netdev - Re: [PATCH] tcp: Fix highest_sack and highest_sack

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Date:   Fri, 27 Dec 2019 19:28:35 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Cambda Zhu <cambda@...ux.alibaba.com>,
        Eric Dumazet <edumazet@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        Yuchung Cheng <ycheng@...gle.com>,
        netdev <netdev@...r.kernel.org>,
        Dust Li <dust.li@...ux.alibaba.com>
Subject: Re: [PATCH] tcp: Fix highest_sack and highest_sack_seq



On 12/27/19 4:49 PM, Eric Dumazet wrote:
> 
> 
> On 12/27/19 12:52 AM, Cambda Zhu wrote:
>> From commit 50895b9de1d3 ("tcp: highest_sack fix"), the logic about
>> setting tp->highest_sack to the head of the send queue was removed.
>> Of course the logic is error prone, but it is logical. Before we
>> remove the pointer to the highest sack skb and use the seq instead,
>> we need to set tp->highest_sack to NULL when there is no skb after
>> the last sack, and then replace NULL with the real skb when new skb
>> inserted into the rtx queue, because the NULL means the highest sack
>> seq is tp->snd_nxt. If tp->highest_sack is NULL and new data sent,
>> the next ACK with sack option will increase tp->reordering unexpectedly.
>>
>> This patch sets tp->highest_sack to the tail of the rtx queue if
>> it's NULL and new data is sent. The patch keeps the rule that the
>> highest_sack can only be maintained by sack processing, except for
>> this only case.
>>
>> Fixes: 50895b9de1d3 ("tcp: highest_sack fix")
>> Signed-off-by: Cambda Zhu <cambda@...ux.alibaba.com>
>> ---
>>  net/ipv4/tcp_output.c | 3 +++
>>  1 file changed, 3 insertions(+)
>>
>> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
>> index 1f7735ca8f22..58c92a7d671c 100644
>> --- a/net/ipv4/tcp_output.c
>> +++ b/net/ipv4/tcp_output.c
>> @@ -72,6 +72,9 @@ static void tcp_event_new_data_sent(struct sock *sk, struct sk_buff *skb)
>>  	__skb_unlink(skb, &sk->sk_write_queue);
>>  	tcp_rbtree_insert(&sk->tcp_rtx_queue, skb);
>>  
>> +	if (tp->highest_sack == NULL)
>> +		tp->highest_sack = skb;
>> +
>>  	tp->packets_out += tcp_skb_pcount(skb);
>>  	if (!prior_packets || icsk->icsk_pending == ICSK_TIME_LOSS_PROBE)
>>  		tcp_rearm_rto(sk);
>>
> 
> 
> This patch seems to keep something in the fast path, even for flows never experiencing
> sacks.
> 
> Why would we always painfully maintain tp->highest_sack to the left most skb in the rtx queue ?
> 
> Given that tcp_highest_sack_seq() has an explicit check about tp->highest_sack being NULL,
> there is something I do not quite understand yet.
> 
> Why keeping this piece of code ?
> 
>     if (tp->highest_sack == NULL)
>             return tp->snd_nxt;
> 
> Defensive programming should be replaced by better knowledge.
> 
> Can you provide more explanations, or maybe a packetdrill test ?
> 
> Maybe some other path (in slow path this time) misses a !tp->highest_sack test.
> 
> Thanks.
> 

Or maybe the real bug has been latent for years.

(added in commit 6859d49475d4 "[TCP]: Abstract tp->highest_sack accessing & point to next skb" )


diff --git a/include/net/tcp.h b/include/net/tcp.h
index e460ea7f767ba627972a63a974cae80357808366..32781fb5cf3a7aa1158c98cb87754b59dc922b1f 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1843,12 +1843,9 @@ static inline void tcp_push_pending_frames(struct sock *sk)
  */
 static inline u32 tcp_highest_sack_seq(struct tcp_sock *tp)
 {
-       if (!tp->sacked_out)
+       if (!tp->sacked_out || !tp->highest_sack)
                return tp->snd_una;
 
-       if (tp->highest_sack == NULL)
-               return tp->snd_nxt;
-
        return TCP_SKB_CB(tp->highest_sack)->seq;
 }