Re: [RFC][PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Thursday, 21 June 2007 15:30, Pavel Machek wrote:
> Hi!
> 
> > From: Rafael J. Wysocki <rjw@sisk.pl>
> > 
> > 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> 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 <rjw@sisk.pl>

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> for reviewing the previous
versions of this patch and for very useful comments.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
---
 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();
 }
-
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