slip/slcan: added locking in wakeup function

The locking is needed, since the the internal buffer for the CAN frames is
changed during the wakeup call. This could cause buffer inconsistencies
under high loads, especially for the outgoing short CAN packet skbuffs.

The needed locks led to deadlocks before commit
"5ede52538ee2b2202d9dff5b06c33bfde421e6e4 tty: Remove extra wakeup from pty
write() path", which removed the direct callback to the wakeup function from the
tty layer.

As slcan.c is based on slip.c the issue in the original code is fixed, too.

Signed-off-by: Andre Naujoks <nautsch2@gmail.com>
Acked-by: Oliver Hartkopp <socketcan@hartkopp.net>
Acked-by: Marc Kleine-Budde  <mkl@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/slip/slip.c

@@ -429,11 +429,13 @@ static void slip_write_wakeup(struct tty_struct *tty)
 	if (!sl || sl->magic != SLIP_MAGIC || !netif_running(sl->dev))
 		return;
 
+	spin_lock(&sl->lock);
 	if (sl->xleft <= 0)  {
 		/* Now serial buffer is almost free & we can start
 		 * transmission of another packet */
 		sl->dev->stats.tx_packets++;
 		clear_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
+		spin_unlock(&sl->lock);
 		sl_unlock(sl);
 		return;
 	}
@@ -441,6 +443,7 @@ static void slip_write_wakeup(struct tty_struct *tty)
 	actual = tty->ops->write(tty, sl->xhead, sl->xleft);
 	sl->xleft -= actual;
 	sl->xhead += actual;
+	spin_unlock(&sl->lock);
 }
 
 static void sl_tx_timeout(struct net_device *dev)