Revert "tty: fix stall caused by missing memory barrier in drivers/tty/n_tty.c"

This reverts commit af32cc7b.

The commit was incorrectly backported and was causing hangs.
Reported-by: Corey Wright <undefined@pobox.com>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>

drivers/tty/n_tty.c

@@ -364,7 +364,7 @@ static void n_tty_packet_mode_flush(struct tty_struct *tty)
 	spin_lock_irqsave(&tty->ctrl_lock, flags);
 	if (tty->link->packet) {
 		tty->ctrl_status |= TIOCPKT_FLUSHREAD;
-		spin_unlock_irqrestore(&tty->ctrl_lock, flags);
+		if (waitqueue_active(&tty->link->read_wait))
 			wake_up_interruptible(&tty->link->read_wait);
 	}
 	spin_unlock_irqrestore(&tty->ctrl_lock, flags);
@@ -1387,6 +1387,7 @@ n_tty_receive_char_special(struct tty_struct *tty, unsigned char c)
 			put_tty_queue(c, ldata);
 			ldata->canon_head = ldata->read_head;
 			kill_fasync(&tty->fasync, SIGIO, POLL_IN);
+			if (waitqueue_active(&tty->read_wait))
 				wake_up_interruptible_poll(&tty->read_wait, POLLIN);
 			return 0;
 		}
@@ -1670,6 +1671,7 @@ static void __receive_buf(struct tty_struct *tty, const unsigned char *cp,
 	if ((!ldata->icanon && (read_cnt(ldata) >= ldata->minimum_to_wake)) ||
 		L_EXTPROC(tty)) {
 		kill_fasync(&tty->fasync, SIGIO, POLL_IN);
+		if (waitqueue_active(&tty->read_wait))
 			wake_up_interruptible_poll(&tty->read_wait, POLLIN);
 	}
 }
@@ -1889,7 +1891,9 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 	}
 
 	/* The termios change make the tty ready for I/O */
+	if (waitqueue_active(&tty->write_wait))
 		wake_up_interruptible(&tty->write_wait);
+	if (waitqueue_active(&tty->read_wait))
 		wake_up_interruptible(&tty->read_wait);
 }