tcp_cubic: fix spurious HYSTART_DELAY exit upon drop in min RTT

Mirja Kuehlewind reported a bug in Linux TCP CUBIC Hystart, where
Hystart HYSTART_DELAY mechanism can exit Slow Start spuriously on an
ACK when the minimum rtt of a connection goes down. From inspection it
is clear from the existing code that this could happen in an example
like the following:

o The first 8 RTT samples in a round trip are 150ms, resulting in a
  curr_rtt of 150ms and a delay_min of 150ms.

o The 9th RTT sample is 100ms. The curr_rtt does not change after the
  first 8 samples, so curr_rtt remains 150ms. But delay_min can be
  lowered at any time, so delay_min falls to 100ms. The code executes
  the HYSTART_DELAY comparison between curr_rtt of 150ms and delay_min
  of 100ms, and the curr_rtt is declared far enough above delay_min to
  force a (spurious) exit of Slow start.

The fix here is simple: allow every RTT sample in a round trip to
lower the curr_rtt.

Fixes: edf0faa01c66 ("[TCP] CUBIC v2.3")
Reported-by: Mirja Kuehlewind <mirja.kuehlewind@ericsson.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/tcp_cubic.c b/net/ipv4/tcp_cubic.c
index 8f8eefd3a3ce116aa8fa2b7ef85c7eb503fa8da7..c7bf5b26bf0c24837034eaee0189847d49ded6e9 100644 (file)
--- a/net/ipv4/tcp_cubic.c
+++ b/net/ipv4/tcp_cubic.c
@@ -432,10 +432,9 @@ static void hystart_update(struct sock *sk, u32 delay)
 
        if (hystart_detect & HYSTART_DELAY) {
                /* obtain the minimum delay of more than sampling packets */
+               if (ca->curr_rtt > delay)
+                       ca->curr_rtt = delay;
                if (ca->sample_cnt < HYSTART_MIN_SAMPLES) {
-                       if (ca->curr_rtt > delay)
-                               ca->curr_rtt = delay;
-
                        ca->sample_cnt++;
                } else {
                        if (ca->curr_rtt > ca->delay_min +