netdev - Re: [PATCH net 3/5] tcp: move false FR condition into tcp_false_fast_retrans

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Date:   Wed, 07 Mar 2018 07:50:39 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
        netdev@...r.kernel.org
Subject: Re: [PATCH net 3/5] tcp: move false FR condition into
 tcp_false_fast_retrans_possible()

On Wed, 2018-03-07 at 14:59 +0200, Ilpo Järvinen wrote:
> No functional changes.
> 
> Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
> ---
>  net/ipv4/tcp_input.c | 21 +++++++++++++++++----
>  1 file changed, 17 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 1a33752..e20f9ad 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -2211,6 +2211,19 @@ static void tcp_update_scoreboard(struct sock
> *sk, int fast_rexmit)
>  	}
>  }
>  
> +/* False fast retransmits may occur when SACK is not in use under
> certain
> + * conditions (RFC6582). The sender MUST hold old state until
> something
> + * *above* high_seq is ACKed to prevent triggering such false fast
> + * retransmits. SACK TCP is safe.
> + */
> +static bool tcp_false_fast_retrans_possible(const struct sock *sk,
> +					    const u32 snd_una)
> +{
> +	const struct tcp_sock *tp = tcp_sk(sk);
> +
> +	return ((snd_una == tp->high_seq) && tcp_is_reno(tp));

return (EXPR);

should use instead :

return EXPR;

> +}
> +
>  static bool tcp_tsopt_ecr_before(const struct tcp_sock *tp, u32
> when)
>  {
>  	return tp->rx_opt.saw_tstamp && tp->rx_opt.rcv_tsecr &&
> @@ -2350,10 +2363,10 @@ static bool tcp_try_undo_recovery(struct sock
> *sk)
>  	} else if (tp->rack.reo_wnd_persist) {
>  		tp->rack.reo_wnd_persist--;
>  	}
> -	if (tp->snd_una == tp->high_seq && tcp_is_reno(tp)) {
> -		/* Hold old state until something *above* high_seq
> -		 * is ACKed. For Reno it is MUST to prevent false
> -		 * fast retransmits (RFC2582). SACK TCP is safe. */
> +	if (tcp_false_fast_retrans_possible(sk, tp->snd_una)) {
> +		/* Hold old state until something *above* high_seq
> is ACKed
> +		 * if false fast retransmit is possible.
> +		 */
>  		if (!tcp_any_retrans_done(sk))
>  			tp->retrans_stamp = 0;
>  		return true;