tty: Fix high cpu load if tty is unreleaseable

commit 37b16457 upstream.

Kernel oops can cause the tty to be unreleaseable (for example, if
n_tty_read() crashes while on the read_wait queue). This will cause
tty_release() to endlessly loop without sleeping.

Use a killable sleep timeout which grows by 2n+1 jiffies over the interval
[0, 120 secs.) and then jumps to forever (but still killable).

NB: killable just allows for the task to be rewoken manually, not
to be terminated.
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/tty/tty_io.c

@@ -1701,6 +1701,7 @@ int tty_release(struct inode *inode, struct file *filp)
 	int	pty_master, tty_closing, o_tty_closing, do_sleep;
 	int	idx;
 	char	buf[64];
+	long	timeout = 0;
 
 	if (tty_paranoia_check(tty, inode, __func__))
 		return 0;
@@ -1785,7 +1786,11 @@ int tty_release(struct inode *inode, struct file *filp)
 				__func__, tty_name(tty, buf));
 		tty_unlock_pair(tty, o_tty);
 		mutex_unlock(&tty_mutex);
-		schedule();
+		schedule_timeout_killable(timeout);
+		if (timeout < 120 * HZ)
+			timeout = 2 * timeout + 1;
+		else
+			timeout = MAX_SCHEDULE_TIMEOUT;
 	}
 
 	/*