Re: [PATCH] tcp: fix false reordering signal in tcp_shifted_skb

[David Miller <davem@davemloft.net>]

From: Neal Cardwell <ncardwell@google.com>
Date: Sun, 26 Feb 2012 15:06:19 -0500

> When tcp_shifted_skb() shifts bytes from the skb that is currently
> pointed to by 'highest_sack' then the increment of
> TCP_SKB_CB(skb)->seq implicitly advances tcp_highest_sack_seq(). This
> implicit advancement, combined with the recent fix to pass the correct
> SACKed range into tcp_sacktag_one(), caused tcp_sacktag_one() to think
> that the newly SACKed range was before the tcp_highest_sack_seq(),
> leading to a call to tcp_update_reordering() with a degree of
> reordering matching the size of the newly SACKed range (typically just
> 1 packet, which is a NOP, but potentially larger).
> 
> This commit fixes this by simply calling tcp_sacktag_one() before the
> TCP_SKB_CB(skb)->seq advancement that can advance our notion of the
> highest SACKed sequence.
> 
> Correspondingly, we can simplify the code a little now that
> tcp_shifted_skb() should update the lost_cnt_hint in all cases where
> skb == tp->lost_skb_hint.
> 
> Signed-off-by: Neal Cardwell <ncardwell@google.com>

Applied and queued up for -stable, thanks Neal.