tty: Remove unnecessary re-test of ldisc ref count

Since the tty->ldisc is prevented from being changed by tty_set_ldisc()
when a tty is being hung up, re-testing the ldisc user count is
unnecessary -- ie, it cannot be a different ldisc and the user count
cannot have increased (assuming the caller meets the precondition that
TTY_LDISC flag is cleared)

Removal of the 'early-out' locking optimization is necessary for
the subsequent patch 'tty: Fix ldisc halt sequence on hangup'.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_ldisc.c

@@ -558,29 +558,29 @@ static int tty_ldisc_wait_idle(struct tty_struct *tty, long timeout)
  *	have been halted for this to guarantee it remains idle.
  *
  *	Caller must hold legacy and ->ldisc_mutex.
+ *
+ *	NB: tty_set_ldisc() is prevented from changing the ldisc concurrently
+ *	with this function by checking the TTY_HUPPING flag.
  */
 static bool tty_ldisc_hangup_wait_idle(struct tty_struct *tty)
 {
-	while (tty->ldisc) {	/* Not yet closed */
-		if (atomic_read(&tty->ldisc->users) != 1) {
-			char cur_n[TASK_COMM_LEN], tty_n[64];
-			long timeout = 3 * HZ;
-			tty_unlock(tty);
-
-			while (tty_ldisc_wait_idle(tty, timeout) == -EBUSY) {
-				timeout = MAX_SCHEDULE_TIMEOUT;
-				printk_ratelimited(KERN_WARNING
-					"%s: waiting (%s) for %s took too long, but we keep waiting...\n",
-					__func__, get_task_comm(cur_n, current),
-					tty_name(tty, tty_n));
-			}
-			/* must reacquire both locks and preserve lock order */
-			mutex_unlock(&tty->ldisc_mutex);
-			tty_lock(tty);
-			mutex_lock(&tty->ldisc_mutex);
-			continue;
+	char cur_n[TASK_COMM_LEN], tty_n[64];
+	long timeout = 3 * HZ;
+
+	if (tty->ldisc) {	/* Not yet closed */
+		tty_unlock(tty);
+
+		while (tty_ldisc_wait_idle(tty, timeout) == -EBUSY) {
+			timeout = MAX_SCHEDULE_TIMEOUT;
+			printk_ratelimited(KERN_WARNING
+				"%s: waiting (%s) for %s took too long, but we keep waiting...\n",
+				__func__, get_task_comm(cur_n, current),
+				tty_name(tty, tty_n));
 		}
-		break;
+		/* must reacquire both locks and preserve lock order */
+		mutex_unlock(&tty->ldisc_mutex);
+		tty_lock(tty);
+		mutex_lock(&tty->ldisc_mutex);
 	}
 	return !!tty->ldisc;
 }