Re: [PATCH] tcp: Fix highest_sack and highest_sack_seq

[Eric Dumazet <eric.dumazet@gmail.com>]

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@linux.alibaba.com>

Signed-off-by: Eric Dumazet <edumazet@google.com>

Sadly I could not come to an alternative solution.

Thanks !