Date: Wed, 16 Sep 2015 10:19:36 -0400 (EDT)
From: Benjamin Coddington <bcodding@redhat.com>
To: Trond Myklebust <trond.myklebust@primarydata.com>
cc: Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH] SUNRPC: Fix transport reset race while TCP is
 connecting
In-Reply-To: <CAHQdGtRQ9RZNrJeYBykQLx1FsWenebQa35MBEHUiCY9EVedwXw@mail.gmail.com>
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References: <d858d15746eeb224f4fb27554771e14b58f31306.1442411712.git.bcodding@redhat.com> <CAHQdGtRQ9RZNrJeYBykQLx1FsWenebQa35MBEHUiCY9EVedwXw@mail.gmail.com>
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On Wed, 16 Sep 2015, Trond Myklebust wrote:

> On Wed, Sep 16, 2015 at 9:58 AM, Benjamin Coddington
> <bcodding@redhat.com> wrote:
> > After a network segmentation that times out the socket, we can get stuck in
> > a loop where the client repeatedly sends a SYN, then a RST, with the result
> > that the client will never successfully reconnect to the server.
> >
> > When there's a need to re-establish a TCP connection, the scheduled
> > connect_worker does xs_tcp_setup_socket(), which sends a SYN, then
> > immediately wakes the waiting task to call_connect_status with a status of
> > EAGAIN, which proceeds to xprt_connect() and kernel_sock_shutdown() which
> > will send a RST.
> >
> > Fix this loop by deferring xprt_clear_connecting() until the TCP state
> > change of ESTALISHED or CLOSED, thereby ensuring that we do not race into
> > xs_connect() while waiting for SYN,ACK from the server.
> >
> > Fixes: 099392048c (SUNRPC: Prevent SYN+SYNACK+RST storms)
> > Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
> > ---
> >  net/sunrpc/xprtsock.c |    6 ++++--
> >  1 files changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c
> > index 7be90bc..6df51df 100644
> > --- a/net/sunrpc/xprtsock.c
> > +++ b/net/sunrpc/xprtsock.c
> > @@ -1450,6 +1450,7 @@ static void xs_tcp_state_change(struct sock *sk)
> >         switch (sk->sk_state) {
> >         case TCP_ESTABLISHED:
> >                 spin_lock(&xprt->transport_lock);
> > +               xprt_clear_connecting(xprt);
> >                 if (!xprt_test_and_set_connected(xprt)) {
> >                         struct sock_xprt *transport = container_of(xprt,
> >                                         struct sock_xprt, xprt);
> > @@ -1496,6 +1497,7 @@ static void xs_tcp_state_change(struct sock *sk)
> >                 smp_mb__after_atomic();
> >                 break;
> >         case TCP_CLOSE:
> > +               xprt_clear_connecting(xprt);
> >                 xs_sock_mark_closed(xprt);
> >         }
> >   out:
> > @@ -2237,10 +2239,10 @@ static void xs_tcp_setup_socket(struct work_struct *work)
> >                 xs_tcp_force_close(xprt);
> >                 break;
> >         case 0:
> > -       case -EINPROGRESS:
> > +               xprt_clear_connecting(xprt);
> >         case -EALREADY:
> > +       case -EINPROGRESS:
> >                 xprt_unlock_connect(xprt, transport);
> > -               xprt_clear_connecting(xprt);
> >                 return;
> >         case -EINVAL:
> >                 /* Happens, for instance, if the user specified a link
> >
>
> This introduces races. The call to xprt_clear_connecting in TCP_CLOSE
> can trigger pretty much at any time, and so could interfere with a
> subsequent connection attempt.

Oh, yeah.  I see that now.

> I have an alternative patch which addresses this problem and which is
> undergoing testing.

Great!