Re: [PATCH net] tcp: initialize send sequence numbers before deferring TFO

[Eric Dumazet]

Le lun. 2 nov. 2026 à 08:41, Qingfang Deng <qingfang.deng@xxxxxxxxx> a écrit :
>
> On 2026/11/2 14:11, Jiayuan Chen wrote:
> > On 11/2/26 11:59 AM, Qingfang Deng wrote:
> >> TCP_FASTOPEN_CONNECT can return from connect() before tcp_connect_init()
> >> when a cached cookie or cookieless Fast Open allows deferring the SYN.
> >> At this point write_seq contains the initial sequence number, while
> >> snd_una and snd_nxt have not been initialized to that sequence number.
> >>
> >> Consequently, inet_diag and /proc/net/tcp{,6} report the initial
> >> sequence number as the send queue length despite no SYN or payload being
> >> queued.
> >>
> >> Initialize snd_una and snd_nxt to write_seq before marking the
> >> connection as deferred. Both sequence-based queue lengths then remain
> >> zero until the first write starts the connection and runs
> >> tcp_connect_init().
> >
> > This patch looks good to me. But idiag_wqueue can still go negative
> > on the normal connect path, since the socket is already hashed and
> > diag does not take the socket lock. We always set write_seq first,
> > then init snd_una later:
> >
> >    tcp_v4_connect
> >      WRITE_ONCE(tp->write_seq, st.seq)
> >      tcp_connect
> >        tcp_connect_init
> >          WRITE_ONCE(tp->snd_una, tp->write_seq)
> >
> > Can you try this instead?
> >
> >    r->idiag_wqueue = READ_ONCE(sk->sk_wmem_queued) ?
> >                      READ_ONCE(tp->write_seq) - READ_ONCE(tp->snd_una) : 0;
>
> This does not guarantee the load order on a weakly-ordered CPU. LLM
> suggests using this load-acquire/store-release sequence:
>
> r->idiag_wqueue = smp_load_acquire(&sk->sk_wmem_queued) ?
> READ_ONCE(tp->write_seq) - READ_ONCE(tp->snd_una) : 0;
>
> Then in tcp_connect_queue_skb(), replace sk_wmem_queued_add()
> with smp_store_release():
>
> /* Paired with smp_load_acquire() in tcp_diag_get_info(). */
> smp_store_release(&sk->sk_wmem_queued, sk->sk_wmem_queued + skb->truesize);
>
> What do you think?

Please do not do this.

/proc/net/tcp and inet_diag dump sockets without holding the socket
lock. They provide a best effort snapshot, and transient inconsistent
values are expected (see the comment about rx_queue in get_tcp4_sock()).

The window in tcp_v4_connect()/tcp_v6_connect() is a few usec, with the
socket lock held by the caller. Adding memory barriers in the TCP stack,
on a field updated in many places, to cover this is not worth it.

The bug you found is different: with deferred TFO, the bogus values stay
forever, until the application calls sendmsg(). Your v1 fixes this.

Note that tcpi_notsent_bytes (tcp_get_info()) is also fixed by your
patch, you might mention this in the changelog. Not a big deal if you
do not want to send a v2.

Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>