netdev - Re: [PATCH] tcp: properly update lost_cnt

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Date:	Wed, 28 Sep 2011 11:17:28 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	"Yan, Zheng" <zheng.z.yan@...el.com>
cc:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Nandita Dukkipati <nanditad@...gle.com>
Subject: Re: [PATCH] tcp: properly update lost_cnt_hint during shifting

On Wed, 28 Sep 2011, Yan, Zheng wrote:

> lost_skb_hint is used by tcp_mark_head_lost() to mark the first
> unhandled skb. lost_cnt_hint is the number of sacked packets before
> the lost_skb_hint. tcp_shifted_skb() shouldn't increase lost_cnt_hint
> when shifting a sacked skb that is before the lost_skb_hint, because
> packets in it are already counted.
> 
> Signed-off-by: Zheng Yan <zheng.z.yan@...el.com>
> ---
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 21fab3e..f712ace 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1390,9 +1390,14 @@ static int tcp_shifted_skb(struct sock *sk, struct sk_buff *skb,
>  	BUG_ON(!pcount);
>  
>  	/* Tweak before seqno plays */
> -	if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint &&
> -	    !before(TCP_SKB_CB(tp->lost_skb_hint)->seq, TCP_SKB_CB(skb)->seq))
> -		tp->lost_cnt_hint += pcount;
> +	if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint) {
> +		if (skb == tp->lost_skb_hint)
> +			tp->lost_cnt_hint += pcount;
> +		else if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
> +			 before(TCP_SKB_CB(skb)->seq,
> +				TCP_SKB_CB(tp->lost_skb_hint)->seq))
> +			tp->lost_cnt_hint += pcount;
> +	}

Ah right, the hole filled case which shifts not only the newly SACKed 
skb but also the next, already SACKed skb?

I fail to see why you needed to change !before into two checks though:
 skb == tp->lost_skb_hint and before(params reversed) ? Shouldn't the 
equality that is provided by the negation cover for the == check (and the 
params reversion isn't necessary in any case)? In fact, isn't the skb == 
tp->lost_skb_hint check strictly wrong without the same TCPCB_SACKED_ACKED 
guard (though I'm not sure, I didn't check, if the hint can ever point to 
such a segment in the first place)?

Added Cc to Nandita as they're hunting (possibly other) bug in 
tcp_mark_head_lost.

-- 
 i.

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