From ccd4b65aef4be2278543fde5b999e55a4d694fd8 Mon Sep 17 00:00:00 2001 From: "Rafael J. Wysocki" Date: Thu, 19 Jul 2007 01:47:37 -0700 Subject: [PATCH] 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 , Pavel Machek and Oleg Nesterov for reviewing the previous versions of this patch and for very useful comments. Signed-off-by: Rafael J. Wysocki Acked-by: Uli Luckas Acked-by: Nigel Cunningham Acked-by: Pavel Machek Cc: Oleg Nesterov Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/kmod.c | 78 ++++++++++++++++++++++++++++++++++++++++++++------- 1 file changed, 68 insertions(+), 10 deletions(-) diff --git a/kernel/kmod.c b/kernel/kmod.c index 928f3678142..beedbdc6460 100644 --- a/kernel/kmod.c +++ b/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 +267,55 @@ 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; - return NOTIFY_OK; + 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(); } -- 2.41.1