int ip_build_and_send_pkt(struct sk_buff *skb, struct sock *sk,
			  __be32 saddr, __be32 daddr, struct ip_options_rcu *opt)
{
	struct inet_sock *inet = inet_sk(sk);
	struct rtable *rt = skb_rtable(skb);
	struct iphdr *iph;

	/* Build the IP header. */
	skb_push(skb, sizeof(struct iphdr) + (opt ? opt->opt.optlen : 0));
	skb_reset_network_header(skb);
	iph = ip_hdr(skb);
	iph->version  = 4;
	iph->ihl      = 5;
	iph->tos      = inet->tos;
	if (ip_dont_fragment(sk, &rt->dst))
		iph->frag_off = htons(IP_DF);
	else
		iph->frag_off = 0;
	iph->ttl      = ip_select_ttl(inet, &rt->dst);
	iph->daddr    = rt->rt_dst;
	iph->saddr    = rt->rt_src;
	iph->protocol = sk->sk_protocol;
	ip_select_ident(iph, &rt->dst, sk);

	if (opt && opt->opt.optlen) {
		iph->ihl += opt->opt.optlen>>2;
		ip_options_build(skb, &opt->opt, daddr, rt, 0);
	}

	skb->priority = sk->sk_priority;
	skb->mark = sk->sk_mark;

	/* Send it out. */
	return ip_local_out(skb);
}
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.