void sctp_generate_heartbeat_event(unsigned long data)
{
	int error = 0;
	struct sctp_transport *transport = (struct sctp_transport *) data;
	struct sctp_association *asoc = transport->asoc;
	struct sock *sk = asoc->base.sk;
	struct net *net = sock_net(sk);

	bh_lock_sock(sk);
	if (sock_owned_by_user(sk)) {
		pr_debug("%s: sock is busy\n", __func__);

		/* Try again later.  */
		if (!mod_timer(&transport->hb_timer, jiffies + (HZ/20)))
			sctp_transport_hold(transport);
		goto out_unlock;
	}

	/* Is this structure just waiting around for us to actually
	 * get destroyed?
	 */
	if (transport->dead)
		goto out_unlock;

	error = sctp_do_sm(net, SCTP_EVENT_T_TIMEOUT,
			   SCTP_ST_TIMEOUT(SCTP_EVENT_TIMEOUT_HEARTBEAT),
			   asoc->state, asoc->ep, asoc,
			   transport, GFP_ATOMIC);

	if (error)
		sk->sk_err = -error;

out_unlock:
	bh_unlock_sock(sk);
	sctp_transport_put(transport);
}
The program contains a code sequence that can run concurrently with other code, and the code sequence 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 that is operating concurrently.