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Commit ccd4b65a authored by Rafael J. Wysocki's avatar Rafael J. Wysocki Committed by Linus Torvalds
Browse files

PM: prevent frozen user mode helpers from failing the freezing of tasks 



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 <u.luckas@road.de>, Pavel Machek
<pavel@ucw.cz> and Oleg Nesterov <oleg@tv-sign.ru> for reviewing the
previous versions of this patch and for very useful comments.

Signed-off-by: default avatarRafael J. Wysocki <rjw@sisk.pl>
Acked-by: default avatarUli Luckas <u.luckas@road.de>
Acked-by: default avatarNigel Cunningham <nigel@nigel.suspend2.net>
Acked-by: default avatarPavel Machek <pavel@ucw.cz>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 8cdd4936

kernel/kmod.c

+68 −10
Original line number Diff line number Diff line
@@ -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 +267,55 @@ static void __call_usermodehelper(struct work_struct *work) 
	}

}



#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;

		smp_mb();

		/*

		 * 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;

		}

	case PM_POST_HIBERNATION:

	case PM_POST_SUSPEND:

		usermodehelper_disabled = 0;
@@ -293,6 +325,30 @@ static int usermodehelper_pm_callback(struct notifier_block *nfb, 
	return NOTIFY_DONE;

}



static void helper_lock(void)

{

	atomic_inc(&running_helpers);

	smp_mb__after_atomic_inc();

}



static void helper_unlock(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 helper_lock(void) {}

static inline void helper_unlock(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 +453,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, 
	DECLARE_COMPLETION_ONSTACK(done);

	int retval;



	helper_lock();

	if (sub_info->path[0] == '\0') {

		retval = 0;

		goto out;
@@ -418,6 +475,7 @@ int call_usermodehelper_exec(struct subprocess_info *sub_info, 


  out:

	call_usermodehelper_freeinfo(sub_info);

	helper_unlock();

	return retval;

}

EXPORT_SYMBOL(call_usermodehelper_exec);
@@ -459,5 +517,5 @@ 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();

}