Re: [PATCH net-next] tcp: limit wake-up for crossed SYN cases with SYN-ACK

[Eric Dumazet]

On Thu, Aug 1, 2024 at 6:39 PM Matthieu Baerts (NGI0)
<matttbe@xxxxxxxxxx> wrote:
>
> In TCP_SYN_RECV states, sk->sk_socket will be assigned in case of
> marginal crossed SYN, but also in other cases, e.g.
>
>  - With TCP Fast Open, if the connection got accept()'ed before
>    receiving the 3rd ACK ;
>
>  - With MPTCP, when accepting additional subflows to an existing MPTCP
>    connection.
>
> In these cases, the switch to TCP_ESTABLISHED is done when receiving the
> 3rd ACK, without the SYN flag then.
>
> To properly restrict the wake-up to crossed SYN cases as expected there,
> it is then required to also limit the check to packets containing the
> SYN-ACK flags.
>
> Without this modification, it looks like the wake-up was not causing any
> visible issue with TFO and MPTCP, apart from not being needed. That's
> why this patch doesn't contain a Cc to stable, and a Fixes tag.
>
> While at it, the attached comment has also been updated: sk->sk_sleep
> has been removed in 2010, and replaced by sk->sk_wq in commit
> 43815482370c ("net: sock_def_readable() and friends RCU conversion").
>
> Suggested-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
> Signed-off-by: Matthieu Baerts (NGI0) <matttbe@xxxxxxxxxx>
> ---
> Notes:
>   - This is the same patch as the one suggested earlier in -net as part
>     of another series, but targeting net-next (Eric), and with an
>     updated commit message. The previous version was visible there:
>     https://lore.kernel.org/20240718-upstream-net-next-20240716-tcp-3rd-ack-consume-sk_socket-v2-2-d653f85639f6@xxxxxxxxxx/
> ---

Note: I am not aware of any tests using FASYNC

sock_wake_async() / kill_fasync() are sending signals, not traditional wakeups.

Do we really want to potentially break some applications still using
pre-multi-thread era async io ?

Not that I really care, but I wonder why you care :)