tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT sample

BugLink: http://bugs.launchpad.net/bugs/990544

[ Upstream commit 18a223e0b9ec8979320ba364b47c9772391d6d05 ]

Fix a code path in tcp_rcv_rtt_update() that was comparing scaled and
unscaled RTT samples.

The intent in the code was to only use the 'm' measurement if it was a
new minimum.  However, since 'm' had not yet been shifted left 3 bits
but 'new_sample' had, this comparison would nearly always succeed,
leading us to erroneously set our receive-side RTT estimate to the 'm'
sample when that sample could be nearly 8x too high to use.

The overall effect is to often cause the receive-side RTT estimate to
be significantly too large (up to 40% too large for brief periods in
my tests).

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

1 files changed, 5 insertions, 2 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 72b1857723d..104b02e3ee2 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -460,8 +460,11 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
                 if (!win_dep) {
                         m -= (new_sample >> 3);
                         new_sample += m;
-                } else if (m < new_sample)
-                        new_sample = m << 3;
+                } else {
+                        m <<= 3;
+                        if (m < new_sample)
+                                new_sample = m;
+                }
         } else {
                 /* No previous measure. */
                 new_sample = m << 3;