Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757539AbXFUOxr (ORCPT ); Thu, 21 Jun 2007 10:53:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754397AbXFUOxi (ORCPT ); Thu, 21 Jun 2007 10:53:38 -0400 Received: from ogre.sisk.pl ([217.79.144.158]:52677 "EHLO ogre.sisk.pl" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753839AbXFUOxh (ORCPT ); Thu, 21 Jun 2007 10:53:37 -0400 From: "Rafael J. Wysocki" To: Pavel Machek Subject: Re: [RFC][PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks Date: Thu, 21 Jun 2007 17:00:20 +0200 User-Agent: KMail/1.9.5 Cc: LKML , Nigel Cunningham , Uli Luckas , pm list References: <200706182329.58917.rjw@sisk.pl> <20070621133015.GG18392@elf.ucw.cz> In-Reply-To: <20070621133015.GG18392@elf.ucw.cz> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200706211700.21488.rjw@sisk.pl> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 6140 Lines: 190 On Thursday, 21 June 2007 15:30, Pavel Machek wrote: > Hi! > > > From: Rafael J. Wysocki > > > > At present, if a user mode helper is running while usermodehelper_pm_callback() > > is executed, the helper may be frozen and the completion in > > call_usermodehelper_exec() won't be completed until user space processes are > > thawed. As a result, the freezing of kernel threads may fail, which is not > > desirable. > > > > Prevent this from happening by introducing a counter of running user mode > > helpers and allowing usermodehelper_pm_callback() to succeed for > > action = PM_HIBERNATION_PREPARE or action = PM_SUSPEND_PREPARE only if there > > are no helpers running. [Namely, usermodehelper_pm_callback() waits for at most > > RUNNING_HELPERS_TIMEOUT for the number of running helpers to become zero and > > fails if that doesn't happen.] > > > > Special thanks to Uli Luckas for reviewing the previous > > versions of this patch and for very useful comments. > > > > > - return NOTIFY_OK; > > + wait_event_timeout(running_helpers_waitq, > > + atomic_read(&running_helpers) == 0, > > + RUNNING_HELPERS_TIMEOUT); > > + if (atomic_read(&running_helpers) == 0) { > > Uff, this is suspect. What happens if running_helpers goes to 1 here.? Hmm, yes, this is racy. We can use the value returned by wait_event_timeout() to close this race. Updated patch follows. Greetings, Rafael --- From: Rafael J. Wysocki At present, if a user mode helper is running while usermodehelper_pm_callback() is executed, the helper may be frozen and the completion in call_usermodehelper_exec() won't be completed until user space processes are thawed. As a result, the freezing of kernel threads may fail, which is not desirable. Prevent this from happening by introducing a counter of running user mode helpers and allowing usermodehelper_pm_callback() to succeed for action = PM_HIBERNATION_PREPARE or action = PM_SUSPEND_PREPARE only if there are no helpers running. [Namely, usermodehelper_pm_callback() waits for at most RUNNING_HELPERS_TIMEOUT for the number of running helpers to become zero and fails if that doesn't happen.] Special thanks to Uli Luckas for reviewing the previous versions of this patch and for very useful comments. Signed-off-by: Rafael J. Wysocki --- kernel/kmod.c | 70 +++++++++++++++++++++++++++++++++++++++++++++++++--------- 1 file changed, 60 insertions(+), 10 deletions(-) Index: linux-2.6.22-rc4-mm2/kernel/kmod.c =================================================================== --- linux-2.6.22-rc4-mm2.orig/kernel/kmod.c +++ linux-2.6.22-rc4-mm2/kernel/kmod.c @@ -41,14 +41,6 @@ extern int max_threads; static struct workqueue_struct *khelper_wq; -/* - * If set, both call_usermodehelper_keys() and call_usermodehelper_pipe() exit - * immediately returning -EBUSY. Used for preventing user land processes from - * being created after the user land has been frozen during a system-wide - * hibernation or suspend operation. - */ -static int usermodehelper_disabled; - #ifdef CONFIG_KMOD /* @@ -275,15 +267,48 @@ static void __call_usermodehelper(struct } } +#ifdef CONFIG_PM +/* + * If set, call_usermodehelper_exec() will exit immediately returning -EBUSY + * (used for preventing user land processes from being created after the user + * land has been frozen during a system-wide hibernation or suspend operation). + */ +static int usermodehelper_disabled; + +/* Number of helpers running */ +static atomic_t running_helpers = ATOMIC_INIT(0); + +/* + * Wait queue head used by usermodehelper_pm_callback() to wait for all running + * helpers to finish. + */ +static DECLARE_WAIT_QUEUE_HEAD(running_helpers_waitq); + +/* + * Time to wait for running_helpers to become zero before the setting of + * usermodehelper_disabled in usermodehelper_pm_callback() fails + */ +#define RUNNING_HELPERS_TIMEOUT (5 * HZ) + 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; + 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; + } case PM_POST_HIBERNATION: case PM_POST_SUSPEND: usermodehelper_disabled = 0; @@ -293,6 +318,29 @@ static int usermodehelper_pm_callback(st return NOTIFY_DONE; } +static void new_helper(void) +{ + atomic_inc(&running_helpers); +} + +static void helper_finished(void) +{ + if (atomic_dec_and_test(&running_helpers)) + wake_up(&running_helpers_waitq); +} + +static void register_pm_notifier_callback(void) +{ + pm_notifier(usermodehelper_pm_callback, 0); +} +#else /* CONFIG_PM */ +#define usermodehelper_disabled 0 + +static inline void new_helper(void) {} +static inline void helper_finished(void) {} +static inline void register_pm_notifier_callback(void) {} +#endif /* CONFIG_PM */ + /** * call_usermodehelper_setup - prepare to call a usermode helper * @path - path to usermode executable @@ -397,6 +445,7 @@ int call_usermodehelper_exec(struct subp DECLARE_COMPLETION_ONSTACK(done); int retval; + new_helper(); if (sub_info->path[0] == '\0') { retval = 0; goto out; @@ -418,6 +467,7 @@ int call_usermodehelper_exec(struct subp out: call_usermodehelper_freeinfo(sub_info); + helper_finished(); return retval; } EXPORT_SYMBOL(call_usermodehelper_exec); @@ -459,5 +509,5 @@ void __init usermodehelper_init(void) { khelper_wq = create_singlethread_workqueue("khelper"); BUG_ON(!khelper_wq); - pm_notifier(usermodehelper_pm_callback, 0); + register_pm_notifier_callback(); } - 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/