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Subject[RC][PATCH -mm] PM: Prevent frozen user mode helpers from failing the freezing of tasks
Date
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>
Acked-by: Nigel Cunningham <nigel@nigel.suspend2.net>
Acked-by: Uli Luckas <u.luckas@road.de>
Reviewed-by: Pavel Machek <pavel@ucw.cz>
---
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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