any new > > + * helpers, so it is sufficient if running_helpers turns out to > > + * be zero at one point (it may be increased later, but that > > + * doesn't matter). > > + */ > > + retval = wait_event_timeout(running_helpers_waitq, > > + atomic_read(&running_helpers) == 0, > > + RUNNING_HELPERS_TIMEOUT); > > + if (retval) { > > + return NOTIFY_OK; > > + } else { > > + usermodehelper_disabled = 0; > > + return NOTIFY_BAD; > > I think this is racy. First, this needs smp_mb() between "usermodehelper_disabled = 1" > and wait_event_timeout(). Yes ... > Second, call_usermodehelper's path should first increment the counter, and only > then check usermodehelper_disabled, It does this already. > and it needs an mb() in between too. Otherwise, the helper can see > usermodehelper_disabled == 0, then PM_SUSPEND_PREPARE comes and > returns NOTIFY_OK, then the helper increments the counter and starts application. > > Sadly, you can't use srcu/qrcu because it doesn't handle timeouts. OK If I understand that correctly, it should suffice to place smp_mb() after usermodehelper_disabled = 1; in usermodehelper_pm_callback() and another smp_mb() after atomic_inc(&running_helpers); in new_helper(). Is that correct? Greetings, Rafael -- "Premature optimization is the root of all evil." - Donald Knuth - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/