[PATCH 3.19 008/123] tcp: fix tcp_cong_avoid_ai() credit accumulation bug with decreases in w

[Greg Kroah-Hartman]

3.19-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Neal Cardwell <ncardwell@xxxxxxxxxx>

[ Upstream commit 9949afa42be0b76f5832db112ce51bb6b35b2abb ]

The recent change to tcp_cong_avoid_ai() to handle stretch ACKs
introduced a bug where snd_cwnd_cnt could accumulate a very large
value while w was large, and then if w was reduced snd_cwnd could be
incremented by a large delta, leading to a large burst and high packet
loss. This was tickled when CUBIC's bictcp_update() sets "ca->cnt =
100 * cwnd".

This bug crept in while preparing the upstream version of
814d488c6126.

Testing: This patch has been tested in datacenter netperf transfers
and live youtube.com and google.com servers.

Fixes: 814d488c6126 ("tcp: fix the timid additive increase on stretch ACKs")
Signed-off-by: Neal Cardwell <ncardwell@xxxxxxxxxx>
Signed-off-by: Yuchung Cheng <ycheng@xxxxxxxxxx>
Signed-off-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/ipv4/tcp_cong.c |    6 ++++++
 1 file changed, 6 insertions(+)

--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -309,6 +309,12 @@ EXPORT_SYMBOL_GPL(tcp_slow_start);
  */
 void tcp_cong_avoid_ai(struct tcp_sock *tp, u32 w, u32 acked)
 {
+	/* If credits accumulated at a higher w, apply them gently now. */
+	if (tp->snd_cwnd_cnt >= w) {
+		tp->snd_cwnd_cnt = 0;
+		tp->snd_cwnd++;
+	}
+
 	tp->snd_cwnd_cnt += acked;
 	if (tp->snd_cwnd_cnt >= w) {
 		u32 delta = tp->snd_cwnd_cnt / w;


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