Fix possible pty line discipline race.

This ain't pretty. Real fix under discussion.

drivers/char/pty.c

@@ -149,13 +149,15 @@ static int pty_write_room(struct tty_struct *tty)
 static int pty_chars_in_buffer(struct tty_struct *tty)
 {
 	struct tty_struct *to = tty->link;
+	ssize_t (*chars_in_buffer)(struct tty_struct *);
 	int count;
 
-	if (!to || !to->ldisc.chars_in_buffer)
+	/* We should get the line discipline lock for "tty->link" */
+	if (!to || !(chars_in_buffer = to->ldisc.chars_in_buffer))
 		return 0;
 
 	/* The ldisc must report 0 if no characters available to be read */
-	count = to->ldisc.chars_in_buffer(to);
+	count = chars_in_buffer(to);
 
 	if (tty->driver->subtype == PTY_TYPE_SLAVE) return count;