[TCP]: MTUprobe: receiver window & data available checks fixed

It seems that the checked range for receiver window check should
begin from the first rather than from the last skb that is going
to be included to the probe. And that can be achieved without
reference to skbs at all, snd_nxt and write_seq provides the
correct seqno already. Plus, it SHOULD account packets that are
necessary to trigger fast retransmit [RFC4821].

Location of snd_wnd < probe_size/size_needed check is bogus
because it will cause the other if() match as well (due to
snd_nxt >= snd_una invariant).

Removed dead obvious comment.
Signed-off-by: NIlpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

net/ipv4/tcp_output.c

@@ -1295,6 +1295,7 @@ static int tcp_mtu_probe(struct sock *sk)
 	struct sk_buff *skb, *nskb, *next;
 	int len;
 	int probe_size;
+	int size_needed;
 	unsigned int pif;
 	int copy;
 	int mss_now;
@@ -1313,6 +1314,7 @@ static int tcp_mtu_probe(struct sock *sk)
 	/* Very simple search strategy: just double the MSS. */
 	mss_now = tcp_current_mss(sk, 0);
 	probe_size = 2*tp->mss_cache;
+	size_needed = probe_size + (tp->reordering + 1) * tp->mss_cache;
 	if (probe_size > tcp_mtu_to_mss(sk, icsk->icsk_mtup.search_high)) {
 		/* TODO: set timer for probe_converge_event */
 		return -1;
@@ -1322,18 +1324,15 @@ static int tcp_mtu_probe(struct sock *sk)
 	len = 0;
 	if ((skb = tcp_send_head(sk)) == NULL)
 		return -1;
-	while ((len += skb->len) < probe_size && !tcp_skb_is_last(sk, skb))
+	while ((len += skb->len) < size_needed && !tcp_skb_is_last(sk, skb))
 		skb = tcp_write_queue_next(sk, skb);
-	if (len < probe_size)
+	if (len < size_needed)
 		return -1;
 
-	/* Receive window check. */
-	if (after(TCP_SKB_CB(skb)->seq + probe_size, tp->snd_una + tp->snd_wnd)) {
-		if (tp->snd_wnd < probe_size)
-			return -1;
-		else
-			return 0;
-	}
+	if (tp->snd_wnd < size_needed)
+		return -1;
+	if (after(tp->snd_nxt + size_needed, tp->snd_una + tp->snd_wnd))
+		return 0;
 
 	/* Do we need to wait to drain cwnd? */
 	pif = tcp_packets_in_flight(tp);