[PATCH] Re: a question on tcp_highspeed.c (in 2.6.16)

[John Heffner <jheffner@psc.edu>]

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Xiaoliang (David) Wei wrote:
> Hi gurus,
> 
>    I am reading the code of tcp_highspeed.c in the kernel and have a
> question on the hstcp_cong_avoid function, specifically the following
> AI part (line 136~143 in net/ipv4/tcp_highspeed.c ):
> 
>                /* Do additive increase */
>                if (tp->snd_cwnd < tp->snd_cwnd_clamp) {
>                        tp->snd_cwnd_cnt += ca->ai;
>                        if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
>                                tp->snd_cwnd++;
>                                tp->snd_cwnd_cnt -= tp->snd_cwnd;
>                        }
>                }
> 
>    In this part, when (tp->snd_cwnd_cnt == tp->snd_cwnd),
> snd_cwnd_cnt will be -1... snd_cwnd_cnt is defined as u16, will this
> small chance of getting -1 becomes a problem?
> Shall we change it by reversing the order of the cwnd++ and cwnd_cnt -= 
> cwnd?

Absolutely correct.  Thanks.

Signed-off-by: John Heffner <jheffner@psc.edu>

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diff --git a/net/ipv4/tcp_highspeed.c b/net/ipv4/tcp_highspeed.c
index e0e9d13..b72fa55 100644
--- a/net/ipv4/tcp_highspeed.c
+++ b/net/ipv4/tcp_highspeed.c
@@ -137,8 +137,8 @@ static void hstcp_cong_avoid(struct sock
 		if (tp->snd_cwnd < tp->snd_cwnd_clamp) {
 			tp->snd_cwnd_cnt += ca->ai;
 			if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
-				tp->snd_cwnd++;
 				tp->snd_cwnd_cnt -= tp->snd_cwnd;
+				tp->snd_cwnd++;
 			}
 		}
 	}