struct dst_entry *inet_csk_route_req(struct sock *sk,
				     const struct request_sock *req)
{
	struct rtable *rt;
	const struct inet_request_sock *ireq = inet_rsk(req);
	struct ip_options *opt = inet_rsk(req)->opt;
	struct net *net = sock_net(sk);
	struct flowi4 fl4;

	flowi4_init_output(&fl4, sk->sk_bound_dev_if, sk->sk_mark,
			   RT_CONN_FLAGS(sk), RT_SCOPE_UNIVERSE,
			   sk->sk_protocol, inet_sk_flowi_flags(sk),
			   (opt && opt->srr) ? opt->faddr : ireq->rmt_addr,
			   ireq->loc_addr, ireq->rmt_port, inet_sk(sk)->inet_sport);
	security_req_classify_flow(req, flowi4_to_flowi(&fl4));
	rt = ip_route_output_flow(net, &fl4, sk);
	if (IS_ERR(rt))
		goto no_route;
	if (opt && opt->is_strictroute && rt->rt_dst != rt->rt_gateway)
		goto route_err;
	return &rt->dst;

route_err:
	ip_rt_put(rt);
no_route:
	IP_INC_STATS_BH(net, IPSTATS_MIB_OUTNOROUTES);
	return NULL;
}
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.