[PATCH] Fix locking for tty drivers when doing urgent characters

If you send a priority character (as is done for flow control) then the tty
driver can either have its own method for "jumping the queue" or the characrer
can be queued normally.  In the latter case we call the write method but
without the atomic_write_lock taken elsewhere.

Make this consistent.  Note that the send_xchar method if implemented remains
outside of the lock as it can jump ahead of a current write so must not be
locked out by it.
Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/tty_ioctl.c

@@ -423,24 +423,28 @@ static int set_ltchars(struct tty_struct * tty, struct ltchars __user * ltchars)
  *
  *	Send a high priority character to the tty even if stopped
  *
- *	Locking: none
- *
- *	FIXME: overlapping calls with start/stop tty lose state of tty
+ *	Locking: none for xchar method, write ordering for write method.
  */
 
-static void send_prio_char(struct tty_struct *tty, char ch)
+static int send_prio_char(struct tty_struct *tty, char ch)
 {
 	int	was_stopped = tty->stopped;
 
 	if (tty->driver->send_xchar) {
 		tty->driver->send_xchar(tty, ch);
-		return;
+		return 0;
 	}
+
+	if (mutex_lock_interruptible(&tty->atomic_write_lock))
+		return -ERESTARTSYS;
+
 	if (was_stopped)
 		start_tty(tty);
 	tty->driver->write(tty, &ch, 1);
 	if (was_stopped)
 		stop_tty(tty);
+	mutex_unlock(&tty->atomic_write_lock);
+	return 0;
 }
 
 int n_tty_ioctl(struct tty_struct * tty, struct file * file,
@@ -514,11 +518,11 @@ int n_tty_ioctl(struct tty_struct * tty, struct file * file,
 				break;
 			case TCIOFF:
 				if (STOP_CHAR(tty) != __DISABLED_CHAR)
-					send_prio_char(tty, STOP_CHAR(tty));
+					return send_prio_char(tty, STOP_CHAR(tty));
 				break;
 			case TCION:
 				if (START_CHAR(tty) != __DISABLED_CHAR)
-					send_prio_char(tty, START_CHAR(tty));
+					return send_prio_char(tty, START_CHAR(tty));
 				break;
 			default:
 				return -EINVAL;