netdev - Re: [PATCH v4] tcp: fix tcp_init_transfer() to not reset icsk_ca

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Date:   Sun, 4 Jul 2021 10:03:08 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     David Miller <davem@...emloft.net>
Cc:     phind.uet@...il.com, yhs@...com, edumazet@...gle.com,
        yoshfuji@...ux-ipv6.org, dsahern@...nel.org, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, andrii@...nel.org,
        kafai@...com, songliubraving@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, ycheng@...gle.com, yyd@...gle.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        syzbot+f1e24a0594d4e3a895d3@...kaller.appspotmail.com
Subject: Re: [PATCH v4] tcp: fix tcp_init_transfer() to not reset icsk_ca_initialized

On Sat, Jul 3, 2021 at 5:49 PM David Miller <davem@...emloft.net> wrote:
>
> From: Nguyen Dinh Phi <phind.uet@...il.com>
> Date: Sat,  3 Jul 2021 17:34:17 +0800
>
> > diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> > index 7d5e59f688de..855ada2be25e 100644
> > --- a/net/ipv4/tcp_input.c
> > +++ b/net/ipv4/tcp_input.c
> > @@ -5922,7 +5922,6 @@ void tcp_init_transfer(struct sock *sk, int bpf_op, struct sk_buff *skb)
> >               tp->snd_cwnd = tcp_init_cwnd(tp, __sk_dst_get(sk));
> >       tp->snd_cwnd_stamp = tcp_jiffies32;
> >
> > -     icsk->icsk_ca_initialized = 0;
> >       bpf_skops_established(sk, bpf_op, skb);
> >       if (!icsk->icsk_ca_initialized)
> >               tcp_init_congestion_control(sk);
>
> Don't you have to make the tcp_init_congestion_control() call unconditional now?

I think we want to keep it conditional, to avoid double-initialization
if the BPF code sets the congestion control algorithm and initializes
it. But that's relatively new and subtle, so it might be nice for this
patch to add a comment about that, since it's touching this part of
the code anyway:

-       icsk->icsk_ca_initialized = 0;
        bpf_skops_established(sk, bpf_op, skb);
+       /* Initialize congestion control unless BPF initialized it already: */
        if (!icsk->icsk_ca_initialized)
                tcp_init_congestion_control(sk);

neal