PM: disable usermode helper before hibernation and suspend

Use a hibernation and suspend notifier to disable the user mode helper before
a hibernation/suspend and enable it after the operation.

[[e-mail]: build fix]
Signed-off-by: Rafael J. Wysocki <[e-mail]>
Acked-by: Pavel Machek <[e-mail]>
Acked-by: Nigel Cunningham <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/kmod.c

···
 #include <linux/kernel.h>
 #include <linux/init.h>
 #include <linux/resource.h>
+#include <linux/notifier.h>
+#include <linux/suspend.h>
 #include <asm/uaccess.h>
 
 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
 
···
 	}
 }
 
+static int usermodehelper_pm_callback(struct notifier_block *nfb,
+					unsigned long action,
+					void *ignored)
+{
+	switch (action) {
+	case PM_HIBERNATION_PREPARE:
+	case PM_SUSPEND_PREPARE:
+		usermodehelper_disabled = 1;
+		return NOTIFY_OK;
+	case PM_POST_HIBERNATION:
+	case PM_POST_SUSPEND:
+		usermodehelper_disabled = 0;
+		return NOTIFY_OK;
+	}
+
+	return NOTIFY_DONE;
+}
+
 /**
  * call_usermodehelper_setup - prepare to call a usermode helper
  * @path - path to usermode executable
···
 		goto out;
 	}
 
-	if (!khelper_wq) {
+	if (!khelper_wq || usermodehelper_disabled) {
 		retval = -EBUSY;
 		goto out;
 	}
···
 {
 	khelper_wq = create_singlethread_workqueue("khelper");
 	BUG_ON(!khelper_wq);
+	pm_notifier(usermodehelper_pm_callback, 0);
 }