net/ipv4/tcp_timer.c

   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  *      The TCP retransmit timer.
 309  */
 310
 311 void tcp_retransmit_timer(struct sock *sk)
 312 {
 313         struct tcp_sock *tp = tcp_sk(sk);
 314         struct inet_connection_sock *icsk = inet_csk(sk);
 315
 316         if (tp->early_retrans_delayed) {
 317                 tcp_resume_early_retransmit(sk);
 318                 return;
 319         }
 320
 321         if (!tp->packets_out)
 322                 goto out;
 323
 324         WARN_ON(tcp_write_queue_empty(sk));
 325
 326         if (!tp->snd_wnd && !sock_flag(sk, SOCK_DEAD) &&
 327             !((1 << sk->sk_state) & (TCPF_SYN_SENT | TCPF_SYN_RECV))) {
 328                 /* Receiver dastardly shrinks window. Our retransmits
 329                  * become zero probes, but we should not timeout this
 330                  * connection. If the socket is an orphan, time it out,
 331                  * we cannot allow such beasts to hang infinitely.
 332                  */
 333                 struct inet_sock *inet = inet_sk(sk);
 334                 if (sk->sk_family == AF_INET) {
 335                         LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("Peer %pI4:%u/%u unexpectedly shrunk window %u:%u (repaired)\n"),
 336                                        &inet->inet_daddr,
 337                                        ntohs(inet->inet_dport), inet->inet_num,
 338                                        tp->snd_una, tp->snd_nxt);
 339                 }
 340 #if IS_ENABLED(CONFIG_IPV6)
 341                 else if (sk->sk_family == AF_INET6) {
 342                         struct ipv6_pinfo *np = inet6_sk(sk);
 343                         LIMIT_NETDEBUG(KERN_DEBUG pr_fmt("Peer %pI6:%u/%u unexpectedly shrunk window %u:%u (repaired)\n"),
 344                                        &np->daddr,
 345                                        ntohs(inet->inet_dport), inet->inet_num,
 346                                        tp->snd_una, tp->snd_nxt);
 347                 }
 348 #endif
 349                 if (tcp_time_stamp - tp->rcv_tstamp > TCP_RTO_MAX) {
 350                         tcp_write_err(sk);
 351                         goto out;
 352                 }
 353                 tcp_enter_loss(sk, 0);
 354                 tcp_retransmit_skb(sk, tcp_write_queue_head(sk));
 355                 __sk_dst_reset(sk);
 356                 goto out_reset_timer;
 357         }
 358
 359         if (tcp_write_timeout(sk))
 360                 goto out;
 361
 362         if (icsk->icsk_retransmits == 0) {
 363                 int mib_idx;
 364
 365                 if (icsk->icsk_ca_state == TCP_CA_Recovery) {
 366                         if (tcp_is_sack(tp))
 367                                 mib_idx = LINUX_MIB_TCPSACKRECOVERYFAIL;
 368                         else
 369                                 mib_idx = LINUX_MIB_TCPRENORECOVERYFAIL;
 370                 } else if (icsk->icsk_ca_state == TCP_CA_Loss) {
 371                         mib_idx = LINUX_MIB_TCPLOSSFAILURES;
 372                 } else if ((icsk->icsk_ca_state == TCP_CA_Disorder) ||
 373                            tp->sacked_out) {
 374                         if (tcp_is_sack(tp))
 375                                 mib_idx = LINUX_MIB_TCPSACKFAILURES;
 376                         else
 377                                 mib_idx = LINUX_MIB_TCPRENOFAILURES;
 378                 } else {
 379                         mib_idx = LINUX_MIB_TCPTIMEOUTS;
 380                 }
 381                 NET_INC_STATS_BH(sock_net(sk), mib_idx);
 382         }
 383
 384         if (tcp_use_frto(sk)) {
 385                 tcp_enter_frto(sk);
 386         } else {
 387                 tcp_enter_loss(sk, 0);
 388         }
 389
 390         if (tcp_retransmit_skb(sk, tcp_write_queue_head(sk)) > 0) {
 391                 /* Retransmission failed because of local congestion,
 392                  * do not backoff.
 393                  */
 394                 if (!icsk->icsk_retransmits)
 395                         icsk->icsk_retransmits = 1;
 396                 inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS,
 397                                           min(icsk->icsk_rto, TCP_RESOURCE_PROBE_INTERVAL),
 398                                           TCP_RTO_MAX);
 399                 goto out;
 400         }
 401
 402         /* Increase the timeout each time we retransmit.  Note that
 403          * we do not increase the rtt estimate.  rto is initialized
 404          * from rtt, but increases here.  Jacobson (SIGCOMM 88) suggests
 405          * that doubling rto each time is the least we can get away with.
 406          * In KA9Q, Karn uses this for the first few times, and then
 407          * goes to quadratic.  netBSD doubles, but only goes up to *64,
 408          * and clamps at 1 to 64 sec afterwards.  Note that 120 sec is
 409          * defined in the protocol as the maximum possible RTT.  I guess
 410          * we'll have to use something other than TCP to talk to the
 411          * University of Mars.
 412          *
 413          * PAWS allows us longer timeouts and large windows, so once
 414          * implemented ftp to mars will work nicely. We will have to fix
 415          * the 120 second clamps though!
 416          */
 417         icsk->icsk_backoff++;
 418         icsk->icsk_retransmits++;
 419
 420 out_reset_timer:
 421         /* If stream is thin, use linear timeouts. Since 'icsk_backoff' is
 422          * used to reset timer, set to 0. Recalculate 'icsk_rto' as this
 423          * might be increased if the stream oscillates between thin and thick,
 424          * thus the old value might already be too high compared to the value
 425          * set by 'tcp_set_rto' in tcp_input.c which resets the rto without
 426          * backoff. Limit to TCP_THIN_LINEAR_RETRIES before initiating
 427          * exponential backoff behaviour to avoid continue hammering
 428          * linear-timeout retransmissions into a black hole
 429          */
 430         if (sk->sk_state == TCP_ESTABLISHED &&
 431             (tp->thin_lto || sysctl_tcp_thin_linear_timeouts) &&
 432             tcp_stream_is_thin(tp) &&
 433             icsk->icsk_retransmits <= TCP_THIN_LINEAR_RETRIES) {
 434                 icsk->icsk_backoff = 0;
 435                 icsk->icsk_rto = min(__tcp_set_rto(tp), TCP_RTO_MAX);
 436         } else {
 437                 /* Use normal (exponential) backoff */
 438                 icsk->icsk_rto = min(icsk->icsk_rto << 1, TCP_RTO_MAX);
 439         }
 440         inet_csk_reset_xmit_timer(sk, ICSK_TIME_RETRANS, icsk->icsk_rto, TCP_RTO_MAX);
 441         if (retransmits_timed_out(sk, sysctl_tcp_retries1 + 1, 0, 0))
 442                 __sk_dst_reset(sk);
 443
 444 out:;
 445 }
 446
 447 void tcp_write_timer_handler(struct sock *sk)
 448 {
 449         struct inet_connection_sock *icsk = inet_csk(sk);
 450         int event;
 451
 452         if (sk->sk_state == TCP_CLOSE || !icsk->icsk_pending)
 453                 goto out;
 454
 455         if (time_after(icsk->icsk_timeout, jiffies)) {
 456                 sk_reset_timer(sk, &icsk->icsk_retransmit_timer, icsk->icsk_timeout);
 457                 goto out;
 458         }
 459
 460         event = icsk->icsk_pending;
 461         icsk->icsk_pending = 0;
 462
 463         switch (event) {
 464         case ICSK_TIME_RETRANS:
 465                 tcp_retransmit_timer(sk);
 466                 break;
 467         case ICSK_TIME_PROBE0:
 468                 tcp_probe_timer(sk);
 469                 break;
 470         }
 471
 472 out:
 473         sk_mem_reclaim(sk);
 474 }
 475
 476 static void tcp_write_timer(unsigned long data)
 477 {
 478         struct sock *sk = (struct sock *)data;
 479
 480         bh_lock_sock(sk);
 481         if (!sock_owned_by_user(sk)) {
 482                 tcp_write_timer_handler(sk);
 483         } else {
 484                 /* deleguate our work to tcp_release_cb() */
 485                 if (!test_and_set_bit(TCP_WRITE_TIMER_DEFERRED, &tcp_sk(sk)->tsq_flags))
 486                         sock_hold(sk);
 487         }
 488         bh_unlock_sock(sk);
 489         sock_put(sk);
 490 }
 491
 492 /*
 493  *      Timer for listening sockets
 494  */
 495
 496 static void tcp_synack_timer(struct sock *sk)
 497 {
 498         inet_csk_reqsk_queue_prune(sk, TCP_SYNQ_INTERVAL,
 499                                    TCP_TIMEOUT_INIT, TCP_RTO_MAX);
 500 }
 501
 502 void tcp_syn_ack_timeout(struct sock *sk, struct request_sock *req)
 503 {
 504         NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPTIMEOUTS);
 505 }
 506 EXPORT_SYMBOL(tcp_syn_ack_timeout);
 507
 508 void tcp_set_keepalive(struct sock *sk, int val)
 509 {
 510         if ((1 << sk->sk_state) & (TCPF_CLOSE | TCPF_LISTEN))
 511                 return;
 512
 513         if (val && !sock_flag(sk, SOCK_KEEPOPEN))
 514                 inet_csk_reset_keepalive_timer(sk, keepalive_time_when(tcp_sk(sk)));
 515         else if (!val)
 516                 inet_csk_delete_keepalive_timer(sk);
 517 }
 518
 519
 520 static void tcp_keepalive_timer (unsigned long data)
 521 {
 522         struct sock *sk = (struct sock *) data;
 523         struct inet_connection_sock *icsk = inet_csk(sk);
 524         struct tcp_sock *tp = tcp_sk(sk);
 525         u32 elapsed;
 526
 527         /* Only process if socket is not in use. */
 528         bh_lock_sock(sk);
 529         if (sock_owned_by_user(sk)) {
 530                 /* Try again later. */
 531                 inet_csk_reset_keepalive_timer (sk, HZ/20);
 532                 goto out;
 533         }
 534
 535         if (sk->sk_state == TCP_LISTEN) {
 536                 tcp_synack_timer(sk);
 537                 goto out;
 538         }
 539
 540         if (sk->sk_state == TCP_FIN_WAIT2 && sock_flag(sk, SOCK_DEAD)) {
 541                 if (tp->linger2 >= 0) {
 542                         const int tmo = tcp_fin_time(sk) - TCP_TIMEWAIT_LEN;
 543
 544                         if (tmo > 0) {
 545                                 tcp_time_wait(sk, TCP_FIN_WAIT2, tmo);
 546                                 goto out;
 547                         }
 548                 }
 549                 tcp_send_active_reset(sk, GFP_ATOMIC);
 550                 goto death;
 551         }
 552
 553         if (!sock_flag(sk, SOCK_KEEPOPEN) || sk->sk_state == TCP_CLOSE)
 554                 goto out;
 555
 556         elapsed = keepalive_time_when(tp);
 557
 558         /* It is alive without keepalive 8) */
 559         if (tp->packets_out || tcp_send_head(sk))
 560                 goto resched;
 561
 562         elapsed = keepalive_time_elapsed(tp);
 563
 564         if (elapsed >= keepalive_time_when(tp)) {
 565                 /* If the TCP_USER_TIMEOUT option is enabled, use that
 566                  * to determine when to timeout instead.
 567                  */
 568                 if ((icsk->icsk_user_timeout != 0 &&
 569                     elapsed >= icsk->icsk_user_timeout &&
 570                     icsk->icsk_probes_out > 0) ||
 571                     (icsk->icsk_user_timeout == 0 &&
 572                     icsk->icsk_probes_out >= keepalive_probes(tp))) {
 573                         tcp_send_active_reset(sk, GFP_ATOMIC);
 574                         tcp_write_err(sk);
 575                         goto out;
 576                 }
 577                 if (tcp_write_wakeup(sk) <= 0) {
 578                         icsk->icsk_probes_out++;
 579                         elapsed = keepalive_intvl_when(tp);
 580                 } else {
 581                         /* If keepalive was lost due to local congestion,
 582                          * try harder.
 583                          */
 584                         elapsed = TCP_RESOURCE_PROBE_INTERVAL;
 585                 }
 586         } else {
 587                 /* It is tp->rcv_tstamp + keepalive_time_when(tp) */
 588                 elapsed = keepalive_time_when(tp) - elapsed;
 589         }
 590
 591         sk_mem_reclaim(sk);
 592
 593 resched:
 594         inet_csk_reset_keepalive_timer (sk, elapsed);
 595         goto out;
 596
 597 death:
 598         tcp_done(sk);
 599
 600 out:
 601         bh_unlock_sock(sk);
 602         sock_put(sk);
 603 }
 604
 605 void tcp_init_xmit_timers(struct sock *sk)
 606 {
 607         inet_csk_init_xmit_timers(sk, &tcp_write_timer, &tcp_delack_timer,
 608                                   &tcp_keepalive_timer);
 609 }
 610 EXPORT_SYMBOL(tcp_init_xmit_timers);