tty: Fix high cpu load if tty is unreleaseable

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.

Cc: <stable@vger.kernel.org>
Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit 37b16457)
Signed-off-by: Willy Tarreau <w@1wt.eu>

drivers/char/tty_io.c

@@ -1482,6 +1482,7 @@ void tty_release_dev(struct file *filp)
 	int	devpts;
 	int	idx;
 	char	buf[64];
+	long	timeout = 0;
 	struct 	inode *inode;
 
 	inode = filp->f_path.dentry->d_inode;
@@ -1602,7 +1603,11 @@ void tty_release_dev(struct file *filp)
 		printk(KERN_WARNING "tty_release_dev: %s: read/write wait queue "
 				    "active!\n", tty_name(tty, buf));
 		mutex_unlock(&tty_mutex);
-		schedule();
+		schedule_timeout_killable(timeout);
+		if (timeout < 120 * HZ)
+			timeout = 2 * timeout + 1;
+		else
+			timeout = MAX_SCHEDULE_TIMEOUT;
 	}
 
 	/*