netdev - Re: [PATCH] net: tcp: add correct check for tcp_retransmit

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Date:   Fri, 30 Nov 2018 10:41:33 -0800
From:   Yuchung Cheng <ycheng@...gle.com>
To:     Sharath Chandra Vurukala <sharathv@...eaurora.org>
Cc:     netdev <netdev@...r.kernel.org>, chinagar@...eaurora.org,
        kapandey@...eaurora.org, subashab@...eaurora.org
Subject: Re: [PATCH] net: tcp: add correct check for tcp_retransmit_skb()

On Fri, Nov 30, 2018 at 10:28 AM Sharath Chandra Vurukala
<sharathv@...eaurora.org> wrote:
>
> when the tcp_retranmission_timer expires and tcp_retranmsit_skb is
> called if the retranmsission fails due to local congestion,
> backoff should not incremented.
>
> tcp_retransmit_skb() returns non-zero negative value in some cases of
> failure but the caller tcp_retransmission_timer() has a check for
> failure which checks if the return value is greater than zero.
> The check is corrected to check for non-zero value.
>
> Signed-off-by: Sharath Chandra Vurukala <sharathv@...eaurora.org>
Perhaps my previous comment was not clear: your bug-fix patch is incorrect.

On local congestion, tcp_retransmit_skb returns positive values
*only*. negative values do not indicate local congestion.

> ---
>  net/ipv4/tcp_timer.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index 091c5392..c19f371 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -511,7 +511,7 @@ void tcp_retransmit_timer(struct sock *sk)
>
>         tcp_enter_loss(sk);
>
> -       if (tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1) > 0) {
> +       if (tcp_retransmit_skb(sk, tcp_rtx_queue_head(sk), 1) != 0) {
>                 /* Retransmission failed because of local congestion,
>                  * do not backoff.
>                  */
> --
> 1.9.1
>