static int tiocgsid(struct tty_struct *tty, struct tty_struct *real_tty, pid_t __user *p)
{
	/*
	 * (tty == real_tty) is a cheap way of
	 * testing if the tty is NOT a master pty.
	*/
	if (tty == real_tty && current->signal->tty != real_tty)
		return -ENOTTY;
	if (!real_tty->session)
		return -ENOTTY;
	return put_user(pid_vnr(real_tty->session), p);
}

The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.