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Subject[PATCH 3.2 089/115] tcp_yeah: don't set ssthresh below 2
3.2.80-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Neal Cardwell <ncardwell@google.com>

[ Upstream commit 83d15e70c4d8909d722c0d64747d8fb42e38a48f ]

For tcp_yeah, use an ssthresh floor of 2, the same floor used by Reno
and CUBIC, per RFC 5681 (equation 4).

tcp_yeah_ssthresh() was sometimes returning a 0 or negative ssthresh
value if the intended reduction is as big or bigger than the current
cwnd. Congestion control modules should never return a zero or
negative ssthresh. A zero ssthresh generally results in a zero cwnd,
causing the connection to stall. A negative ssthresh value will be
interpreted as a u32 and will set a target cwnd for PRR near 4
billion.

Oleksandr Natalenko reported that a system using tcp_yeah with ECN
could see a warning about a prior_cwnd of 0 in
tcp_cwnd_reduction(). Testing verified that this was due to
tcp_yeah_ssthresh() misbehaving in this way.

Reported-by: Oleksandr Natalenko <oleksandr@natalenko.name>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
net/ipv4/tcp_yeah.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/net/ipv4/tcp_yeah.c
+++ b/net/ipv4/tcp_yeah.c
@@ -222,7 +222,7 @@ static u32 tcp_yeah_ssthresh(struct sock
yeah->fast_count = 0;
yeah->reno_count = max(yeah->reno_count>>1, 2U);

- return tp->snd_cwnd - reduction;
+ return max_t(int, tp->snd_cwnd - reduction, 2);
}

static struct tcp_congestion_ops tcp_yeah __read_mostly = {
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