Subject: Re: [PATCH] tcp: properly update lost_cnt_hint during shifting

From: "Yan, Zheng" <[email-masked]>
Date: Wed, 28 Sep 2011 17:15:33 +0800

On 09/28/2011 04:55 PM, Yan, Zheng wrote:
> On 09/28/2011 04:17 PM, Ilpo Järvinen wrote:
>> 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 <[email-masked]>
>>> ---
>>> 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)?
> 
> Thanks you for your reply.
> 
> skb == tp->lost_skb_hint is special.
> 
> If the skb is sacked and we shift 'pcount' packets to previous skb,
> these packets will not be counted by future tcp_mark_head_lost() call.
> So we should increase lost_cnt_hint.
> 
> If the skb is not sacked, the skb will be sacked soon by tcp_sacktag_one(),
> So we should not increase lost_cnt_hint.
> 
> I didn't think out the second case. I think the correct patch should be:
> ---
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 21fab3e..dcc2411 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -1390,9 +1390,15 @@ 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 ((TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) &&
> +		    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;
> +	}
>  
>  	TCP_SKB_CB(prev)->end_seq += shifted;
>  	TCP_SKB_CB(skb)->seq += shifted;
> ---

Sorry, I didn't think out the "skb before lost_skb_hint" case neither.
If the skb isn't sacked, tcp_sacktag_one() will increase the lost_cnt_hint.
So tcp_shifted_skb() shouldn't adjust the the lost_cnt_hint.

I hope my patch is correct this time.

---
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 21fab3e..697ce5f 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1390,8 +1390,8 @@ 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))
+	if (!tcp_is_fack(tp) && tcp_is_sack(tp) && tp->lost_skb_hint == skb &&
+	    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
 		tp->lost_cnt_hint += pcount;
 
 	TCP_SKB_CB(prev)->end_seq += shifted;