Re: [PATCH net] tcp: update skb->skb_mstamp more carefully

[David Miller <davem@davemloft.net>]

From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Wed, 13 Sep 2017 20:30:39 -0700

> From: Eric Dumazet <edumazet@googl.com>
> 
> liujian reported a problem in TCP_USER_TIMEOUT processing with a patch
> in tcp_probe_timer() :
>       https://www.spinics.net/lists/netdev/msg454496.html
> 
> After investigations, the root cause of the problem is that we update
> skb->skb_mstamp of skbs in write queue, even if the attempt to send a
> clone or copy of it failed. One reason being a routing problem.
> 
> This patch prevents this, solving liujian issue.
> 
> It also removes a potential RTT miscalculation, since
> __tcp_retransmit_skb() is not OR-ing TCP_SKB_CB(skb)->sacked with
> TCPCB_EVER_RETRANS if a failure happens, but skb->skb_mstamp has
> been changed.
> 
> A future ACK would then lead to a very small RTT sample and min_rtt
> would then be lowered to this too small value.
> 
> Tested:
...
> Signed-off-by: Eric Dumazet <edumazet@googl.com>
> Reported-by: liujian <liujian56@huawei.com>

Applied and queued up for -stable, thanks.