tty: Simplify tty_set_ldisc() exit handling

BugLink: http://bugs.launchpad.net/bugs/1709126

Perform common exit for both successful and error exit handling
in tty_set_ldisc(). Fixes unlikely possibility of failing to restart
input kworker when switching to the same line discipline (noop case).
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 63d8cb3f)
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Acked-by: Marcelo Cerri <marcelo.cerri@canonical.com>
Acked-by: Benjamin M Romer <benjamin.romer@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/tty/tty_ldisc.c

@@ -536,34 +536,21 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 
 	tty_lock(tty);
 	retval = tty_ldisc_lock(tty, 5 * HZ);
-	if (retval) {
-		tty_ldisc_put(new_ldisc);
-		tty_unlock(tty);
-		return retval;
-	}
+	if (retval)
+		goto err;
 
-	/*
-	 *	Check the no-op case
-	 */
+	/* Check the no-op case */
+	if (tty->ldisc->ops->num == ldisc)
+		goto out;
 
-	if (tty->ldisc->ops->num == ldisc) {
-		tty_ldisc_unlock(tty);
-		tty_ldisc_put(new_ldisc);
-		tty_unlock(tty);
-		return 0;
+	if (test_bit(TTY_HUPPED, &tty->flags)) {
+		/* We were raced by hangup */
+		retval = -EIO;
+		goto out;
 	}
 
 	old_ldisc = tty->ldisc;
 
-	if (test_bit(TTY_HUPPED, &tty->flags)) {
-		/* We were raced by the hangup method. It will have stomped
-		   the ldisc data and closed the ldisc down */
-		tty_ldisc_unlock(tty);
-		tty_ldisc_put(new_ldisc);
-		tty_unlock(tty);
-		return -EIO;
-	}
-
 	/* Shutdown the old discipline. */
 	tty_ldisc_close(tty, old_ldisc);
 
@@ -589,18 +576,15 @@ int tty_set_ldisc(struct tty_struct *tty, int ldisc)
 	   the old ldisc (if it was restored as part of error cleanup
 	   above). In either case, releasing a single reference from
 	   the old ldisc is correct. */
-
-	tty_ldisc_put(old_ldisc);
-
-	/*
-	 *	Allow ldisc referencing to occur again
-	 */
+	new_ldisc = old_ldisc;
+out:
 	tty_ldisc_unlock(tty);
 
 	/* Restart the work queue in case no characters kick it off. Safe if
 	   already running */
 	tty_buffer_restart_work(tty->port);
-
+err:
+	tty_ldisc_put(new_ldisc);	/* drop the extra reference */
 	tty_unlock(tty);
 	return retval;
 }