netdev - Re: [PATCH net-next] tcp: drop dst in tcp_add

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Date:   Tue, 20 Nov 2018 10:08:12 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Miller <davem@...emloft.net>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next] tcp: drop dst in tcp_add_backlog()

On Tue, Nov 20, 2018 at 10:01 AM David Miller <davem@...emloft.net> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
> Date: Mon, 19 Nov 2018 17:45:55 -0800
>
> > Under stress, softirq rx handler often hits a socket owned by the user,
> > and has to queue the packet into socket backlog.
> >
> > When this happens, skb dst refcount is taken before we escape rcu
> > protected region. This is done from __sk_add_backlog() calling
> > skb_dst_force().
> >
> > Consumer will have to perform the opposite costly operation.
> >
> > AFAIK nothing in tcp stack requests the dst after skb was stored
> > in the backlog. If this was the case, we would have had failures
> > already since skb_dst_force() can end up clearing skb dst anyway.
> >
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
>
> Hmmm, it seems to be used by connection completion code to setup the
> socket cached rx dst, right?
>
> For example tcp_finish_connect() --> icsk->icsk_af_ops->sk_rx_dst_set(sk, skb)

We already cope with skb->dst being NULL there I believe.

For reference look at

commit 5037e9ef9454917b047f9f3a19b4dd179fbf7cd4    net: fix IP early demux races