[PATCH net-next] tcp: fix ABC in tcp_slow_start()

[Eric Dumazet <eric.dumazet@gmail.com>]

From: Eric Dumazet <edumazet@google.com>

When/if sysctl_tcp_abc > 1, we expect to increase cwnd by 2 if the
received ACK acknowledges more than 2*MSS bytes, in tcp_slow_start()

Problem is this RFC 3465 statement is not correctly coded, as
the while () loop increases snd_cwnd one by one.

So to reach the "cwnd += 2" goal, we need to use "cnt = 2*cnt + 1"

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Tom Herbert <therbert@google.com>
Cc: Yuchung Cheng <ycheng@google.com>
Cc: Neal Cardwell <ncardwell@google.com>
Cc: Nandita Dukkipati <nanditad@google.com>
Cc: John Heffner <johnwheffner@gmail.com>
Cc: Stephen Hemminger <shemminger@vyatta.com>
---
 net/ipv4/tcp_cong.c |   13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 04dbd7a..486379a 100644
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -306,7 +306,7 @@ EXPORT_SYMBOL_GPL(tcp_is_cwnd_limited);
  */
 void tcp_slow_start(struct tcp_sock *tp)
 {
-	int cnt; /* increase in packets */
+	unsigned int cnt; /* increase in packets */
 
 	/* RFC3465: ABC Slow start
 	 * Increase only after a full MSS of bytes is acked
@@ -318,16 +318,19 @@ void tcp_slow_start(struct tcp_sock *tp)
 	if (sysctl_tcp_abc && tp->bytes_acked < tp->mss_cache)
 		return;
 
-	if (sysctl_tcp_max_ssthresh > 0 && tp->snd_cwnd > sysctl_tcp_max_ssthresh)
+	cnt = tp->snd_cwnd;			/* exponential increase */
+	if (sysctl_tcp_max_ssthresh > 0 &&
+	    tp->snd_cwnd > sysctl_tcp_max_ssthresh)
 		cnt = sysctl_tcp_max_ssthresh >> 1;	/* limited slow start */
-	else
-		cnt = tp->snd_cwnd;			/* exponential increase */
 
 	/* RFC3465: ABC
 	 * We MAY increase by 2 if discovered delayed ack
+	 * The "+ 1" in the expression is needed if we want to increase
+	 * cwnd by 2, because the way is coded the following loop.
 	 */
 	if (sysctl_tcp_abc > 1 && tp->bytes_acked >= 2*tp->mss_cache)
-		cnt <<= 1;
+		cnt = 2*cnt + 1;
+
 	tp->bytes_acked = 0;
 
 	tp->snd_cwnd_cnt += cnt;