[PATCH] Fix a race condition in pty.c

There is a race condition int pty.c when pty_close wakes up waiter on its
pair device before set TTY_OTHER_CLOSED flag.

It is possible on SMP or preempt kernel, waiter wakes up too early that it
will not get TTY_OTHER_CLOSED flag then fall into sleep again - missed wakeup.

hjl reports that this bug will hang some expect scripts on SMP machines.
Signed-off-by: Zou Nan hai <Nanhai.zou@intel.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/char/pty.c

@@ -55,9 +55,9 @@ static void pty_close(struct tty_struct * tty, struct file * filp)
 	if (!tty->link)
 		return;
 	tty->link->packet = 0;
+	set_bit(TTY_OTHER_CLOSED, &tty->link->flags);
 	wake_up_interruptible(&tty->link->read_wait);
 	wake_up_interruptible(&tty->link->write_wait);
-	set_bit(TTY_OTHER_CLOSED, &tty->link->flags);
 	if (tty->driver->subtype == PTY_TYPE_MASTER) {
 		set_bit(TTY_OTHER_CLOSED, &tty->flags);
 #ifdef CONFIG_UNIX98_PTYS