staging: dgnc: take a lock when storing value in dgnc_poll_tick

Reads of dgnc_poll_tick are protected by dgnc_poll_lock spinlock, but the write
to dgnc_poll_tick is not. It could theoretically race.
Signed-off-by: Salah Triki <salah.triki@acm.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/dgnc/dgnc_driver.c

@@ -63,6 +63,7 @@ static const struct file_operations dgnc_BoardFops = {
 uint			dgnc_NumBoards;
 struct dgnc_board		*dgnc_Board[MAXBOARDS];
 DEFINE_SPINLOCK(dgnc_global_lock);
+DEFINE_SPINLOCK(dgnc_poll_lock); /* Poll scheduling lock */
 uint			dgnc_Major;
 int			dgnc_poll_tick = 20;	/* Poll interval - 20 ms */
 
@@ -74,7 +75,6 @@ static struct class *dgnc_class;
 /*
  * Poller stuff
  */
-static DEFINE_SPINLOCK(dgnc_poll_lock); /* Poll scheduling lock */
 static ulong		dgnc_poll_time; /* Time of next poll */
 static uint		dgnc_poll_stop; /* Used to tell poller to stop */
 static struct timer_list dgnc_poll_timer;

drivers/staging/dgnc/dgnc_driver.h

@@ -390,6 +390,7 @@ struct channel_t {
 extern uint		dgnc_Major;		/* Our driver/mgmt major */
 extern int		dgnc_poll_tick;		/* Poll interval - 20 ms */
 extern spinlock_t	dgnc_global_lock;	/* Driver global spinlock */
+extern spinlock_t	dgnc_poll_lock;		/* Poll scheduling lock */
 extern uint		dgnc_NumBoards;		/* Total number of boards */
 extern struct dgnc_board	*dgnc_Board[MAXBOARDS];	/* Array of board structs */
 

drivers/staging/dgnc/dgnc_sysfs.c

@@ -56,11 +56,18 @@ static ssize_t dgnc_driver_pollrate_show(struct device_driver *ddp, char *buf)
 static ssize_t dgnc_driver_pollrate_store(struct device_driver *ddp,
 					  const char *buf, size_t count)
 {
+	unsigned long flags;
+	int tick;
 	int ret;
 
-	ret = sscanf(buf, "%d\n", &dgnc_poll_tick);
+	ret = sscanf(buf, "%d\n", &tick);
 	if (ret != 1)
 		return -EINVAL;
+
+	spin_lock_irqsave(&dgnc_poll_lock, flags);
+	dgnc_poll_tick = tick;
+	spin_unlock_irqrestore(&dgnc_poll_lock, flags);
+
 	return count;
 }
 static DRIVER_ATTR(pollrate, (S_IRUSR | S_IWUSR), dgnc_driver_pollrate_show,