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: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Uli Luckas <u.luckas@road.de>
Acked-by: Nigel Cunningham <nigel@nigel.suspend2.net>
Acked-by: Pavel Machek <pavel@ucw.cz>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 68 insertions, 10 deletions

diff --git a/kernel/kmod.c b/kernel/kmod.c
index 928f3678142a..beedbdc64608 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();
 }