xref: /openbmc/linux/net/ipv4/tcp_timer.c (revision e23feb16)
1 /*
2  * INET		An implementation of the TCP/IP protocol suite for the LINUX
3  *		operating system.  INET is implemented using the  BSD Socket
4  *		interface as the means of communication with the user level.
5  *
6  *		Implementation of the Transmission Control Protocol(TCP).
7  *
8  * Authors:	Ross Biro
9  *		Fred N. van Kempen, <waltje@uWalt.NL.Mugnet.ORG>
10  *		Mark Evans, <evansmp@uhura.aston.ac.uk>
11  *		Corey Minyard <wf-rch!minyard@relay.EU.net>
12  *		Florian La Roche, <flla@stud.uni-sb.de>
13  *		Charles Hedrick, <hedrick@klinzhai.rutgers.edu>
14  *		Linus Torvalds, <torvalds@cs.helsinki.fi>
15  *		Alan Cox, <gw4pts@gw4pts.ampr.org>
16  *		Matthew Dillon, <dillon@apollo.west.oic.com>
17  *		Arnt Gulbrandsen, <agulbra@nvg.unit.no>
18  *		Jorge Cwik, <jorge@laser.satlink.net>
19  */
20 
21 #include <linux/module.h>
22 #include <linux/gfp.h>
23 #include <net/tcp.h>
24 
25 int sysctl_tcp_syn_retries __read_mostly = TCP_SYN_RETRIES;
26 int sysctl_tcp_synack_retries __read_mostly = TCP_SYNACK_RETRIES;
27 int sysctl_tcp_keepalive_time __read_mostly = TCP_KEEPALIVE_TIME;
28 int sysctl_tcp_keepalive_probes __read_mostly = TCP_KEEPALIVE_PROBES;
29 int sysctl_tcp_keepalive_intvl __read_mostly = TCP_KEEPALIVE_INTVL;
30 int sysctl_tcp_retries1 __read_mostly = TCP_RETR1;
31 int sysctl_tcp_retries2 __read_mostly = TCP_RETR2;
32 int sysctl_tcp_orphan_retries __read_mostly;
33 int sysctl_tcp_thin_linear_timeouts __read_mostly;
34 
35 static void tcp_write_err(struct sock *sk)
36 {
37 	sk->sk_err = sk->sk_err_soft ? : ETIMEDOUT;
38 	sk->sk_error_report(sk);
39 
40 	tcp_done(sk);
41 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONTIMEOUT);
42 }
43 
44 /* Do not allow orphaned sockets to eat all our resources.
45  * This is direct violation of TCP specs, but it is required
46  * to prevent DoS attacks. It is called when a retransmission timeout
47  * or zero probe timeout occurs on orphaned socket.
48  *
49  * Criteria is still not confirmed experimentally and may change.
50  * We kill the socket, if:
51  * 1. If number of orphaned sockets exceeds an administratively configured
52  *    limit.
53  * 2. If we have strong memory pressure.
54  */
55 static int tcp_out_of_resources(struct sock *sk, int do_reset)
56 {
57 	struct tcp_sock *tp = tcp_sk(sk);
58 	int shift = 0;
59 
60 	/* If peer does not open window for long time, or did not transmit
61 	 * anything for long time, penalize it. */
62 	if ((s32)(tcp_time_stamp - tp->lsndtime) > 2*TCP_RTO_MAX || !do_reset)
63 		shift++;
64 
65 	/* If some dubious ICMP arrived, penalize even more. */
66 	if (sk->sk_err_soft)
67 		shift++;
68 
69 	if (tcp_check_oom(sk, shift)) {
70 		/* Catch exceptional cases, when connection requires reset.
71 		 *      1. Last segment was sent recently. */
72 		if ((s32)(tcp_time_stamp - tp->lsndtime) <= TCP_TIMEWAIT_LEN ||
73 		    /*  2. Window is closed. */
74 		    (!tp->snd_wnd && !tp->packets_out))
75 			do_reset = 1;
76 		if (do_reset)
77 			tcp_send_active_reset(sk, GFP_ATOMIC);
78 		tcp_done(sk);
79 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPABORTONMEMORY);
80 		return 1;
81 	}
82 	return 0;
83 }
84 
85 /* Calculate maximal number or retries on an orphaned socket. */
86 static int tcp_orphan_retries(struct sock *sk, int alive)
87 {
88 	int retries = sysctl_tcp_orphan_retries; /* May be zero. */
89 
90 	/* We know from an ICMP that something is wrong. */
91 	if (sk->sk_err_soft && !alive)
92 		retries = 0;
93 
94 	/* However, if socket sent something recently, select some safe
95 	 * number of retries. 8 corresponds to >100 seconds with minimal
96 	 * RTO of 200msec. */
97 	if (retries == 0 && alive)
98 		retries = 8;
99 	return retries;
100 }
101 
102 static void tcp_mtu_probing(struct inet_connection_sock *icsk, struct sock *sk)
103 {
104 	/* Black hole detection */
105 	if (sysctl_tcp_mtu_probing) {
106 		if (!icsk->icsk_mtup.enabled) {
107 			icsk->icsk_mtup.enabled = 1;
108 			tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
109 		} else {
110 			struct tcp_sock *tp = tcp_sk(sk);
111 			int mss;
112 
113 			mss = tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_low) >> 1;
114 			mss = min(sysctl_tcp_base_mss, mss);
115 			mss = max(mss, 68 - tp->tcp_header_len);
116 			icsk->icsk_mtup.search_low = tcp_mss_to_mtu(sk, mss);
117 			tcp_sync_mss(sk, icsk->icsk_pmtu_cookie);
118 		}
119 	}
120 }
121 
122 /* This function calculates a "timeout" which is equivalent to the timeout of a
123  * TCP connection after "boundary" unsuccessful, exponentially backed-off
124  * retransmissions with an initial RTO of TCP_RTO_MIN or TCP_TIMEOUT_INIT if
125  * syn_set flag is set.
126  */
127 static bool retransmits_timed_out(struct sock *sk,
128 				  unsigned int boundary,
129 				  unsigned int timeout,
130 				  bool syn_set)
131 {
132 	unsigned int linear_backoff_thresh, start_ts;
133 	unsigned int rto_base = syn_set ? TCP_TIMEOUT_INIT : TCP_RTO_MIN;
134 
135 	if (!inet_csk(sk)->icsk_retransmits)
136 		return false;
137 
138 	if (unlikely(!tcp_sk(sk)->retrans_stamp))
139 		start_ts = TCP_SKB_CB(tcp_write_queue_head(sk))->when;
140 	else
141 		start_ts = tcp_sk(sk)->retrans_stamp;
142 
143 	if (likely(timeout == 0)) {
144 		linear_backoff_thresh = ilog2(TCP_RTO_MAX/rto_base);
145 
146 		if (boundary <= linear_backoff_thresh)
147 			timeout = ((2 << boundary) - 1) * rto_base;
148 		else
149 			timeout = ((2 << linear_backoff_thresh) - 1) * rto_base +
150 				(boundary - linear_backoff_thresh) * TCP_RTO_MAX;
151 	}
152 	return (tcp_time_stamp - start_ts) >= timeout;
153 }
154 
155 /* A write timeout has occurred. Process the after effects. */
156 static int tcp_write_timeout(struct sock *sk)
157 {
158 	struct inet_connection_sock *icsk = inet_csk(sk);
159 	int retry_until;
160 	bool do_reset, syn_set = false;
161 
162 	if ((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV)) {
163 		if (icsk->icsk_retransmits)
164 			dst_negative_advice(sk);
165 		retry_until = icsk->icsk_syn_retries ? : sysctl_tcp_syn_retries;
166 		syn_set = true;
167 	} else {
168 		if (retransmits_timed_out(sk, sysctl_tcp_retries1, 0, 0)) {
169 			/* Black hole detection */
170 			tcp_mtu_probing(icsk, sk);
171 
172 			dst_negative_advice(sk);
173 		}
174 
175 		retry_until = sysctl_tcp_retries2;
176 		if (sock_flag(sk, SOCK_DEAD)) {
177 			const int alive = (icsk->icsk_rto < TCP_RTO_MAX);
178 
179 			retry_until = tcp_orphan_retries(sk, alive);
180 			do_reset = alive ||
181 				!retransmits_timed_out(sk, retry_until, 0, 0);
182 
183 			if (tcp_out_of_resources(sk, do_reset))
184 				return 1;
185 		}
186 	}
187 
188 	if (retransmits_timed_out(sk, retry_until,
189 				  syn_set ? 0 : icsk->icsk_user_timeout, syn_set)) {
190 		/* Has it gone just too far? */
191 		tcp_write_err(sk);
192 		return 1;
193 	}
194 	return 0;
195 }
196 
197 void tcp_delack_timer_handler(struct sock *sk)
198 {
199 	struct tcp_sock *tp = tcp_sk(sk);
200 	struct inet_connection_sock *icsk = inet_csk(sk);
201 
202 	sk_mem_reclaim_partial(sk);
203 
204 	if (sk->sk_state == TCP_CLOSE || !(icsk->icsk_ack.pending & ICSK_ACK_TIMER))
205 		goto out;
206 
207 	if (time_after(icsk->icsk_ack.timeout, jiffies)) {
208 		sk_reset_timer(sk, &icsk->icsk_delack_timer, icsk->icsk_ack.timeout);
209 		goto out;
210 	}
211 	icsk->icsk_ack.pending &= ~ICSK_ACK_TIMER;
212 
213 	if (!skb_queue_empty(&tp->ucopy.prequeue)) {
214 		struct sk_buff *skb;
215 
216 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPSCHEDULERFAILED);
217 
218 		while ((skb = __skb_dequeue(&tp->ucopy.prequeue)) != NULL)
219 			sk_backlog_rcv(sk, skb);
220 
221 		tp->ucopy.memory = 0;
222 	}
223 
224 	if (inet_csk_ack_scheduled(sk)) {
225 		if (!icsk->icsk_ack.pingpong) {
226 			/* Delayed ACK missed: inflate ATO. */
227 			icsk->icsk_ack.ato = min(icsk->icsk_ack.ato << 1, icsk->icsk_rto);
228 		} else {
229 			/* Delayed ACK missed: leave pingpong mode and
230 			 * deflate ATO.
231 			 */
232 			icsk->icsk_ack.pingpong = 0;
233 			icsk->icsk_ack.ato      = TCP_ATO_MIN;
234 		}
235 		tcp_send_ack(sk);
236 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKS);
237 	}
238 
239 out:
240 	if (sk_under_memory_pressure(sk))
241 		sk_mem_reclaim(sk);
242 }
243 
244 static void tcp_delack_timer(unsigned long data)
245 {
246 	struct sock *sk = (struct sock *)data;
247 
248 	bh_lock_sock(sk);
249 	if (!sock_owned_by_user(sk)) {
250 		tcp_delack_timer_handler(sk);
251 	} else {
252 		inet_csk(sk)->icsk_ack.blocked = 1;
253 		NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_DELAYEDACKLOCKED);
254 		/* deleguate our work to tcp_release_cb() */
255 		if (!test_and_set_bit(TCP_DELACK_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
256 			sock_hold(sk);
257 	}
258 	bh_unlock_sock(sk);
259 	sock_put(sk);
260 }
261 
262 static void tcp_probe_timer(struct sock *sk)
263 {
264 	struct inet_connection_sock *icsk = inet_csk(sk);
265 	struct tcp_sock *tp = tcp_sk(sk);
266 	int max_probes;
267 
268 	if (tp->packets_out || !tcp_send_head(sk)) {
269 		icsk->icsk_probes_out = 0;
270 		return;
271 	}
272 
273 	/* *WARNING* RFC 1122 forbids this
274 	 *
275 	 * It doesn't AFAIK, because we kill the retransmit timer -AK
276 	 *
277 	 * FIXME: We ought not to do it, Solaris 2.5 actually has fixing
278 	 * this behaviour in Solaris down as a bug fix. [AC]
279 	 *
280 	 * Let me to explain. icsk_probes_out is zeroed by incoming ACKs
281 	 * even if they advertise zero window. Hence, connection is killed only
282 	 * if we received no ACKs for normal connection timeout. It is not killed
283 	 * only because window stays zero for some time, window may be zero
284 	 * until armageddon and even later. We are in full accordance
285 	 * with RFCs, only probe timer combines both retransmission timeout
286 	 * and probe timeout in one bottle.				--ANK
287 	 */
288 	max_probes = sysctl_tcp_retries2;
289 
290 	if (sock_flag(sk, SOCK_DEAD)) {
291 		const int alive = ((icsk->icsk_rto << icsk->icsk_backoff) < TCP_RTO_MAX);
292 
293 		max_probes = tcp_orphan_retries(sk, alive);
294 
295 		if (tcp_out_of_resources(sk, alive || icsk->icsk_probes_out <= max_probes))
296 			return;
297 	}
298 
299 	if (icsk->icsk_probes_out > max_probes) {
300 		tcp_write_err(sk);
301 	} else {
302 		/* Only send another probe if we didn't close things up. */
303 		tcp_send_probe0(sk);
304 	}
305 }
306 
307 /*
308  *	Timer for Fast Open socket to retransmit SYNACK. Note that the
309  *	sk here is the child socket, not the parent (listener) socket.
310  */
311 static void tcp_fastopen_synack_timer(struct sock *sk)
312 {
313 	struct inet_connection_sock *icsk = inet_csk(sk);
314 	int max_retries = icsk->icsk_syn_retries ? :
315 	    sysctl_tcp_synack_retries + 1; /* add one more retry for fastopen */
316 	struct request_sock *req;
317 
318 	req = tcp_sk(sk)->fastopen_rsk;
319 	req->rsk_ops->syn_ack_timeout(sk, req);
320 
321 	if (req->num_timeout >= max_retries) {
322 		tcp_write_err(sk);
323 		return;
324 	}
325 	/* XXX (TFO) - Unlike regular SYN-ACK retransmit, we ignore error
326 	 * returned from rtx_syn_ack() to make it more persistent like
327 	 * regular retransmit because if the child socket has been accepted
328 	 * it's not good to give up too easily.
329 	 */
330 	inet_rtx_syn_ack(sk, req);
331 	req->num_timeout++;
332 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
333 			  TCP_TIMEOUT_INIT << req->num_timeout, TCP_RTO_MAX);
334 }
335 
336 /*
337  *	The TCP retransmit timer.
338  */
339 
340 void tcp_retransmit_timer(struct sock *sk)
341 {
342 	struct tcp_sock *tp = tcp_sk(sk);
343 	struct inet_connection_sock *icsk = inet_csk(sk);
344 
345 	if (tp->fastopen_rsk) {
346 		WARN_ON_ONCE(sk->sk_state != TCP_SYN_RECV &&
347 			     sk->sk_state != TCP_FIN_WAIT1);
348 		tcp_fastopen_synack_timer(sk);
349 		/* Before we receive ACK to our SYN-ACK don't retransmit
350 		 * anything else (e.g., data or FIN segments).
351 		 */
352 		return;
353 	}
354 	if (!tp->packets_out)
355 		goto out;
356 
357 	WARN_ON(tcp_write_queue_empty(sk));
358 
359 	tp->tlp_high_seq = 0;
360 
361 	if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) &&
362 	    !((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))) {
363 		/* Receiver dastardly shrinks window. Our retransmits
364 		 * become zero probes, but we should not timeout this
365 		 * connection. If the socket is an orphan, time it out,
366 		 * we cannot allow such beasts to hang infinitely.
367 		 */
368 		struct inet_sock *inet = inet_sk(sk);
369 		if (sk->sk_family == AF_INET) {
370 			LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("Peer %pI4:%u/%u unexpectedly shrunk window %u:%u (repaired)\n"),
371 				       &inet->inet_daddr,
372 				       ntohs(inet->inet_dport), inet->inet_num,
373 				       tp->snd_una, tp->snd_nxt);
374 		}
375 #if IS_ENABLED(CONFIG_IPV6)
376 		else if (sk->sk_family == AF_INET6) {
377 			struct ipv6_pinfo *np = inet6_sk(sk);
378 			LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("Peer %pI6:%u/%u unexpectedly shrunk window %u:%u (repaired)\n"),
379 				       &np->daddr,
380 				       ntohs(inet->inet_dport), inet->inet_num,
381 				       tp->snd_una, tp->snd_nxt);
382 		}
383 #endif
384 		if (tcp_time_stamp - tp->rcv_tstamp > TCP_RTO_MAX) {
385 			tcp_write_err(sk);
386 			goto out;
387 		}
388 		tcp_enter_loss(sk, 0);
389 		tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
390 		__sk_dst_reset(sk);
391 		goto out_reset_timer;
392 	}
393 
394 	if (tcp_write_timeout(sk))
395 		goto out;
396 
397 	if (icsk->icsk_retransmits == 0) {
398 		int mib_idx;
399 
400 		if (icsk->icsk_ca_state == TCP_CA_Recovery) {
401 			if (tcp_is_sack(tp))
402 				mib_idx = LINUX_MIB_TCPSACKRECOVERYFAIL;
403 			else
404 				mib_idx = LINUX_MIB_TCPRENORECOVERYFAIL;
405 		} else if (icsk->icsk_ca_state == TCP_CA_Loss) {
406 			mib_idx = LINUX_MIB_TCPLOSSFAILURES;
407 		} else if ((icsk->icsk_ca_state == TCP_CA_Disorder) ||
408 			   tp->sacked_out) {
409 			if (tcp_is_sack(tp))
410 				mib_idx = LINUX_MIB_TCPSACKFAILURES;
411 			else
412 				mib_idx = LINUX_MIB_TCPRENOFAILURES;
413 		} else {
414 			mib_idx = LINUX_MIB_TCPTIMEOUTS;
415 		}
416 		NET_INC_STATS_BH(sock_net(sk), mib_idx);
417 	}
418 
419 	tcp_enter_loss(sk, 0);
420 
421 	if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk)) > 0) {
422 		/* Retransmission failed because of local congestion,
423 		 * do not backoff.
424 		 */
425 		if (!icsk->icsk_retransmits)
426 			icsk->icsk_retransmits = 1;
427 		inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
428 					  min(icsk->icsk_rto, TCP_RESOURCE_PROBE_INTERVAL),
429 					  TCP_RTO_MAX);
430 		goto out;
431 	}
432 
433 	/* Increase the timeout each time we retransmit.  Note that
434 	 * we do not increase the rtt estimate.  rto is initialized
435 	 * from rtt, but increases here.  Jacobson (SIGCOMM 88) suggests
436 	 * that doubling rto each time is the least we can get away with.
437 	 * In KA9Q, Karn uses this for the first few times, and then
438 	 * goes to quadratic.  netBSD doubles, but only goes up to *64,
439 	 * and clamps at 1 to 64 sec afterwards.  Note that 120 sec is
440 	 * defined in the protocol as the maximum possible RTT.  I guess
441 	 * we'll have to use something other than TCP to talk to the
442 	 * University of Mars.
443 	 *
444 	 * PAWS allows us longer timeouts and large windows, so once
445 	 * implemented ftp to mars will work nicely. We will have to fix
446 	 * the 120 second clamps though!
447 	 */
448 	icsk->icsk_backoff++;
449 	icsk->icsk_retransmits++;
450 
451 out_reset_timer:
452 	/* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
453 	 * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
454 	 * might be increased if the stream oscillates between thin and thick,
455 	 * thus the old value might already be too high compared to the value
456 	 * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
457 	 * backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
458 	 * exponential backoff behaviour to avoid continue hammering
459 	 * linear-timeout retransmissions into a black hole
460 	 */
461 	if (sk->sk_state == TCP_ESTABLISHED &&
462 	    (tp->thin_lto || sysctl_tcp_thin_linear_timeouts) &&
463 	    tcp_stream_is_thin(tp) &&
464 	    icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
465 		icsk->icsk_backoff = 0;
466 		icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
467 	} else {
468 		/* Use normal (exponential) backoff */
469 		icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
470 	}
471 	inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
472 	if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1, 0, 0))
473 		__sk_dst_reset(sk);
474 
475 out:;
476 }
477 
478 void tcp_write_timer_handler(struct sock *sk)
479 {
480 	struct inet_connection_sock *icsk = inet_csk(sk);
481 	int event;
482 
483 	if (sk->sk_state == TCP_CLOSE || !icsk->icsk_pending)
484 		goto out;
485 
486 	if (time_after(icsk->icsk_timeout, jiffies)) {
487 		sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
488 		goto out;
489 	}
490 
491 	event = icsk->icsk_pending;
492 
493 	switch (event) {
494 	case ICSK_TIME_EARLY_RETRANS:
495 		tcp_resume_early_retransmit(sk);
496 		break;
497 	case ICSK_TIME_LOSS_PROBE:
498 		tcp_send_loss_probe(sk);
499 		break;
500 	case ICSK_TIME_RETRANS:
501 		icsk->icsk_pending = 0;
502 		tcp_retransmit_timer(sk);
503 		break;
504 	case ICSK_TIME_PROBE0:
505 		icsk->icsk_pending = 0;
506 		tcp_probe_timer(sk);
507 		break;
508 	}
509 
510 out:
511 	sk_mem_reclaim(sk);
512 }
513 
514 static void tcp_write_timer(unsigned long data)
515 {
516 	struct sock *sk = (struct sock *)data;
517 
518 	bh_lock_sock(sk);
519 	if (!sock_owned_by_user(sk)) {
520 		tcp_write_timer_handler(sk);
521 	} else {
522 		/* deleguate our work to tcp_release_cb() */
523 		if (!test_and_set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
524 			sock_hold(sk);
525 	}
526 	bh_unlock_sock(sk);
527 	sock_put(sk);
528 }
529 
530 /*
531  *	Timer for listening sockets
532  */
533 
534 static void tcp_synack_timer(struct sock *sk)
535 {
536 	inet_csk_reqsk_queue_prune(sk, TCP_SYNQ_INTERVAL,
537 				   TCP_TIMEOUT_INIT, TCP_RTO_MAX);
538 }
539 
540 void tcp_syn_ack_timeout(struct sock *sk, struct request_sock *req)
541 {
542 	NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEOUTS);
543 }
544 EXPORT_SYMBOL(tcp_syn_ack_timeout);
545 
546 void tcp_set_keepalive(struct sock *sk, int val)
547 {
548 	if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
549 		return;
550 
551 	if (val && !sock_flag(sk, SOCK_KEEPOPEN))
552 		inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
553 	else if (!val)
554 		inet_csk_delete_keepalive_timer(sk);
555 }
556 
557 
558 static void tcp_keepalive_timer (unsigned long data)
559 {
560 	struct sock *sk = (struct sock *) data;
561 	struct inet_connection_sock *icsk = inet_csk(sk);
562 	struct tcp_sock *tp = tcp_sk(sk);
563 	u32 elapsed;
564 
565 	/* Only process if socket is not in use. */
566 	bh_lock_sock(sk);
567 	if (sock_owned_by_user(sk)) {
568 		/* Try again later. */
569 		inet_csk_reset_keepalive_timer (sk, HZ/20);
570 		goto out;
571 	}
572 
573 	if (sk->sk_state == TCP_LISTEN) {
574 		tcp_synack_timer(sk);
575 		goto out;
576 	}
577 
578 	if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
579 		if (tp->linger2 >= 0) {
580 			const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;
581 
582 			if (tmo > 0) {
583 				tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
584 				goto out;
585 			}
586 		}
587 		tcp_send_active_reset(sk, GFP_ATOMIC);
588 		goto death;
589 	}
590 
591 	if (!sock_flag(sk, SOCK_KEEPOPEN) || sk->sk_state == TCP_CLOSE)
592 		goto out;
593 
594 	elapsed = keepalive_time_when(tp);
595 
596 	/* It is alive without keepalive 8) */
597 	if (tp->packets_out || tcp_send_head(sk))
598 		goto resched;
599 
600 	elapsed = keepalive_time_elapsed(tp);
601 
602 	if (elapsed >= keepalive_time_when(tp)) {
603 		/* If the TCP_USER_TIMEOUT option is enabled, use that
604 		 * to determine when to timeout instead.
605 		 */
606 		if ((icsk->icsk_user_timeout != 0 &&
607 		    elapsed >= icsk->icsk_user_timeout &&
608 		    icsk->icsk_probes_out > 0) ||
609 		    (icsk->icsk_user_timeout == 0 &&
610 		    icsk->icsk_probes_out >= keepalive_probes(tp))) {
611 			tcp_send_active_reset(sk, GFP_ATOMIC);
612 			tcp_write_err(sk);
613 			goto out;
614 		}
615 		if (tcp_write_wakeup(sk) <= 0) {
616 			icsk->icsk_probes_out++;
617 			elapsed = keepalive_intvl_when(tp);
618 		} else {
619 			/* If keepalive was lost due to local congestion,
620 			 * try harder.
621 			 */
622 			elapsed = TCP_RESOURCE_PROBE_INTERVAL;
623 		}
624 	} else {
625 		/* It is tp->rcv_tstamp + keepalive_time_when(tp) */
626 		elapsed = keepalive_time_when(tp) - elapsed;
627 	}
628 
629 	sk_mem_reclaim(sk);
630 
631 resched:
632 	inet_csk_reset_keepalive_timer (sk, elapsed);
633 	goto out;
634 
635 death:
636 	tcp_done(sk);
637 
638 out:
639 	bh_unlock_sock(sk);
640 	sock_put(sk);
641 }
642 
643 void tcp_init_xmit_timers(struct sock *sk)
644 {
645 	inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
646 				  &tcp_keepalive_timer);
647 }
648 EXPORT_SYMBOL(tcp_init_xmit_timers);
649