Re: [PATCH net-next] sctp: fix busy polling

[Eric Dumazet <edumazet@google.com>]

On Fri, Dec 22, 2023 at 5:08 PM Xin Long <lucien.xin@gmail.com> wrote:
>
> On Tue, Dec 19, 2023 at 12:00 PM Eric Dumazet <edumazet@google.com> wrote:
> >
> > Busy polling while holding the socket lock makes litle sense,
> > because incoming packets wont reach our receive queue.
> >
> > Fixes: 8465a5fcd1ce ("sctp: add support for busy polling to sctp protocol")
> > Reported-by: Jacob Moroni <jmoroni@google.com>
> > Signed-off-by: Eric Dumazet <edumazet@google.com>
> > Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
> > Cc: Xin Long <lucien.xin@gmail.com>
> > ---
> >  net/sctp/socket.c | 10 ++++------
> >  1 file changed, 4 insertions(+), 6 deletions(-)
> >
> > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > index 5fb02bbb4b349ef9ab9c2790cccb30fb4c4e897c..6b9fcdb0952a0fe599ae5d1d1cc6fa9557a3a3bc 100644
> > --- a/net/sctp/socket.c
> > +++ b/net/sctp/socket.c
> > @@ -2102,6 +2102,10 @@ static int sctp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
> >         if (unlikely(flags & MSG_ERRQUEUE))
> >                 return inet_recv_error(sk, msg, len, addr_len);
> >
> > +       if (sk_can_busy_loop(sk) &&
> > +           skb_queue_empty_lockless(&sk->sk_receive_queue))
> > +               sk_busy_loop(sk, flags & MSG_DONTWAIT);
> > +
> Here is no any sk_state check, if the SCTP socket(TCP type) has been
> already closed by peer, will sctp_recvmsg() block here?

Busy polling is only polling the NIC queue, hoping to feed this socket
for incoming packets.

Using more than a lockless read of sk->sk_receive_queue is not really necessary,
and racy anyway.

Eliezer Tamir added a check against sk_state for no good reason in
TCP, my plan is to remove it.

There are other states where it still makes sense to allow busy polling.


>
> Maybe here it needs a `!(sk->sk_shutdown & RCV_SHUTDOWN)` check,
> which is set when it's closed by the peer.

See above. Keep this as simple as possible...


>
> Thanks
>
> >         lock_sock(sk);
> >
> >         if (sctp_style(sk, TCP) && !sctp_sstate(sk, ESTABLISHED) &&
> > @@ -9046,12 +9050,6 @@ struct sk_buff *sctp_skb_recv_datagram(struct sock *sk, int flags, int *err)
> >                 if (sk->sk_shutdown & RCV_SHUTDOWN)
> >                         break;
> >
> > -               if (sk_can_busy_loop(sk)) {
> > -                       sk_busy_loop(sk, flags & MSG_DONTWAIT);
> > -
> > -                       if (!skb_queue_empty_lockless(&sk->sk_receive_queue))
> > -                               continue;
> > -               }
> >
> >                 /* User doesn't want to wait.  */
> >                 error = -EAGAIN;
> > --
> > 2.43.0.472.g3155946c3a-goog
> >