dccp: Extend CCID packet dequeueing interface

This extends the packet dequeuing interface of dccp_write_xmit() to allow
 1. CCIDs to take care of timing when the next packet may be sent;
 2. delayed sending (as before, with an inter-packet gap up to 65.535 seconds).

The main purpose is to take CCID2 out of its polling mode (when it is network-
limited, it tries every millisecond to send, without interruption).
The interface can also be used to support other CCIDs.

The mode of operation for (2) is as follows:
 * new packet is enqueued via dccp_sendmsg() => dccp_write_xmit(),
 * ccid_hc_tx_send_packet() detects that it may not send (e.g. window full), 
 * it signals this condition via `CCID_PACKET_WILL_DEQUEUE_LATER',
 * dccp_write_xmit() returns without further action;
 * after some time the wait-condition for CCID becomes true,
 * that CCID schedules the tasklet,
 * tasklet function calls ccid_hc_tx_send_packet() via dccp_write_xmit(),
 * since the wait-condition is now true, ccid_hc_tx_packet() returns "send now",
 * packet is sent, and possibly more (since dccp_write_xmit() loops).

Code reuse: the taskled function calls dccp_write_xmit(), the timer function
            reduces to a wrapper around the same code.

If the tasklet finds that the socket is locked, it re-schedules the tasklet
function (not the tasklet) after one jiffy.

Changed DCCP_BUG to dccp_pr_debug when transmit_skb returns an error (e.g. when a
local qdisc is used, NET_XMIT_DROP=1 can be returned for many packets).

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

2 files changed, 95 insertions, 59 deletions

diff --git a/net/dccp/output.c b/net/dccp/output.c
index bfda071559f4..9afd58e39e23 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -251,65 +251,98 @@ do_interrupted:
         goto out;
 }
 
+/**
+ * dccp_xmit_packet  -  Send data packet under control of CCID
+ * Transmits next-queued payload and informs CCID to account for the packet.
+ */
+static void dccp_xmit_packet(struct sock *sk)
+{
+        int err, len;
+        struct dccp_sock *dp = dccp_sk(sk);
+        struct sk_buff *skb = skb_dequeue(&sk->sk_write_queue);
+
+        if (unlikely(skb == NULL))
+                return;
+        len = skb->len;
+
+        if (sk->sk_state == DCCP_PARTOPEN) {
+                const u32 cur_mps = dp->dccps_mss_cache - DCCP_FEATNEG_OVERHEAD;
+                /*
+                 * See 8.1.5 - Handshake Completion.
+                 *
+                 * For robustness we resend Confirm options until the client has
+                 * entered OPEN. During the initial feature negotiation, the MPS
+                 * is smaller than usual, reduced by the Change/Confirm options.
+                 */
+                if (!list_empty(&dp->dccps_featneg) && len > cur_mps) {
+                        DCCP_WARN("Payload too large (%d) for featneg.\n", len);
+                        dccp_send_ack(sk);
+                        dccp_feat_list_purge(&dp->dccps_featneg);
+                }
+
+                inet_csk_schedule_ack(sk);
+                inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
+                                              inet_csk(sk)->icsk_rto,
+                                              DCCP_RTO_MAX);
+                DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATAACK;
+        } else if (dccp_ack_pending(sk)) {
+                DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATAACK;
+        } else {
+                DCCP_SKB_CB(skb)->dccpd_type = DCCP_PKT_DATA;
+        }
+
+        err = dccp_transmit_skb(sk, skb);
+        if (err)
+                dccp_pr_debug("transmit_skb() returned err=%d\n", err);
+        /*
+         * Register this one as sent even if an error occurred. To the remote
+         * end a local packet drop is indistinguishable from network loss, i.e.
+         * any local drop will eventually be reported via receiver feedback.
+         */
+        ccid_hc_tx_packet_sent(dp->dccps_hc_tx_ccid, sk, len);
+
+        /*
+         * If the CCID needs to transfer additional header options out-of-band
+         * (e.g. Ack Vectors or feature-negotiation options), it activates this
+         * flag to schedule a Sync. The Sync will automatically incorporate all
+         * currently pending header options, thus clearing the backlog.
+         */
+        if (dp->dccps_sync_scheduled)
+                dccp_send_sync(sk, dp->dccps_gsr, DCCP_PKT_SYNC);
+}
+
 void dccp_write_xmit(struct sock *sk, int block)
 {
         struct dccp_sock *dp = dccp_sk(sk);
         struct sk_buff *skb;
 
         while ((skb = skb_peek(&sk->sk_write_queue))) {
-                int err = ccid_hc_tx_send_packet(dp->dccps_hc_tx_ccid, sk, skb);
+                int rc = ccid_hc_tx_send_packet(dp->dccps_hc_tx_ccid, sk, skb);
 
-                if (err > 0) {
+                switch (ccid_packet_dequeue_eval(rc)) {
+                case CCID_PACKET_WILL_DEQUEUE_LATER:
+                        return;
+                case CCID_PACKET_DELAY:
                         if (!block) {
                                 sk_reset_timer(sk, &dp->dccps_xmit_timer,
-                                                msecs_to_jiffies(err)+jiffies);
+                                                msecs_to_jiffies(rc)+jiffies);
+                                return;
+                        }
+                        rc = dccp_wait_for_ccid(sk, skb, rc);
+                        if (rc && rc != -EINTR) {
+                                DCCP_BUG("err=%d after dccp_wait_for_ccid", rc);
+                                skb_dequeue(&sk->sk_write_queue);
+                                kfree_skb(skb);
                                 break;
-                        } else
-                                err = dccp_wait_for_ccid(sk, skb, err);
-                        if (err && err != -EINTR)
-                                DCCP_BUG("err=%d after dccp_wait_for_ccid", err);
-                }
-
-                skb_dequeue(&sk->sk_write_queue);
-                if (err == 0) {
-                        struct dccp_skb_cb *dcb = DCCP_SKB_CB(skb);
-                        const int len = skb->len;
-
-                        if (sk->sk_state == DCCP_PARTOPEN) {
-                                const u32 cur_mps = dp->dccps_mss_cache - DCCP_FEATNEG_OVERHEAD;
-                                /*
-                                 * See 8.1.5 - Handshake Completion.
-                                 *
-                                 * For robustness we resend Confirm options until the client has
-                                 * entered OPEN. During the initial feature negotiation, the MPS
-                                 * is smaller than usual, reduced by the Change/Confirm options.
-                                 */
-                                if (!list_empty(&dp->dccps_featneg) && len > cur_mps) {
-                                        DCCP_WARN("Payload too large (%d) for featneg.\n", len);
-                                        dccp_send_ack(sk);
-                                        dccp_feat_list_purge(&dp->dccps_featneg);
-                                }
-
-                                inet_csk_schedule_ack(sk);
-                                inet_csk_reset_xmit_timer(sk, ICSK_TIME_DACK,
-                                                  inet_csk(sk)->icsk_rto,
-                                                  DCCP_RTO_MAX);
-                                dcb->dccpd_type = DCCP_PKT_DATAACK;
-                        } else if (dccp_ack_pending(sk))
-                                dcb->dccpd_type = DCCP_PKT_DATAACK;
-                        else
-                                dcb->dccpd_type = DCCP_PKT_DATA;
-
-                        err = dccp_transmit_skb(sk, skb);
-                        ccid_hc_tx_packet_sent(dp->dccps_hc_tx_ccid, sk, len);
-                        if (err)
-                                DCCP_BUG("err=%d after ccid_hc_tx_packet_sent",
-                                         err);
-                        if (dp->dccps_sync_scheduled)
-                                dccp_send_sync(sk, dp->dccps_gsr, DCCP_PKT_SYNC);
-                } else {
-                        dccp_pr_debug("packet discarded due to err=%d\n", err);
+                        }
+                        /* fall through */
+                case CCID_PACKET_SEND_AT_ONCE:
+                        dccp_xmit_packet(sk);
+                        break;
+                case CCID_PACKET_ERR:
+                        skb_dequeue(&sk->sk_write_queue);
                         kfree_skb(skb);
+                        dccp_pr_debug("packet discarded due to err=%d\n", rc);
                 }
         }
 }
diff --git a/net/dccp/timer.c b/net/dccp/timer.c
index 162d1e683c39..9369aca4b0e9 100644
--- a/net/dccp/timer.c
+++ b/net/dccp/timer.c
@@ -237,32 +237,35 @@ out:
         sock_put(sk);
 }
 
-/* Transmit-delay timer: used by the CCIDs to delay actual send time */
-static void dccp_write_xmit_timer(unsigned long data)
+/**
+ * dccp_write_xmitlet  -  Workhorse for CCID packet dequeueing interface
+ * See the comments above %ccid_dequeueing_decision for supported modes.
+ */
+static void dccp_write_xmitlet(unsigned long data)
 {
         struct sock *sk = (struct sock *)data;
-        struct dccp_sock *dp = dccp_sk(sk);
 
         bh_lock_sock(sk);
         if (sock_owned_by_user(sk))
-                sk_reset_timer(sk, &dp->dccps_xmit_timer, jiffies+1);
+                sk_reset_timer(sk, &dccp_sk(sk)->dccps_xmit_timer, jiffies + 1);
         else
                 dccp_write_xmit(sk, 0);
         bh_unlock_sock(sk);
-        sock_put(sk);
 }
 
-static void dccp_init_write_xmit_timer(struct sock *sk)
+static void dccp_write_xmit_timer(unsigned long data)
 {
-        struct dccp_sock *dp = dccp_sk(sk);
-
-        setup_timer(&dp->dccps_xmit_timer, dccp_write_xmit_timer,
-                        (unsigned long)sk);
+        dccp_write_xmitlet(data);
+        sock_put((struct sock *)data);
 }
 
 void dccp_init_xmit_timers(struct sock *sk)
 {
-        dccp_init_write_xmit_timer(sk);
+        struct dccp_sock *dp = dccp_sk(sk);
+
+        tasklet_init(&dp->dccps_xmitlet, dccp_write_xmitlet, (unsigned long)sk);
+        setup_timer(&dp->dccps_xmit_timer, dccp_write_xmit_timer,
+                                                             (unsigned long)sk);
         inet_csk_init_xmit_timers(sk, &dccp_write_timer, &dccp_delack_timer,
                                   &dccp_keepalive_timer);
 }