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 <hdegoede@redhat.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

1 files changed, 5 insertions, 0 deletions

diff --git a/include/linux/watchdog.h b/include/linux/watchdog.h
index c3545c5d918a..da1dc1b52744 100644
--- a/include/linux/watchdog.h
+++ b/include/linux/watchdog.h
@@ -104,6 +104,7 @@ struct watchdog_ops {
  * @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
@@ -111,6 +112,9 @@ struct watchdog_ops {
  *
  * 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;
@@ -124,6 +128,7 @@ struct watchdog_device {
         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 */