net/ipv4/tcp_timer.c - kernel/msm-4.9 - Gitiles

 /*
  * INET		An implementation of the TCP/IP protocol suite for the LINUX
  *		operating system.  INET is implemented using the  BSD Socket
  *		interface as the means of communication with the user level.
  *
  *		Implementation of the Transmission Control Protocol(TCP).
  *
  * Authors:	Ross Biro
  *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
  *		Mark Evans, <evansmp@uhura.aston.ac.uk>
  *		Corey Minyard <wf-rch!minyard@relay.EU.net>
  *		Florian La Roche, <flla@stud.uni-sb.de>
  *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
  *		Linus Torvalds, <torvalds@cs.helsinki.fi>
  *		Alan Cox, <gw4pts@gw4pts.ampr.org>
  *		Matthew Dillon, <dillon@apollo.west.oic.com>
  *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
  *		Jorge Cwik, <jorge@laser.satlink.net>
  */

 #include <linux/module.h>
 #include <linux/gfp.h>
 #include <net/tcp.h>

 int sysctl_tcp_syn_retries __read_mostly = TCP_SYN_RETRIES;
 int sysctl_tcp_synack_retries __read_mostly = TCP_SYNACK_RETRIES;
 int sysctl_tcp_keepalive_time __read_mostly = TCP_KEEPALIVE_TIME;
 int sysctl_tcp_keepalive_probes __read_mostly = TCP_KEEPALIVE_PROBES;
 int sysctl_tcp_keepalive_intvl __read_mostly = TCP_KEEPALIVE_INTVL;
 int sysctl_tcp_retries1 __read_mostly = TCP_RETR1;
 int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
 int sysctl_tcp_orphan_retries __read_mostly;
 int sysctl_tcp_thin_linear_timeouts __read_mostly;

 static void tcp_write_err(struct sock *sk)
 {
 	sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
 	sk->sk_error_report(sk);

 	tcp_done(sk);
 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
 }

 /* Do not allow orphaned sockets to eat all our resources.
  * This is direct violation of TCP specs, but it is required
  * to prevent DoS attacks. It is called when a retransmission timeout
  * or zero probe timeout occurs on orphaned socket.
  *
  * Criteria is still not confirmed experimentally and may change.
  * We kill the socket, if:
  * 1. If number of orphaned sockets exceeds an administratively configured
  *    limit.
  * 2. If we have strong memory pressure.
  */
 static int tcp_out_of_resources(struct sock *sk, bool do_reset)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	int shift = 0;

 	/* If peer does not open window for long time, or did not transmit
 	 * anything for long time, penalize it. */
 	if ((s32)(tcp_time_stamp - tp->lsndtime) > 2*TCP_RTO_MAX || !do_reset)
 		shift++;

 	/* If some dubious ICMP arrived, penalize even more. */
 	if (sk->sk_err_soft)
 		shift++;

 	if (tcp_check_oom(sk, shift)) {
 		/* Catch exceptional cases, when connection requires reset.
 		 *      1. Last segment was sent recently. */
 		if ((s32)(tcp_time_stamp - tp->lsndtime) <= TCP_TIMEWAIT_LEN ||
 		    /*  2. Window is closed. */
 		    (!tp->snd_wnd && !tp->packets_out))
 			do_reset = true;
 		if (do_reset)
 			tcp_send_active_reset(sk, GFP_ATOMIC);
 		tcp_done(sk);
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONMEMORY);
 		return 1;
 	}
 	return 0;
 }

 /* Calculate maximal number or retries on an orphaned socket. */
 static int tcp_orphan_retries(struct sock *sk, int alive)
 {
 	int retries = sysctl_tcp_orphan_retries; /* May be zero. */

 	/* We know from an ICMP that something is wrong. */
 	if (sk->sk_err_soft && !alive)
 		retries = 0;

 	/* However, if socket sent something recently, select some safe
 	 * number of retries. 8 corresponds to >100 seconds with minimal
 	 * RTO of 200msec. */
 	if (retries == 0 && alive)
 		retries = 8;
 	return retries;
 }

 static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
 {
 	struct net *net = sock_net(sk);

 	/* Black hole detection */
 	if (net->ipv4.sysctl_tcp_mtu_probing) {
 		if (!icsk->icsk_mtup.enabled) {
 			icsk->icsk_mtup.enabled = 1;
 			icsk->icsk_mtup.probe_timestamp = tcp_time_stamp;
 			tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 		} else {
 			struct net *net = sock_net(sk);
 			struct tcp_sock *tp = tcp_sk(sk);
 			int mss;

 			mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
 			mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
 			mss = max(mss, 68 - tp->tcp_header_len);
 			icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
 			tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
 		}
 	}
 }

 /* This function calculates a "timeout" which is equivalent to the timeout of a
  * TCP connection after "boundary" unsuccessful, exponentially backed-off
  * retransmissions with an initial RTO of TCP_RTO_MIN or TCP_TIMEOUT_INIT if
  * syn_set flag is set.
  */
 static bool retransmits_timed_out(struct sock *sk,
 				  unsigned int boundary,
 				  unsigned int timeout,
 				  bool syn_set)
 {
 	unsigned int linear_backoff_thresh, start_ts;
 	unsigned int rto_base = syn_set ? TCP_TIMEOUT_INIT : TCP_RTO_MIN;

 	if (!inet_csk(sk)->icsk_retransmits)
 		return false;

 	start_ts = tcp_sk(sk)->retrans_stamp;
 	if (unlikely(!start_ts))
 		start_ts = tcp_skb_timestamp(tcp_write_queue_head(sk));

 	if (likely(timeout == 0)) {
 		linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);

 		if (boundary <= linear_backoff_thresh)
 			timeout = ((2 << boundary) - 1) * rto_base;
 		else
 			timeout = ((2 << linear_backoff_thresh) - 1) * rto_base +
 				(boundary - linear_backoff_thresh) * TCP_RTO_MAX;
 	}
 	return (tcp_time_stamp - start_ts) >= timeout;
 }

 /* A write timeout has occurred. Process the after effects. */
 static int tcp_write_timeout(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	int retry_until;
 	bool do_reset, syn_set = false;

 	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
 		if (icsk->icsk_retransmits) {
 			dst_negative_advice(sk);
 			if (tp->syn_fastopen || tp->syn_data)
 				tcp_fastopen_cache_set(sk, 0, NULL, true);
 			if (tp->syn_data)
 				NET_INC_STATS_BH(sock_net(sk),
 						 LINUX_MIB_TCPFASTOPENACTIVEFAIL);
 		}
 		retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
 		syn_set = true;
 	} else {
 		if (retransmits_timed_out(sk, sysctl_tcp_retries1, 0, 0)) {
 			/* Black hole detection */
 			tcp_mtu_probing(icsk, sk);

 			dst_negative_advice(sk);
 		}

 		retry_until = sysctl_tcp_retries2;
 		if (sock_flag(sk, SOCK_DEAD)) {
 			const int alive = icsk->icsk_rto < TCP_RTO_MAX;

 			retry_until = tcp_orphan_retries(sk, alive);
 			do_reset = alive ||
 				!retransmits_timed_out(sk, retry_until, 0, 0);

 			if (tcp_out_of_resources(sk, do_reset))
 				return 1;
 		}
 	}

 	if (retransmits_timed_out(sk, retry_until,
 				  syn_set ? 0 : icsk->icsk_user_timeout, syn_set)) {
 		/* Has it gone just too far? */
 		tcp_write_err(sk);
 		return 1;
 	}
 	return 0;
 }

 void tcp_delack_timer_handler(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct inet_connection_sock *icsk = inet_csk(sk);

 	sk_mem_reclaim_partial(sk);

 	if (sk->sk_state == TCP_CLOSE || !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
 		goto out;

 	if (time_after(icsk->icsk_ack.timeout, jiffies)) {
 		sk_reset_timer(sk, &icsk->icsk_delack_timer, icsk->icsk_ack.timeout);
 		goto out;
 	}
 	icsk->icsk_ack.pending &= ~ICSK_ACK_TIMER;

 	if (!skb_queue_empty(&tp->ucopy.prequeue)) {
 		struct sk_buff *skb;

 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSCHEDULERFAILED);

 		while ((skb = __skb_dequeue(&tp->ucopy.prequeue)) != NULL)
 			sk_backlog_rcv(sk, skb);

 		tp->ucopy.memory = 0;
 	}

 	if (inet_csk_ack_scheduled(sk)) {
 		if (!icsk->icsk_ack.pingpong) {
 			/* Delayed ACK missed: inflate ATO. */
 			icsk->icsk_ack.ato = min(icsk->icsk_ack.ato << 1, icsk->icsk_rto);
 		} else {
 			/* Delayed ACK missed: leave pingpong mode and
 			 * deflate ATO.
 			 */
 			icsk->icsk_ack.pingpong = 0;
 			icsk->icsk_ack.ato      = TCP_ATO_MIN;
 		}
 		tcp_send_ack(sk);
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKS);
 	}

 out:
 	if (sk_under_memory_pressure(sk))
 		sk_mem_reclaim(sk);
 }

 static void tcp_delack_timer(unsigned long data)
 {
 	struct sock *sk = (struct sock *)data;

 	bh_lock_sock(sk);
 	if (!sock_owned_by_user(sk)) {
 		tcp_delack_timer_handler(sk);
 	} else {
 		inet_csk(sk)->icsk_ack.blocked = 1;
 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
 		/* deleguate our work to tcp_release_cb() */
 		if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
 			sock_hold(sk);
 	}
 	bh_unlock_sock(sk);
 	sock_put(sk);
 }

 static void tcp_probe_timer(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	int max_probes;
 	u32 start_ts;

 	if (tp->packets_out || !tcp_send_head(sk)) {
 		icsk->icsk_probes_out = 0;
 		return;
 	}

 	/* RFC 1122 4.2.2.17 requires the sender to stay open indefinitely as
 	 * long as the receiver continues to respond probes. We support this by
 	 * default and reset icsk_probes_out with incoming ACKs. But if the
 	 * socket is orphaned or the user specifies TCP_USER_TIMEOUT, we
 	 * kill the socket when the retry count and the time exceeds the
 	 * corresponding system limit. We also implement similar policy when
 	 * we use RTO to probe window in tcp_retransmit_timer().
 	 */
 	start_ts = tcp_skb_timestamp(tcp_send_head(sk));
 	if (!start_ts)
 		skb_mstamp_get(&tcp_send_head(sk)->skb_mstamp);
 	else if (icsk->icsk_user_timeout &&
 		 (s32)(tcp_time_stamp - start_ts) > icsk->icsk_user_timeout)
 		goto abort;

 	max_probes = sysctl_tcp_retries2;
 	if (sock_flag(sk, SOCK_DEAD)) {
 		const int alive = inet_csk_rto_backoff(icsk, TCP_RTO_MAX) < TCP_RTO_MAX;

 		max_probes = tcp_orphan_retries(sk, alive);
 		if (!alive && icsk->icsk_backoff >= max_probes)
 			goto abort;
 		if (tcp_out_of_resources(sk, true))
 			return;
 	}

 	if (icsk->icsk_probes_out > max_probes) {
 abort:		tcp_write_err(sk);
 	} else {
 		/* Only send another probe if we didn't close things up. */
 		tcp_send_probe0(sk);
 	}
 }

 /*
  *	Timer for Fast Open socket to retransmit SYNACK. Note that the
  *	sk here is the child socket, not the parent (listener) socket.
  */
 static void tcp_fastopen_synack_timer(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	int max_retries = icsk->icsk_syn_retries ? :
 	    sysctl_tcp_synack_retries + 1; /* add one more retry for fastopen */
 	struct request_sock *req;

 	req = tcp_sk(sk)->fastopen_rsk;
 	req->rsk_ops->syn_ack_timeout(req);

 	if (req->num_timeout >= max_retries) {
 		tcp_write_err(sk);
 		return;
 	}
 	/* XXX (TFO) - Unlike regular SYN-ACK retransmit, we ignore error
 	 * returned from rtx_syn_ack() to make it more persistent like
 	 * regular retransmit because if the child socket has been accepted
 	 * it's not good to give up too easily.
 	 */
 	inet_rtx_syn_ack(sk, req);
 	req->num_timeout++;
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
 			  TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
 }

 /*
  *	The TCP retransmit timer.
  */

 void tcp_retransmit_timer(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 	struct inet_connection_sock *icsk = inet_csk(sk);

 	if (tp->fastopen_rsk) {
 		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
 			     sk->sk_state != TCP_FIN_WAIT1);
 		tcp_fastopen_synack_timer(sk);
 		/* Before we receive ACK to our SYN-ACK don't retransmit
 		 * anything else (e.g., data or FIN segments).
 		 */
 		return;
 	}
 	if (!tp->packets_out)
 		goto out;

 	WARN_ON(tcp_write_queue_empty(sk));

 	tp->tlp_high_seq = 0;

 	if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) &&
 	    !((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))) {
 		/* Receiver dastardly shrinks window. Our retransmits
 		 * become zero probes, but we should not timeout this
 		 * connection. If the socket is an orphan, time it out,
 		 * we cannot allow such beasts to hang infinitely.
 		 */
 		struct inet_sock *inet = inet_sk(sk);
 		if (sk->sk_family == AF_INET) {
 			net_dbg_ratelimited("Peer %pI4:%u/%u unexpectedly shrunk window %u:%u (repaired)\n",
 					    &inet->inet_daddr,
 					    ntohs(inet->inet_dport),
 					    inet->inet_num,
 					    tp->snd_una, tp->snd_nxt);
 		}
 #if IS_ENABLED(CONFIG_IPV6)
 		else if (sk->sk_family == AF_INET6) {
 			net_dbg_ratelimited("Peer %pI6:%u/%u unexpectedly shrunk window %u:%u (repaired)\n",
 					    &sk->sk_v6_daddr,
 					    ntohs(inet->inet_dport),
 					    inet->inet_num,
 					    tp->snd_una, tp->snd_nxt);
 		}
 #endif
 		if (tcp_time_stamp - tp->rcv_tstamp > TCP_RTO_MAX) {
 			tcp_write_err(sk);
 			goto out;
 		}
 		tcp_enter_loss(sk);
 		tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
 		__sk_dst_reset(sk);
 		goto out_reset_timer;
 	}

 	if (tcp_write_timeout(sk))
 		goto out;

 	if (icsk->icsk_retransmits == 0) {
 		int mib_idx;

 		if (icsk->icsk_ca_state == TCP_CA_Recovery) {
 			if (tcp_is_sack(tp))
 				mib_idx = LINUX_MIB_TCPSACKRECOVERYFAIL;
 			else
 				mib_idx = LINUX_MIB_TCPRENORECOVERYFAIL;
 		} else if (icsk->icsk_ca_state == TCP_CA_Loss) {
 			mib_idx = LINUX_MIB_TCPLOSSFAILURES;
 		} else if ((icsk->icsk_ca_state == TCP_CA_Disorder) ||
 			   tp->sacked_out) {
 			if (tcp_is_sack(tp))
 				mib_idx = LINUX_MIB_TCPSACKFAILURES;
 			else
 				mib_idx = LINUX_MIB_TCPRENOFAILURES;
 		} else {
 			mib_idx = LINUX_MIB_TCPTIMEOUTS;
 		}
 		NET_INC_STATS_BH(sock_net(sk), mib_idx);
 	}

 	tcp_enter_loss(sk);

 	if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk)) > 0) {
 		/* Retransmission failed because of local congestion,
 		 * do not backoff.
 		 */
 		if (!icsk->icsk_retransmits)
 			icsk->icsk_retransmits = 1;
 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
 					  min(icsk->icsk_rto, TCP_RESOURCE_PROBE_INTERVAL),
 					  TCP_RTO_MAX);
 		goto out;
 	}

 	/* Increase the timeout each time we retransmit.  Note that
 	 * we do not increase the rtt estimate.  rto is initialized
 	 * from rtt, but increases here.  Jacobson (SIGCOMM 88) suggests
 	 * that doubling rto each time is the least we can get away with.
 	 * In KA9Q, Karn uses this for the first few times, and then
 	 * goes to quadratic.  netBSD doubles, but only goes up to *64,
 	 * and clamps at 1 to 64 sec afterwards.  Note that 120 sec is
 	 * defined in the protocol as the maximum possible RTT.  I guess
 	 * we'll have to use something other than TCP to talk to the
 	 * University of Mars.
 	 *
 	 * PAWS allows us longer timeouts and large windows, so once
 	 * implemented ftp to mars will work nicely. We will have to fix
 	 * the 120 second clamps though!
 	 */
 	icsk->icsk_backoff++;
 	icsk->icsk_retransmits++;

 out_reset_timer:
 	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
 	 * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
 	 * might be increased if the stream oscillates between thin and thick,
 	 * thus the old value might already be too high compared to the value
 	 * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
 	 * backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
 	 * exponential backoff behaviour to avoid continue hammering
 	 * linear-timeout retransmissions into a black hole
 	 */
 	if (sk->sk_state == TCP_ESTABLISHED &&
 	    (tp->thin_lto || sysctl_tcp_thin_linear_timeouts) &&
 	    tcp_stream_is_thin(tp) &&
 	    icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
 		icsk->icsk_backoff = 0;
 		icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
 	} else {
 		/* Use normal (exponential) backoff */
 		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
 	}
 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
 	if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1, 0, 0))
 		__sk_dst_reset(sk);

 out:;
 }

 void tcp_write_timer_handler(struct sock *sk)
 {
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	int event;

 	if (sk->sk_state == TCP_CLOSE || !icsk->icsk_pending)
 		goto out;

 	if (time_after(icsk->icsk_timeout, jiffies)) {
 		sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
 		goto out;
 	}

 	event = icsk->icsk_pending;

 	switch (event) {
 	case ICSK_TIME_EARLY_RETRANS:
 		tcp_resume_early_retransmit(sk);
 		break;
 	case ICSK_TIME_LOSS_PROBE:
 		tcp_send_loss_probe(sk);
 		break;
 	case ICSK_TIME_RETRANS:
 		icsk->icsk_pending = 0;
 		tcp_retransmit_timer(sk);
 		break;
 	case ICSK_TIME_PROBE0:
 		icsk->icsk_pending = 0;
 		tcp_probe_timer(sk);
 		break;
 	}

 out:
 	sk_mem_reclaim(sk);
 }

 static void tcp_write_timer(unsigned long data)
 {
 	struct sock *sk = (struct sock *)data;

 	bh_lock_sock(sk);
 	if (!sock_owned_by_user(sk)) {
 		tcp_write_timer_handler(sk);
 	} else {
 		/* deleguate our work to tcp_release_cb() */
 		if (!test_and_set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
 			sock_hold(sk);
 	}
 	bh_unlock_sock(sk);
 	sock_put(sk);
 }

 void tcp_syn_ack_timeout(const struct request_sock *req)
 {
 	struct net *net = read_pnet(&inet_rsk(req)->ireq_net);

 	NET_INC_STATS_BH(net, LINUX_MIB_TCPTIMEOUTS);
 }
 EXPORT_SYMBOL(tcp_syn_ack_timeout);

 void tcp_set_keepalive(struct sock *sk, int val)
 {
 	if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
 		return;

 	if (val && !sock_flag(sk, SOCK_KEEPOPEN))
 		inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
 	else if (!val)
 		inet_csk_delete_keepalive_timer(sk);
 }


 static void tcp_keepalive_timer (unsigned long data)
 {
 	struct sock *sk = (struct sock *) data;
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct tcp_sock *tp = tcp_sk(sk);
 	u32 elapsed;

 	/* Only process if socket is not in use. */
 	bh_lock_sock(sk);
 	if (sock_owned_by_user(sk)) {
 		/* Try again later. */
 		inet_csk_reset_keepalive_timer (sk, HZ/20);
 		goto out;
 	}

 	if (sk->sk_state == TCP_LISTEN) {
 		pr_err("Hmm... keepalive on a LISTEN ???\n");
 		goto out;
 	}

 	if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
 		if (tp->linger2 >= 0) {
 			const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;

 			if (tmo > 0) {
 				tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
 				goto out;
 			}
 		}
 		tcp_send_active_reset(sk, GFP_ATOMIC);
 		goto death;
 	}

 	if (!sock_flag(sk, SOCK_KEEPOPEN) || sk->sk_state == TCP_CLOSE)
 		goto out;

 	elapsed = keepalive_time_when(tp);

 	/* It is alive without keepalive 8) */
 	if (tp->packets_out || tcp_send_head(sk))
 		goto resched;

 	elapsed = keepalive_time_elapsed(tp);

 	if (elapsed >= keepalive_time_when(tp)) {
 		/* If the TCP_USER_TIMEOUT option is enabled, use that
 		 * to determine when to timeout instead.
 		 */
 		if ((icsk->icsk_user_timeout != 0 &&
 		    elapsed >= icsk->icsk_user_timeout &&
 		    icsk->icsk_probes_out > 0) ||
 		    (icsk->icsk_user_timeout == 0 &&
 		    icsk->icsk_probes_out >= keepalive_probes(tp))) {
 			tcp_send_active_reset(sk, GFP_ATOMIC);
 			tcp_write_err(sk);
 			goto out;
 		}
 		if (tcp_write_wakeup(sk) <= 0) {
 			icsk->icsk_probes_out++;
 			elapsed = keepalive_intvl_when(tp);
 		} else {
 			/* If keepalive was lost due to local congestion,
 			 * try harder.
 			 */
 			elapsed = TCP_RESOURCE_PROBE_INTERVAL;
 		}
 	} else {
 		/* It is tp->rcv_tstamp + keepalive_time_when(tp) */
 		elapsed = keepalive_time_when(tp) - elapsed;
 	}

 	sk_mem_reclaim(sk);

 resched:
 	inet_csk_reset_keepalive_timer (sk, elapsed);
 	goto out;

 death:
 	tcp_done(sk);

 out:
 	bh_unlock_sock(sk);
 	sock_put(sk);
 }

 void tcp_init_xmit_timers(struct sock *sk)
 {
 	inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
 				  &tcp_keepalive_timer);
 }
 EXPORT_SYMBOL(tcp_init_xmit_timers);
	/*
	* INET An implementation of the TCP/IP protocol suite for the LINUX
	* operating system. INET is implemented using the BSD Socket
	* interface as the means of communication with the user level.
	*
	* Implementation of the Transmission Control Protocol(TCP).
	*
	* Authors: Ross Biro
	* Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
	* Mark Evans, <evansmp@uhura.aston.ac.uk>
	* Corey Minyard <wf-rch!minyard@relay.EU.net>
	* Florian La Roche, <flla@stud.uni-sb.de>
	* Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
	* Linus Torvalds, <torvalds@cs.helsinki.fi>
	* Alan Cox, <gw4pts@gw4pts.ampr.org>
	* Matthew Dillon, <dillon@apollo.west.oic.com>
	* Arnt Gulbrandsen, <agulbra@nvg.unit.no>
	* Jorge Cwik, <jorge@laser.satlink.net>
	*/

	#include <linux/module.h>
	#include <linux/gfp.h>
	#include <net/tcp.h>

	int sysctl_tcp_syn_retries __read_mostly = TCP_SYN_RETRIES;
	int sysctl_tcp_synack_retries __read_mostly = TCP_SYNACK_RETRIES;
	int sysctl_tcp_keepalive_time __read_mostly = TCP_KEEPALIVE_TIME;
	int sysctl_tcp_keepalive_probes __read_mostly = TCP_KEEPALIVE_PROBES;
	int sysctl_tcp_keepalive_intvl __read_mostly = TCP_KEEPALIVE_INTVL;
	int sysctl_tcp_retries1 __read_mostly = TCP_RETR1;
	int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
	int sysctl_tcp_orphan_retries __read_mostly;
	int sysctl_tcp_thin_linear_timeouts __read_mostly;

	static void tcp_write_err(struct sock *sk)
	{
	sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
	sk->sk_error_report(sk);

	tcp_done(sk);
	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
	}

	/* Do not allow orphaned sockets to eat all our resources.
	* This is direct violation of TCP specs, but it is required
	* to prevent DoS attacks. It is called when a retransmission timeout
	* or zero probe timeout occurs on orphaned socket.
	*
	* Criteria is still not confirmed experimentally and may change.
	* We kill the socket, if:
	* 1. If number of orphaned sockets exceeds an administratively configured
	* limit.
	* 2. If we have strong memory pressure.
	*/
	static int tcp_out_of_resources(struct sock *sk, bool do_reset)
	{
	struct tcp_sock *tp = tcp_sk(sk);
	int shift = 0;

	/* If peer does not open window for long time, or did not transmit
	* anything for long time, penalize it. */
	if ((s32)(tcp_time_stamp - tp->lsndtime) > 2*TCP_RTO_MAX \|\| !do_reset)
	shift++;

	/* If some dubious ICMP arrived, penalize even more. */
	if (sk->sk_err_soft)
	shift++;

	if (tcp_check_oom(sk, shift)) {
	/* Catch exceptional cases, when connection requires reset.
	* 1. Last segment was sent recently. */
	if ((s32)(tcp_time_stamp - tp->lsndtime) <= TCP_TIMEWAIT_LEN \|\|
	/* 2. Window is closed. */
	(!tp->snd_wnd && !tp->packets_out))
	do_reset = true;
	if (do_reset)
	tcp_send_active_reset(sk, GFP_ATOMIC);
	tcp_done(sk);
	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONMEMORY);
	return 1;
	}
	return 0;
	}

	/* Calculate maximal number or retries on an orphaned socket. */
	static int tcp_orphan_retries(struct sock *sk, int alive)
	{
	int retries = sysctl_tcp_orphan_retries; /* May be zero. */

	/* We know from an ICMP that something is wrong. */
	if (sk->sk_err_soft && !alive)
	retries = 0;

	/* However, if socket sent something recently, select some safe
	* number of retries. 8 corresponds to >100 seconds with minimal
	* RTO of 200msec. */
	if (retries == 0 && alive)
	retries = 8;
	return retries;
	}

	static void tcp_mtu_probing(struct inet_connection_sock icsk, struct sock sk)
	{
	struct net *net = sock_net(sk);

	/* Black hole detection */
	if (net->ipv4.sysctl_tcp_mtu_probing) {
	if (!icsk->icsk_mtup.enabled) {
	icsk->icsk_mtup.enabled = 1;
	icsk->icsk_mtup.probe_timestamp = tcp_time_stamp;
	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
	} else {
	struct net *net = sock_net(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	int mss;

	mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
	mss = min(net->ipv4.sysctl_tcp_base_mss, mss);
	mss = max(mss, 68 - tp->tcp_header_len);
	icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
	tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
	}
	}
	}

	/* This function calculates a "timeout" which is equivalent to the timeout of a
	* TCP connection after "boundary" unsuccessful, exponentially backed-off
	* retransmissions with an initial RTO of TCP_RTO_MIN or TCP_TIMEOUT_INIT if
	* syn_set flag is set.
	*/
	static bool retransmits_timed_out(struct sock *sk,
	unsigned int boundary,
	unsigned int timeout,
	bool syn_set)
	{
	unsigned int linear_backoff_thresh, start_ts;
	unsigned int rto_base = syn_set ? TCP_TIMEOUT_INIT : TCP_RTO_MIN;

	if (!inet_csk(sk)->icsk_retransmits)
	return false;

	start_ts = tcp_sk(sk)->retrans_stamp;
	if (unlikely(!start_ts))
	start_ts = tcp_skb_timestamp(tcp_write_queue_head(sk));

	if (likely(timeout == 0)) {
	linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);

	if (boundary <= linear_backoff_thresh)
	timeout = ((2 << boundary) - 1) * rto_base;
	else
	timeout = ((2 << linear_backoff_thresh) - 1) * rto_base +
	(boundary - linear_backoff_thresh) * TCP_RTO_MAX;
	}
	return (tcp_time_stamp - start_ts) >= timeout;
	}

	/* A write timeout has occurred. Process the after effects. */
	static int tcp_write_timeout(struct sock *sk)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	int retry_until;
	bool do_reset, syn_set = false;

	if ((1 << sk->sk_state) & (TCPF_SYN_SENT \| TCPF_SYN_RECV)) {
	if (icsk->icsk_retransmits) {
	dst_negative_advice(sk);
	if (tp->syn_fastopen \|\| tp->syn_data)
	tcp_fastopen_cache_set(sk, 0, NULL, true);
	if (tp->syn_data)
	NET_INC_STATS_BH(sock_net(sk),
	LINUX_MIB_TCPFASTOPENACTIVEFAIL);
	}
	retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
	syn_set = true;
	} else {
	if (retransmits_timed_out(sk, sysctl_tcp_retries1, 0, 0)) {
	/* Black hole detection */
	tcp_mtu_probing(icsk, sk);

	dst_negative_advice(sk);
	}

	retry_until = sysctl_tcp_retries2;
	if (sock_flag(sk, SOCK_DEAD)) {
	const int alive = icsk->icsk_rto < TCP_RTO_MAX;

	retry_until = tcp_orphan_retries(sk, alive);
	do_reset = alive \|\|
	!retransmits_timed_out(sk, retry_until, 0, 0);

	if (tcp_out_of_resources(sk, do_reset))
	return 1;
	}
	}

	if (retransmits_timed_out(sk, retry_until,
	syn_set ? 0 : icsk->icsk_user_timeout, syn_set)) {
	/* Has it gone just too far? */
	tcp_write_err(sk);
	return 1;
	}
	return 0;
	}

	void tcp_delack_timer_handler(struct sock *sk)
	{
	struct tcp_sock *tp = tcp_sk(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);

	sk_mem_reclaim_partial(sk);

	if (sk->sk_state == TCP_CLOSE \|\| !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
	goto out;

	if (time_after(icsk->icsk_ack.timeout, jiffies)) {
	sk_reset_timer(sk, &icsk->icsk_delack_timer, icsk->icsk_ack.timeout);
	goto out;
	}
	icsk->icsk_ack.pending &= ~ICSK_ACK_TIMER;

	if (!skb_queue_empty(&tp->ucopy.prequeue)) {
	struct sk_buff *skb;

	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSCHEDULERFAILED);

	while ((skb = __skb_dequeue(&tp->ucopy.prequeue)) != NULL)
	sk_backlog_rcv(sk, skb);

	tp->ucopy.memory = 0;
	}

	if (inet_csk_ack_scheduled(sk)) {
	if (!icsk->icsk_ack.pingpong) {
	/* Delayed ACK missed: inflate ATO. */
	icsk->icsk_ack.ato = min(icsk->icsk_ack.ato << 1, icsk->icsk_rto);
	} else {
	/* Delayed ACK missed: leave pingpong mode and
	* deflate ATO.
	*/
	icsk->icsk_ack.pingpong = 0;
	icsk->icsk_ack.ato = TCP_ATO_MIN;
	}
	tcp_send_ack(sk);
	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKS);
	}

	out:
	if (sk_under_memory_pressure(sk))
	sk_mem_reclaim(sk);
	}

	static void tcp_delack_timer(unsigned long data)
	{
	struct sock sk = (struct sock )data;

	bh_lock_sock(sk);
	if (!sock_owned_by_user(sk)) {
	tcp_delack_timer_handler(sk);
	} else {
	inet_csk(sk)->icsk_ack.blocked = 1;
	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
	/* deleguate our work to tcp_release_cb() */
	if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
	sock_hold(sk);
	}
	bh_unlock_sock(sk);
	sock_put(sk);
	}

	static void tcp_probe_timer(struct sock *sk)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	int max_probes;
	u32 start_ts;

	if (tp->packets_out \|\| !tcp_send_head(sk)) {
	icsk->icsk_probes_out = 0;
	return;
	}

	/* RFC 1122 4.2.2.17 requires the sender to stay open indefinitely as
	* long as the receiver continues to respond probes. We support this by
	* default and reset icsk_probes_out with incoming ACKs. But if the
	* socket is orphaned or the user specifies TCP_USER_TIMEOUT, we
	* kill the socket when the retry count and the time exceeds the
	* corresponding system limit. We also implement similar policy when
	* we use RTO to probe window in tcp_retransmit_timer().
	*/
	start_ts = tcp_skb_timestamp(tcp_send_head(sk));
	if (!start_ts)
	skb_mstamp_get(&tcp_send_head(sk)->skb_mstamp);
	else if (icsk->icsk_user_timeout &&
	(s32)(tcp_time_stamp - start_ts) > icsk->icsk_user_timeout)
	goto abort;

	max_probes = sysctl_tcp_retries2;
	if (sock_flag(sk, SOCK_DEAD)) {
	const int alive = inet_csk_rto_backoff(icsk, TCP_RTO_MAX) < TCP_RTO_MAX;

	max_probes = tcp_orphan_retries(sk, alive);
	if (!alive && icsk->icsk_backoff >= max_probes)
	goto abort;
	if (tcp_out_of_resources(sk, true))
	return;
	}

	if (icsk->icsk_probes_out > max_probes) {
	abort: tcp_write_err(sk);
	} else {
	/* Only send another probe if we didn't close things up. */
	tcp_send_probe0(sk);
	}
	}

	/*
	* Timer for Fast Open socket to retransmit SYNACK. Note that the
	* sk here is the child socket, not the parent (listener) socket.
	*/
	static void tcp_fastopen_synack_timer(struct sock *sk)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);
	int max_retries = icsk->icsk_syn_retries ? :
	sysctl_tcp_synack_retries + 1; /* add one more retry for fastopen */
	struct request_sock *req;

	req = tcp_sk(sk)->fastopen_rsk;
	req->rsk_ops->syn_ack_timeout(req);

	if (req->num_timeout >= max_retries) {
	tcp_write_err(sk);
	return;
	}
	/* XXX (TFO) - Unlike regular SYN-ACK retransmit, we ignore error
	* returned from rtx_syn_ack() to make it more persistent like
	* regular retransmit because if the child socket has been accepted
	* it's not good to give up too easily.
	*/
	inet_rtx_syn_ack(sk, req);
	req->num_timeout++;
	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
	TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
	}

	/*
	* The TCP retransmit timer.
	*/

	void tcp_retransmit_timer(struct sock *sk)
	{
	struct tcp_sock *tp = tcp_sk(sk);
	struct inet_connection_sock *icsk = inet_csk(sk);

	if (tp->fastopen_rsk) {
	WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
	sk->sk_state != TCP_FIN_WAIT1);
	tcp_fastopen_synack_timer(sk);
	/* Before we receive ACK to our SYN-ACK don't retransmit
	* anything else (e.g., data or FIN segments).
	*/
	return;
	}
	if (!tp->packets_out)
	goto out;

	WARN_ON(tcp_write_queue_empty(sk));

	tp->tlp_high_seq = 0;

	if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) &&
	!((1 << sk->sk_state) & (TCPF_SYN_SENT \| TCPF_SYN_RECV))) {
	/* Receiver dastardly shrinks window. Our retransmits
	* become zero probes, but we should not timeout this
	* connection. If the socket is an orphan, time it out,
	* we cannot allow such beasts to hang infinitely.
	*/
	struct inet_sock *inet = inet_sk(sk);
	if (sk->sk_family == AF_INET) {
	net_dbg_ratelimited("Peer %pI4:%u/%u unexpectedly shrunk window %u:%u (repaired)\n",
	&inet->inet_daddr,
	ntohs(inet->inet_dport),
	inet->inet_num,
	tp->snd_una, tp->snd_nxt);
	}
	#if IS_ENABLED(CONFIG_IPV6)
	else if (sk->sk_family == AF_INET6) {
	net_dbg_ratelimited("Peer %pI6:%u/%u unexpectedly shrunk window %u:%u (repaired)\n",
	&sk->sk_v6_daddr,
	ntohs(inet->inet_dport),
	inet->inet_num,
	tp->snd_una, tp->snd_nxt);
	}
	#endif
	if (tcp_time_stamp - tp->rcv_tstamp > TCP_RTO_MAX) {
	tcp_write_err(sk);
	goto out;
	}
	tcp_enter_loss(sk);
	tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
	__sk_dst_reset(sk);
	goto out_reset_timer;
	}

	if (tcp_write_timeout(sk))
	goto out;

	if (icsk->icsk_retransmits == 0) {
	int mib_idx;

	if (icsk->icsk_ca_state == TCP_CA_Recovery) {
	if (tcp_is_sack(tp))
	mib_idx = LINUX_MIB_TCPSACKRECOVERYFAIL;
	else
	mib_idx = LINUX_MIB_TCPRENORECOVERYFAIL;
	} else if (icsk->icsk_ca_state == TCP_CA_Loss) {
	mib_idx = LINUX_MIB_TCPLOSSFAILURES;
	} else if ((icsk->icsk_ca_state == TCP_CA_Disorder) \|\|
	tp->sacked_out) {
	if (tcp_is_sack(tp))
	mib_idx = LINUX_MIB_TCPSACKFAILURES;
	else
	mib_idx = LINUX_MIB_TCPRENOFAILURES;
	} else {
	mib_idx = LINUX_MIB_TCPTIMEOUTS;
	}
	NET_INC_STATS_BH(sock_net(sk), mib_idx);
	}

	tcp_enter_loss(sk);

	if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk)) > 0) {
	/* Retransmission failed because of local congestion,
	* do not backoff.
	*/
	if (!icsk->icsk_retransmits)
	icsk->icsk_retransmits = 1;
	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
	min(icsk->icsk_rto, TCP_RESOURCE_PROBE_INTERVAL),
	TCP_RTO_MAX);
	goto out;
	}

	/* Increase the timeout each time we retransmit. Note that
	* we do not increase the rtt estimate. rto is initialized
	* from rtt, but increases here. Jacobson (SIGCOMM 88) suggests
	* that doubling rto each time is the least we can get away with.
	* In KA9Q, Karn uses this for the first few times, and then
	* goes to quadratic. netBSD doubles, but only goes up to *64,
	* and clamps at 1 to 64 sec afterwards. Note that 120 sec is
	* defined in the protocol as the maximum possible RTT. I guess
	* we'll have to use something other than TCP to talk to the
	* University of Mars.
	*
	* PAWS allows us longer timeouts and large windows, so once
	* implemented ftp to mars will work nicely. We will have to fix
	* the 120 second clamps though!
	*/
	icsk->icsk_backoff++;
	icsk->icsk_retransmits++;

	out_reset_timer:
	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
	* used to reset timer, set to 0. Recalculate 'icsk_rto' as this
	* might be increased if the stream oscillates between thin and thick,
	* thus the old value might already be too high compared to the value
	* set by 'tcp_set_rto' in tcp_input.c which resets the rto without
	* backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
	* exponential backoff behaviour to avoid continue hammering
	* linear-timeout retransmissions into a black hole
	*/
	if (sk->sk_state == TCP_ESTABLISHED &&
	(tp->thin_lto \|\| sysctl_tcp_thin_linear_timeouts) &&
	tcp_stream_is_thin(tp) &&
	icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
	icsk->icsk_backoff = 0;
	icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
	} else {
	/* Use normal (exponential) backoff */
	icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
	}
	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
	if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1, 0, 0))
	__sk_dst_reset(sk);

	out:;
	}

	void tcp_write_timer_handler(struct sock *sk)
	{
	struct inet_connection_sock *icsk = inet_csk(sk);
	int event;

	if (sk->sk_state == TCP_CLOSE \|\| !icsk->icsk_pending)
	goto out;

	if (time_after(icsk->icsk_timeout, jiffies)) {
	sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
	goto out;
	}

	event = icsk->icsk_pending;

	switch (event) {
	case ICSK_TIME_EARLY_RETRANS:
	tcp_resume_early_retransmit(sk);
	break;
	case ICSK_TIME_LOSS_PROBE:
	tcp_send_loss_probe(sk);
	break;
	case ICSK_TIME_RETRANS:
	icsk->icsk_pending = 0;
	tcp_retransmit_timer(sk);
	break;
	case ICSK_TIME_PROBE0:
	icsk->icsk_pending = 0;
	tcp_probe_timer(sk);
	break;
	}

	out:
	sk_mem_reclaim(sk);
	}

	static void tcp_write_timer(unsigned long data)
	{
	struct sock sk = (struct sock )data;

	bh_lock_sock(sk);
	if (!sock_owned_by_user(sk)) {
	tcp_write_timer_handler(sk);
	} else {
	/* deleguate our work to tcp_release_cb() */
	if (!test_and_set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
	sock_hold(sk);
	}
	bh_unlock_sock(sk);
	sock_put(sk);
	}

	void tcp_syn_ack_timeout(const struct request_sock *req)
	{
	struct net *net = read_pnet(&inet_rsk(req)->ireq_net);

	NET_INC_STATS_BH(net, LINUX_MIB_TCPTIMEOUTS);
	}
	EXPORT_SYMBOL(tcp_syn_ack_timeout);

	void tcp_set_keepalive(struct sock *sk, int val)
	{
	if ((1 << sk->sk_state) & (TCPF_CLOSE \| TCPF_LISTEN))
	return;

	if (val && !sock_flag(sk, SOCK_KEEPOPEN))
	inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
	else if (!val)
	inet_csk_delete_keepalive_timer(sk);
	}


	static void tcp_keepalive_timer (unsigned long data)
	{
	struct sock sk = (struct sock ) data;
	struct inet_connection_sock *icsk = inet_csk(sk);
	struct tcp_sock *tp = tcp_sk(sk);
	u32 elapsed;

	/* Only process if socket is not in use. */
	bh_lock_sock(sk);
	if (sock_owned_by_user(sk)) {
	/* Try again later. */
	inet_csk_reset_keepalive_timer (sk, HZ/20);
	goto out;
	}

	if (sk->sk_state == TCP_LISTEN) {
	pr_err("Hmm... keepalive on a LISTEN ???\n");
	goto out;
	}

	if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
	if (tp->linger2 >= 0) {
	const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;

	if (tmo > 0) {
	tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
	goto out;
	}
	}
	tcp_send_active_reset(sk, GFP_ATOMIC);
	goto death;
	}

	if (!sock_flag(sk, SOCK_KEEPOPEN) \|\| sk->sk_state == TCP_CLOSE)
	goto out;

	elapsed = keepalive_time_when(tp);

	/* It is alive without keepalive 8) */
	if (tp->packets_out \|\| tcp_send_head(sk))
	goto resched;

	elapsed = keepalive_time_elapsed(tp);

	if (elapsed >= keepalive_time_when(tp)) {
	/* If the TCP_USER_TIMEOUT option is enabled, use that
	* to determine when to timeout instead.
	*/
	if ((icsk->icsk_user_timeout != 0 &&
	elapsed >= icsk->icsk_user_timeout &&
	icsk->icsk_probes_out > 0) \|\|
	(icsk->icsk_user_timeout == 0 &&
	icsk->icsk_probes_out >= keepalive_probes(tp))) {
	tcp_send_active_reset(sk, GFP_ATOMIC);
	tcp_write_err(sk);
	goto out;
	}
	if (tcp_write_wakeup(sk) <= 0) {
	icsk->icsk_probes_out++;
	elapsed = keepalive_intvl_when(tp);
	} else {
	/* If keepalive was lost due to local congestion,
	* try harder.
	*/
	elapsed = TCP_RESOURCE_PROBE_INTERVAL;
	}
	} else {
	/* It is tp->rcv_tstamp + keepalive_time_when(tp) */
	elapsed = keepalive_time_when(tp) - elapsed;
	}

	sk_mem_reclaim(sk);

	resched:
	inet_csk_reset_keepalive_timer (sk, elapsed);
	goto out;

	death:
	tcp_done(sk);

	out:
	bh_unlock_sock(sk);
	sock_put(sk);
	}

	void tcp_init_xmit_timers(struct sock *sk)
	{
	inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
	&tcp_keepalive_timer);
	}
	EXPORT_SYMBOL(tcp_init_xmit_timers);