Re: [PATCH 1/2] SUNRPC: accept() may return sockets that are still in SYN_RECV

On Wed, 2016-07-27 at 14:48 -0400, Fields Bruce James wrote:

On Tue, Jul 26, 2016 at 04:08:29PM +0000, Trond Myklebust wrote:

quoted

quoted

On Jul 26, 2016, at 11:43, J. Bruce Fields [off-list ref] wrote:

On Tue, Jul 26, 2016 at 09:51:19AM -0400, Trond Myklebust wrote:

quoted

We're seeing traces of the following form:

[10952.396347] svc: transport ffff88042ba4a 000 dequeued, inuse=2
[10952.396351] svc: tcp_accept ffff88042ba4 a000 sock ffff88042a6e4c80
[10952.396362] nfsd: connect from 10.2.6.1, port=187
[10952.396364] svc: svc_setup_socket ffff8800b99bcf00
[10952.396368] setting up TCP socket for reading
[10952.396370] svc: svc_setup_socket created ffff8803eb10a000 (inet ffff88042b75b800)
[10952.396373] svc: transport ffff8803eb10a000 put into queue
[10952.396375] svc: transport ffff88042ba4a000 put into queue
[10952.396377] svc: server ffff8800bb0ec000 waiting for data (to = 3600000)
[10952.396380] svc: transport ffff8803eb10a000 dequeued, inuse=2
[10952.396381] svc_recv: found XPT_CLOSE
[10952.396397] svc: svc_delete_xprt(ffff8803eb10a000)
[10952.396398] svc: svc_tcp_sock_detach(ffff8803eb10a000)
[10952.396399] svc: svc_sock_detach(ffff8803eb10a000)
[10952.396412] svc: svc_sock_free(ffff8803eb10a000)

i.e. an immediate close of the socket after initialisation.

Interesting, thanks!

So the one thing I don't understand is why this is correct behavior for
accept--I thought it wasn't supposed to return a socket until it was
fully established.

inet_accept() appears to allow SYN_RECV:

OK.  Cc'ing netdev just to make sure we didn't overlook anything.

SYN_RECV after accept() is a TCP Fast Open property I think.

Maybe you are playing with some global TCP Fast Open settings ?

(Also: what were user-visible symptoms?  Mounts failing, or unexpected
delays?)

--b.

quoted

int inet_accept(struct socket *sock, struct socket *newsock, int flags)
{
        struct sock *sk1 = sock->sk;
        int err = -EINVAL;
        struct sock *sk2 = sk1->sk_prot->accept(sk1, flags, &err);

        if (!sk2)
                goto do_err;

        lock_sock(sk2);

        sock_rps_record_flow(sk2);
        WARN_ON(!((1 << sk2->sk_state) &
                  (TCPF_ESTABLISHED | TCPF_SYN_RECV |
                  TCPF_CLOSE_WAIT | TCPF_CLOSE)));

        sock_graft(sk2, newsock);

        newsock->state = SS_CONNECTED;
        err = 0;
        release_sock(sk2);
do_err:
        return err;
}

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