Re: [PATCH 1/1] SUNRPC: fix handling of half-closed connection

Trond Myklebust
2019-02-22T15:06:07Z

On Fri, 2019-02-22 at 09:46 -0500, Olga Kornievskaia wrote:
> On Fri, Feb 22, 2019 at 8:45 AM Trond Myklebust <
> trondmy@hammerspace.com> wrote:
> > On Fri, 2019-02-22 at 07:12 -0500, Dave Wysochanski wrote:
> > > Hi Olga,
> > >
> > > Do you have a reproducer for this? A number of months ago I did
> > > a
> > > significant amount of testing with half-closed connections, after
> > > we
> > > had reports of connections stuck in FIN_WAIT2 in some older
> > > kernels.
> > > What I found was with kernels that had the tcp keepalives (commit
> > > 7f260e8575bf53b93b77978c1e39f8e67612759c), I could only reproduce
> > > a
> > > hang of a few minutes, after which time the tcp keepalive code
> > > would
> > > reset the connection.
> > >
> > > That said it was a while ago and something subtle may have
> > > changed.
> > > Also I'm not not sure if your header implies an indefinite hang
> > > or
> > > just
> > > a few minutes.
> > >
> > > Thanks.
> > >
> > >
> > > On Wed, 2019-02-20 at 09:56 -0500, Olga Kornievskaia wrote:
> > > > From: Olga Kornievskaia <kolga@netapp.com>
> > > >
> > > > When server replies with an ACK to client's FIN/ACK, client
> > > > ends
> > > > up stuck in a TCP_FIN_WAIT2 state and client's mount hangs.
> > > > Instead, make sure to close and reset client's socket and
> > > > transport
> > > > when transitioned into that state.
>
> Hi Trond,
>
> > So, please do note that we do not want to ignore the FIN_WAIT2
> > state
>
> But we do ignore the FIN_WAIT2 state.

We do not. We wait for the server to send a FIN, which is precisely the
reason for which FIN_WAIT2 exists.

>
> > because it implies that the server has not closed the socket on its
> > side.
>
> That's correct.
>
> > That again means that we cannot re-establish a connection using
> > the same source IP+port to the server, which is problematic for
> > protocols such as NFSv3 which rely on standard duplicate reply
> > cache
> > for correct replay semantics.
>
> that's exactly what's happening that a client is unable to establish
> a
> new connection to the server. With the patch, the client does an RST
> and it re-uses the port and all is well for NFSv3.

RST is not guaranteed to be delivered to the recipient. That's why the
TCP protocol defines FIN: it is a guaranteed to be delivered because it
is ACKed.

> > This is why we don't just set the TCP_LINGER2 socket option and
> > call
> > sock_release(). The choice to try to wait it out is deliberate
> > because
> > the alternative is that we end up with busy-waiting re-connection
> > attempts.
>
> Why would it busy-wait? In my testing, RST happens and new connection
> is established?

Only if the server has dropped the connection without notifying the
client.

--
Trond Myklebust
Linux NFS client maintainer, Hammerspace
trond.myklebust@hammerspace.com