Subject: [PATCH 3/3 (spin 3)] TCP_DEFER_ACCEPT updates - process as established

From: Patrick McManus <[email-masked]>
Date: Sun, 02 Mar 2008 16:56:03 -0500

Ilpo, thanks for the feedback! This spin contains corrections for
codingstyle issues with help from checkpatch.pl, replacement of a flag
used in dccp too, and a fix for cases where the fin piggybacks on real
data.
=====

    Change TCP_DEFER_ACCEPT implementation so that it transitions a
    connection to ESTABLISHED after handshake is complete instead of
    leaving it in SYN-RECV until some data arrvies. Place connection in
    accept queue when first data packet arrives from slow path.

    Benefits:
      - established connection is now reset if it never makes it
       to the accept queue

     - diagnostic state of established matches with the packet traces
       showing completed handshake

     - TCP_DEFER_ACCEPT timeouts are expressed in seconds and can now be
       enforced with reasonable accuracy instead of rounding up to next
       exponential back-off of syn-ack retry.

    Signed-off-by: Patrick McManus <[email-masked]>
---

diff --git a/include/linux/tcp.h b/include/linux/tcp.h
index 08027f1..d96d9b1 100644
--- a/include/linux/tcp.h
+++ b/include/linux/tcp.h
@@ -239,6 +239,11 @@ static inline struct tcp_request_sock *tcp_rsk(const struct request_sock *req)
        return (struct tcp_request_sock *)req;
 }

+struct tcp_deferred_accept_info {
+       struct sock *listen_sk;
+       struct request_sock *request;
+};
+
 struct tcp_sock {
        /* inet_connection_sock has to be the first member of tcp_sock */
        struct inet_connection_sock     inet_conn;
@@ -374,6 +379,8 @@ struct tcp_sock {
        unsigned int            keepalive_intvl;  /* time interval between keep alive probes */
        int                     linger2;

+       struct tcp_deferred_accept_info defer_tcp_accept;
+
        unsigned long last_synq_overflow;

        u32     tso_deferred;
diff --git a/include/net/request_sock.h b/include/net/request_sock.h
index cff4608..389abc4 100644
--- a/include/net/request_sock.h
+++ b/include/net/request_sock.h
@@ -116,8 +116,8 @@ struct request_sock_queue {
        struct request_sock     *rskq_accept_head;
        struct request_sock     *rskq_accept_tail;
        rwlock_t                syn_wait_lock;
-       u8                      rskq_defer_accept;
-       /* 3 bytes hole, try to pack */
+       u16                     rskq_defer_accept;
+       /* 2 bytes hole, try to pack */
        struct listen_sock      *listen_opt;
 };

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 7de4ea3..5780f62 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -138,6 +138,7 @@ extern void tcp_time_wait(struct sock *sk, int state, int timeo);
 #define MAX_TCP_KEEPINTVL      32767
 #define MAX_TCP_KEEPCNT                127
 #define MAX_TCP_SYNCNT         127
+#define MAX_TCP_ACCEPT_DEFERRED 65535

 #define TCP_SYNQ_INTERVAL      (HZ/5)  /* Period of SYNACK timer */

diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
index 7216f5e..9323405 100644
--- a/net/ipv4/inet_connection_sock.c
+++ b/net/ipv4/inet_connection_sock.c
@@ -414,8 +414,7 @@ void inet_csk_reqsk_queue_prune(struct sock *parent,
        struct inet_connection_sock *icsk = inet_csk(parent);
        struct request_sock_queue *queue = &icsk->icsk_accept_queue;
        struct listen_sock *lopt = queue->listen_opt;
-       int max_retries = icsk->icsk_syn_retries ? : sysctl_tcp_synack_retries;
-       int thresh = max_retries;
+       int thresh = icsk->icsk_syn_retries ? : sysctl_tcp_synack_retries;
        unsigned long now = jiffies;
        struct request_sock **reqp, *req;
        int i, budget;
@@ -451,9 +450,6 @@ void inet_csk_reqsk_queue_prune(struct sock *parent,
                }
        }

-       if (queue->rskq_defer_accept)
-               max_retries = queue->rskq_defer_accept;
-
        budget = 2 * (lopt->nr_table_entries / (timeout / interval));
        i = lopt->clock_hand;

@@ -461,9 +457,8 @@ void inet_csk_reqsk_queue_prune(struct sock *parent,
                reqp=&lopt->syn_table[i];
                while ((req = *reqp) != NULL) {
                        if (time_after_eq(now, req->expires)) {
-                               if ((req->retrans < (inet_rsk(req)->acked ? max_retries : thresh))
-                                   &&  (inet_rsk(req)->acked ||
-                                        !req->rsk_ops->rtx_syn_ack(parent, req, NULL))) {
+                               if (req->retrans < thresh &&
+                                   !req->rsk_ops->rtx_syn_ack(parent, req, NULL)) {
                                        unsigned long timeo;

                                        if (req->retrans++ == 0)
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 071e83a..e0fbc25 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -2105,15 +2105,12 @@ static int do_tcp_setsockopt(struct sock *sk, int level,
                break;

        case TCP_DEFER_ACCEPT:
-               icsk->icsk_accept_queue.rskq_defer_accept = 0;
-               if (val > 0) {
-                       /* Translate value in seconds to number of
-                        * retransmits */
-                       while (icsk->icsk_accept_queue.rskq_defer_accept < 32 &&
-                              val > ((TCP_TIMEOUT_INIT / HZ) <<
-                                      icsk->icsk_accept_queue.rskq_defer_accept))
-                               icsk->icsk_accept_queue.rskq_defer_accept++;
-                       icsk->icsk_accept_queue.rskq_defer_accept++;
+               if (val < 0) {
+                       err = -EINVAL;
+               } else {
+                       if (val > MAX_TCP_ACCEPT_DEFERRED)
+                               val = MAX_TCP_ACCEPT_DEFERRED;
+                       icsk->icsk_accept_queue.rskq_defer_accept = val;
                }
                break;

@@ -2295,8 +2292,7 @@ static int do_tcp_getsockopt(struct sock *sk, int level,
                        val = (val ? : sysctl_tcp_fin_timeout) / HZ;
                break;
        case TCP_DEFER_ACCEPT:
-               val = !icsk->icsk_accept_queue.rskq_defer_accept ? 0 :
-                       ((TCP_TIMEOUT_INIT / HZ) << (icsk->icsk_accept_queue.rskq_defer_accept - 1));
+               val = icsk->icsk_accept_queue.rskq_defer_accept;
                break;
        case TCP_WINDOW_CLAMP:
                val = tp->window_clamp;
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 19c449f..11aea98 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4447,6 +4447,49 @@ static void tcp_urg(struct sock *sk, struct sk_buff *skb, struct tcphdr *th)
        }
 }

+static int tcp_defer_accept_check(struct sock *sk)
+{
+       struct tcp_sock *tp = tcp_sk(sk);
+
+       if (tp->defer_tcp_accept.request) {
+               int queued_data =  tp->rcv_nxt - tp->copied_seq;
+               int hasfin =  !skb_queue_empty(&sk->sk_receive_queue) ?
+                       tcp_hdr((struct sk_buff *)
+                               sk->sk_receive_queue.prev)->fin : 0;
+
+               if (queued_data && hasfin)
+                       queued_data--;
+
+               if (queued_data &&
+                   tp->defer_tcp_accept.listen_sk->sk_state == TCP_LISTEN) {
+                       if (sock_flag(sk, SOCK_KEEPOPEN)) {
+                               inet_csk_reset_keepalive_timer(sk,
+                                                              keepalive_time_when(tp));
+                       } else {
+                               inet_csk_delete_keepalive_timer(sk);
+                       }
+
+                       inet_csk_reqsk_queue_add(
+                               tp->defer_tcp_accept.listen_sk,
+                               tp->defer_tcp_accept.request,
+                               sk);
+
+                       tp->defer_tcp_accept.listen_sk->sk_data_ready(
+                               tp->defer_tcp_accept.listen_sk, 0);
+
+                       sock_put(tp->defer_tcp_accept.listen_sk);
+                       sock_put(sk);
+                       tp->defer_tcp_accept.listen_sk = NULL;
+                       tp->defer_tcp_accept.request = NULL;
+               } else if (hasfin ||
+                          tp->defer_tcp_accept.listen_sk->sk_state != TCP_LISTEN) {
+                       tcp_reset(sk);
+                       return -1;
+               }
+       }
+       return 0;
+}
+
 static int tcp_copy_to_iovec(struct sock *sk, struct sk_buff *skb, int hlen)
 {
        struct tcp_sock *tp = tcp_sk(sk);
@@ -4807,6 +4850,9 @@ step5:

        tcp_data_snd_check(sk);
        tcp_ack_snd_check(sk);
+
+       if (tcp_defer_accept_check(sk))
+               return -1;
        return 0;

 csum_error:
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 00156bf..d4c9d03 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1921,6 +1921,14 @@ int tcp_v4_destroy_sock(struct sock *sk)
                sk->sk_sndmsg_page = NULL;
        }

+       if (tp->defer_tcp_accept.request) {
+               reqsk_free(tp->defer_tcp_accept.request);
+               sock_put(tp->defer_tcp_accept.listen_sk);
+               sock_put(sk);
+               tp->defer_tcp_accept.listen_sk = NULL;
+               tp->defer_tcp_accept.request = NULL;
+       }
+
        atomic_dec(&tcp_sockets_allocated);

        return 0;
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index b61b768..ab519d8 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -569,10 +569,8 @@ struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb,
           does sequence test, SYN is truncated, and thus we consider
           it a bare ACK.

-          If icsk->icsk_accept_queue.rskq_defer_accept, we silently drop this
-          bare ACK.  Otherwise, we create an established connection.  Both
-          ends (listening sockets) accept the new incoming connection and try
-          to talk to each other. 8-)
+          Both ends (listening sockets) accept the new incoming
+          connection and try to talk to each other. 8-)

           Note: This case is both harmless, and rare.  Possibility is about the
           same as us discovering intelligent life on another plant tomorrow.
@@ -640,13 +638,6 @@ struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb,
                if (!(flg & TCP_FLAG_ACK))
                        return NULL;

-               /* If TCP_DEFER_ACCEPT is set, drop bare ACK. */
-               if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
-                   TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
-                       inet_rsk(req)->acked = 1;
-                       return NULL;
-               }
-
                /* OK, ACK is valid, create big socket and
                 * feed this segment to it. It will repeat all
                 * the tests. THIS SEGMENT MUST MOVE SOCKET TO
@@ -685,7 +676,24 @@ struct sock *tcp_check_req(struct sock *sk,struct sk_buff *skb,
                inet_csk_reqsk_queue_unlink(sk, req, prev);
                inet_csk_reqsk_queue_removed(sk, req);

-               inet_csk_reqsk_queue_add(sk, req, child);
+               if (inet_csk(sk)->icsk_accept_queue.rskq_defer_accept &&
+                   TCP_SKB_CB(skb)->end_seq == tcp_rsk(req)->rcv_isn + 1) {
+
+                       /* the accept queue handling is done is est recv slow
+                        * path so lets make sure to start there
+                        */
+                       tcp_sk(child)->pred_flags = 0;
+                       sock_hold(sk);
+                       sock_hold(child);
+                       tcp_sk(child)->defer_tcp_accept.listen_sk = sk;
+                       tcp_sk(child)->defer_tcp_accept.request = req;
+
+                       inet_csk_reset_keepalive_timer(child,
+                                                      inet_csk(sk)->icsk_accept_queue.rskq_defer_accept * HZ);
+               } else {
+                       inet_csk_reqsk_queue_add(sk, req, child);
+               }
+
                return child;

        listen_overflow:
diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
index 803d758..160d16f 100644
--- a/net/ipv4/tcp_timer.c
+++ b/net/ipv4/tcp_timer.c
@@ -481,6 +481,11 @@ static void tcp_keepalive_timer (unsigned long data)
                goto death;
        }

+       if (tp->defer_tcp_accept.request && sk->sk_state == TCP_ESTABLISHED) {
+               tcp_send_active_reset(sk, GFP_ATOMIC);
+               goto death;
+       }
+
        if (!sock_flag(sk, SOCK_KEEPOPEN) || sk->sk_state == TCP_CLOSE)
                goto out;