Received-SPF: pass (google.com: domain of linux-nfs-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Fri, 26 Jun 2020 18:10:52 +0300
From:   Dan Aloni <dan@kernelim.com>
To:     Chuck Lever <chuck.lever@oracle.com>
Cc:     linux-rdma@vger.kernel.org,
        Linux NFS Mailing List <linux-nfs@vger.kernel.org>
Subject: Re: [PATCH] xprtrdma: fix EP destruction logic
Message-ID: <20200626151052.6cckaquyu7k3nd6b@gmail.com>
References: <0E2AA9D9-2503-462C-952D-FC0DD5111BD1@oracle.com>
 <20200626071034.34805-1-dan@kernelim.com>
 <FEB41A86-87EB-44BD-BEC4-6EAB3723B426@oracle.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <FEB41A86-87EB-44BD-BEC4-6EAB3723B426@oracle.com>
Sender: linux-nfs-owner@vger.kernel.org
Precedence: bulk

On Fri, Jun 26, 2020 at 08:56:41AM -0400, Chuck Lever wrote:
> > On Jun 26, 2020, at 3:10 AM, Dan Aloni <dan@kernelim.com> wrote:
[..]
> > - Add a mutex in `rpcrdma_ep_destroy` to guard against concurrent calls
> >  to `rpcrdma_xprt_disconnect` coming from either `rpcrdma_xprt_connect`
> >  or `xprt_rdma_close`.
> 
> NAK. The RPC client provides appropriate exclusion, please let's not
> add more serialization that can introduce further deadlocks.

It appeared to me that this exclusion does not works well. As for my
considerations, if I am not mistaken from analyzing crashes I've
seen:

   -> xprt_autoclose (running on xprtiod)
     -> xprt->ops->close
       -> xprt_rdma_close
         -> rpcrdma_xprt_disconnect

and:

    -> xprt_rdma_connect_worker (running on xprtiod)
      -> rpcrdma_xprt_connect
	-> rpcrdma_xprt_disconnect

I understand the rationale or at least the aim that `close` and
`connect` ops should not be concurrent on the same `xprt`, however:

* `xprt_force_disconnect`, is called from various places, queues
  a call to `xprt_autoclose` to the background on `xprtiod` workqueue item,
  conditioned that `!XPRT_LOCKED` which is the case for connect that went
  to the background.
* `xprt_rdma_connect` also sends `xprt_rdma_connect_worker` as an `xprtiod`
  workqueue item, unconditionally.

So we have two work items that can run in parallel, and I don't see
clear gating on this from the code.

Maybe there's a simpler fix for this. Perhaps a
`cancel_delayed_work_sync(&r_xprt->rx_connect_worker);` is appropriate
in `xprt_rdma_close`?

-- 
Dan Aloni