Re: [PATCH] xprtrdma: fix EP destruction logic

[Dan Aloni <dan@kernelim.com>]

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