Re: [PATCH net-next 01/19] net: tcp: introduce tcp_drop_reason()

[Eric Dumazet]

On Tue, Feb 15, 2022 at 3:30 AM <menglong8.dong@xxxxxxxxx> wrote:
>
> From: Menglong Dong <imagedong@xxxxxxxxxxx>
>
> For TCP protocol, tcp_drop() is used to free the skb when it needs
> to be dropped. To make use of kfree_skb_reason() and collect drop
> reasons, introduce the function tcp_drop_reason().
>
> tcp_drop_reason() will finally call kfree_skb_reason() and pass the
> drop reason to 'kfree_skb' tracepoint.
>
> PS: __kfree_skb() was used in tcp_drop(), I'm not sure if it's ok
> to replace it with kfree_skb_reason().
>
> Signed-off-by: Menglong Dong <imagedong@xxxxxxxxxxx>
> ---
>  net/ipv4/tcp_input.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index af94a6d22a9d..e3811afd1756 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4684,10 +4684,19 @@ static bool tcp_ooo_try_coalesce(struct sock *sk,
>         return res;
>  }
>
> -static void tcp_drop(struct sock *sk, struct sk_buff *skb)
> +static void tcp_drop_reason(struct sock *sk, struct sk_buff *skb,
> +                           enum skb_drop_reason reason)
>  {
>         sk_drops_add(sk, skb);
> -       __kfree_skb(skb);
> +       /* why __kfree_skb() used here before, other than kfree_skb()?
> +        * confusing......

Do not add comments like that if you do not know the difference...

__kfree_skb() is used by TCP stack because it owns skb in receive
queues, and avoids touching skb->users
because it must be one already.

(We made sure not using skb_get() in TCP)

It seems fine to use kfree_skb() in tcp_drop(), it is hardly fast
path, and the added cost is pure noise.