[CCID3]: Call sk->sk_write_space(sk) when receiving a feedback packet

This makes the send rate calculations behave way more closely to what
is specified, with the jitter previously seen on x and x_recv
disappearing completely on non lossy setups.

This resembles the tcp_data_snd_check code, that possibly we'll end up
using in DCCP as well, perhaps moving this code to
inet_connection_sock.

For now I'm doing the simplest implementation tho.

Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

net/dccp/ccids/ccid3.c

···
 		/* remove all packets older than the one acked from history */
 		dccp_tx_hist_purge_older(ccid3_tx_hist,
 					 &hctx->ccid3hctx_hist, packet);
+		/*
+		 * As we have calculated new ipi, delta, t_nom it is possible that
+		 * we now can send a packet, so wake up dccp_wait_for_ccids.
+		 */
+		sk->sk_write_space(sk);
 
 		/*
 		 * Schedule no feedback timer to expire in

net/dccp/dccp.h

···
 			   const enum dccp_pkt_type pkt_type);
 
 extern int dccp_write_xmit(struct sock *sk, struct sk_buff *skb, long *timeo);
+extern void dccp_write_space(struct sock *sk);
 
 extern void dccp_init_xmit_timers(struct sock *sk);
 static inline void dccp_clear_xmit_timers(struct sock *sk)

net/dccp/ipv4.c

···
 	dccp_init_xmit_timers(sk);
 	inet_csk(sk)->icsk_rto = DCCP_TIMEOUT_INIT;
 	sk->sk_state = DCCP_CLOSED;
+	sk->sk_write_space = dccp_write_space;
 	dp->dccps_mss_cache = 536;
 	dp->dccps_role = DCCP_ROLE_UNDEFINED;
 

net/dccp/output.c

···
 	return mss_now;
 }
 
+void dccp_write_space(struct sock *sk)
+{
+	read_lock(&sk->sk_callback_lock);
+
+	if (sk->sk_sleep && waitqueue_active(sk->sk_sleep))
+		wake_up_interruptible(sk->sk_sleep);
+	/* Should agree with poll, otherwise some programs break */
+	if (sock_writeable(sk))
+		sk_wake_async(sk, 2, POLL_OUT);
+
+	read_unlock(&sk->sk_callback_lock);
+}
+
 /**
  * dccp_wait_for_ccid - Wait for ccid to tell us we can send a packet
  * @sk: socket to wait for