int __tcp_retransmit_skb(struct sock *sk, struct sk_buff *skb, int segs)
{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	unsigned int cur_mss;
	int diff, len, err;


	/* Inconclusive MTU probe */
	if (icsk->icsk_mtup.probe_size)
		icsk->icsk_mtup.probe_size = 0;

	/* Do not sent more than we queued. 1/4 is reserved for possible
	 * copying overhead: fragmentation, tunneling, mangling etc.
	 */
	if (refcount_read(&sk->sk_wmem_alloc) >
	    min_t(u32, sk->sk_wmem_queued + (sk->sk_wmem_queued >> 2),
		  sk->sk_sndbuf))
		return -EAGAIN;

	if (skb_still_in_host_queue(sk, skb))
		return -EBUSY;

	if (before(TCP_SKB_CB(skb)->seq, tp->snd_una)) {
		if (unlikely(before(TCP_SKB_CB(skb)->end_seq, tp->snd_una))) {
			WARN_ON_ONCE(1);
			return -EINVAL;
		}
		if (tcp_trim_head(sk, skb, tp->snd_una - TCP_SKB_CB(skb)->seq))
			return -ENOMEM;
	}

	if (inet_csk(sk)->icsk_af_ops->rebuild_header(sk))
		return -EHOSTUNREACH; /* Routing failure or similar. */

	cur_mss = tcp_current_mss(sk);

	/* If receiver has shrunk his window, and skb is out of
	 * new window, do not retransmit it. The exception is the
	 * case, when window is shrunk to zero. In this case
	 * our retransmit serves as a zero window probe.
	 */
	if (!before(TCP_SKB_CB(skb)->seq, tcp_wnd_end(tp)) &&
	    TCP_SKB_CB(skb)->seq != tp->snd_una)
		return -EAGAIN;

	len = cur_mss * segs;
	if (skb->len > len) {
		if (tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb, len,
				 cur_mss, GFP_ATOMIC))
			return -ENOMEM; /* We'll try again later. */
	} else {
		if (skb_unclone(skb, GFP_ATOMIC))
			return -ENOMEM;

		diff = tcp_skb_pcount(skb);
		tcp_set_skb_tso_segs(skb, cur_mss);
		diff -= tcp_skb_pcount(skb);
		if (diff)
			tcp_adjust_pcount(sk, skb, diff);
		if (skb->len < cur_mss)
			tcp_retrans_try_collapse(sk, skb, cur_mss);
	}

	/* RFC3168, section 6.1.1.1. ECN fallback */
	if ((TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN_ECN) == TCPHDR_SYN_ECN)
		tcp_ecn_clear_syn(sk, skb);

	/* Update global and local TCP statistics. */
	segs = tcp_skb_pcount(skb);
	TCP_ADD_STATS(sock_net(sk), TCP_MIB_RETRANSSEGS, segs);
	if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_SYN)
		__NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPSYNRETRANS);
	tp->total_retrans += segs;
	tp->bytes_retrans += skb->len;

	/* make sure skb->data is aligned on arches that require it
	 * and check if ack-trimming & collapsing extended the headroom
	 * beyond what csum_start can cover.
	 */
	if (unlikely((NET_IP_ALIGN && ((unsigned long)skb->data & 3)) ||
		     skb_headroom(skb) >= 0xFFFF)) {
		struct sk_buff *nskb;

		tcp_skb_tsorted_save(skb) {
			nskb = __pskb_copy(skb, MAX_TCP_HEADER, GFP_ATOMIC);
			err = nskb ? tcp_transmit_skb(sk, nskb, 0, GFP_ATOMIC) :
				     -ENOBUFS;
		} tcp_skb_tsorted_restore(skb);

		if (!err) {
			tcp_update_skb_after_send(sk, skb, tp->tcp_wstamp_ns);
			tcp_rate_skb_sent(sk, skb);
		}
	} else {
		err = tcp_transmit_skb(sk, skb, 1, GFP_ATOMIC);
	}

	/* To avoid taking spuriously low RTT samples based on a timestamp
	 * for a transmit that never happened, always mark EVER_RETRANS
	 */
	TCP_SKB_CB(skb)->sacked |= TCPCB_EVER_RETRANS;

	if (BPF_SOCK_OPS_TEST_FLAG(tp, BPF_SOCK_OPS_RETRANS_CB_FLAG))
		tcp_call_bpf_3arg(sk, BPF_SOCK_OPS_RETRANS_CB,
				  TCP_SKB_CB(skb)->seq, segs, err);

	if (likely(!err)) {
		trace_tcp_retransmit_skb(sk, skb);
	} else if (err != -EBUSY) {
		NET_ADD_STATS(sock_net(sk), LINUX_MIB_TCPRETRANSFAIL, segs);
	}
	return err;
}