watchdog: Add Locking support

This patch fixes some potential multithreading issues, despite only
allowing one process to open the /dev/watchdog device, we can still get
called multiple times at the same time, since a program could be using thread,
or could share the fd after a fork.

This causes 2 potential problems:
1) watchdog_start / open do an unlocked test_n_set / test_n_clear,
if these 2 race, the watchdog could be stopped while the active
bit indicates it is running or visa versa.

2) Most watchdog_dev drivers probably assume that only one
watchdog-op will get called at a time, this is not necessary
true atm.

Signed-off-by: Hans de Goede <[e-mail]>
Signed-off-by: Wim Van Sebroeck <[e-mail]>

Documentation/watchdog/watchdog-kernel-api.txt

···
 	unsigned int min_timeout;
 	unsigned int max_timeout;
 	void *driver_data;
+	struct mutex lock;
 	unsigned long status;
 };
 
···
 * driver_data: a pointer to the drivers private data of a watchdog device.
   This data should only be accessed via the watchdog_set_drvdata and
   watchdog_get_drvdata routines.
+* lock: Mutex for WatchDog Timer Driver Core internal use only.
 * status: this field contains a number of status bits that give extra
   information about the status of the device (Like: is the watchdog timer
   running/active, is the nowayout bit set, is the device opened via

drivers/watchdog/watchdog_core.c

···
 	 * corrupted in a later stage then we expect a kernel panic!
 	 */
 
+	mutex_init(&wdd->lock);
 	id = ida_simple_get(&watchdog_ida, 0, MAX_DOGS, GFP_KERNEL);
 	if (id < 0)
 		return id;

drivers/watchdog/watchdog_dev.c

···
 {
 	int err = 0;
 
+	mutex_lock(&wddev->lock);
+
 	if (!watchdog_active(wddev))
 		goto out_ping;
 
···
 		err = wddev->ops->start(wddev); /* restart watchdog */
 
 out_ping:
+	mutex_unlock(&wddev->lock);
 	return err;
 }
 
···
 {
 	int err = 0;
 
+	mutex_lock(&wddev->lock);
+
 	if (watchdog_active(wddev))
 		goto out_start;
 
···
 		set_bit(WDOG_ACTIVE, &wddev->status);
 
 out_start:
+	mutex_unlock(&wddev->lock);
 	return err;
 }
 
···
 {
 	int err = 0;
 
+	mutex_lock(&wddev->lock);
+
 	if (!watchdog_active(wddev))
 		goto out_stop;
 
···
 		clear_bit(WDOG_ACTIVE, &wddev->status);
 
 out_stop:
+	mutex_unlock(&wddev->lock);
 	return err;
 }
 
···
 	if (!wddev->ops->status)
 		return -EOPNOTSUPP;
 
+	mutex_lock(&wddev->lock);
+
 	*status = wddev->ops->status(wddev);
 
+	mutex_unlock(&wddev->lock);
 	return err;
 }
 
···
 	    (timeout < wddev->min_timeout || timeout > wddev->max_timeout))
 		return -EINVAL;
 
+	mutex_lock(&wddev->lock);
+
 	err = wddev->ops->set_timeout(wddev, timeout);
 
+	mutex_unlock(&wddev->lock);
 	return err;
 }
 
···
 	if (!wddev->ops->get_timeleft)
 		return -EOPNOTSUPP;
 
+	mutex_lock(&wddev->lock);
+
 	*timeleft = wddev->ops->get_timeleft(wddev);
 
+	mutex_unlock(&wddev->lock);
 	return err;
 }
 
···
 	if (!wddev->ops->ioctl)
 		return -ENOIOCTLCMD;
 
+	mutex_lock(&wddev->lock);
+
 	err = wddev->ops->ioctl(wddev, cmd, arg);
 
+	mutex_unlock(&wddev->lock);
 	return err;
 }
 

include/linux/watchdog.h

···
  * @min_timeout:The watchdog devices minimum timeout value.
  * @max_timeout:The watchdog devices maximum timeout value.
  * @driver-data:Pointer to the drivers private data.
+ * @lock:	Lock for watchdog core internal use only.
  * @status:	Field that contains the devices internal status bits.
  *
  * The watchdog_device structure contains all information about a
···
  *
  * The driver-data field may not be accessed directly. It must be accessed
  * via the watchdog_set_drvdata and watchdog_get_drvdata helpers.
+ *
+ * The lock field is for watchdog core internal use only and should not be
+ * touched.
  */
 struct watchdog_device {
 	int id;
···
 	unsigned int min_timeout;
 	unsigned int max_timeout;
 	void *driver_data;
+	struct mutex lock;
 	unsigned long status;
 /* Bit numbers for status flags */
 #define WDOG_ACTIVE		0	/* Is the watchdog running/active */