tty: Fix transient pty write() EIO

Commit 69939035
('pty: Ignore slave pty close() if never successfully opened')
introduced a bug with ptys whereby a write() in parallel with an
open() on an existing pty could mistakenly indicate an I/O error.

Only indicate an I/O error if the condition on open() actually exists.
Reported-by: NMarkus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Tested-by: NMikael Pettersson <mikpe@it.uu.se>
Cc: stable <stable@vger.kernel.org> # 3.9
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/pty.c

@@ -244,14 +244,9 @@ static void pty_flush_buffer(struct tty_struct *tty)
 
 static int pty_open(struct tty_struct *tty, struct file *filp)
 {
-	int	retval = -ENODEV;
-
 	if (!tty || !tty->link)
-		goto out;
-
-	set_bit(TTY_IO_ERROR, &tty->flags);
+		return -ENODEV;
 
-	retval = -EIO;
 	if (test_bit(TTY_OTHER_CLOSED, &tty->flags))
 		goto out;
 	if (test_bit(TTY_PTY_LOCK, &tty->link->flags))
@@ -262,9 +257,11 @@ static int pty_open(struct tty_struct *tty, struct file *filp)
 	clear_bit(TTY_IO_ERROR, &tty->flags);
 	clear_bit(TTY_OTHER_CLOSED, &tty->link->flags);
 	set_bit(TTY_THROTTLED, &tty->flags);
-	retval = 0;
+	return 0;
+
 out:
-	return retval;
+	set_bit(TTY_IO_ERROR, &tty->flags);
+	return -EIO;
 }
 
 static void pty_set_termios(struct tty_struct *tty,