RE: [PATCH v2] hv_sock: Add support for delayed close

[Dexuan Cui]

> From: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>
> Sent: Thursday, May 16, 2019 11:11 AM
> > Hi Sunil,
> > To make it clear, your patch itself is good, and I was just talking about
> > the next change we're going to make. Once we make the next change,
> > IMO we need a further patch to schedule hvs_close_timeout() to the new
> > single-threaded workqueue rather than the global "system_wq".
> >
> Thanks for your review. Can you add a 'signed-off' from your side to the patch.

I have provided my Reviewed-by. I guess this should be enough. Of course,
David makes the final call. It would be great if the maintaners of the Hyper-V
drivers listed in the "To:" could provide their Signed-off-by.

> > > Next, we're going to remove the "channel->rescind" check in
> > > vmbus_hvsock_device_unregister() -- when doing that, IMO we need to
> > > fix a potential race revealed by the schedule_delayed_work() in this
> > > patch:
> > >
> > > When hvs_close_timeout() finishes, the "sk" struct has been freed, but
> > > vmbus_onoffer_rescind() -> channel->chn_rescind_callback(), i.e.
> > > hvs_close_connection(), may be still running and referencing the "chan"
> > > and "sk" structs (), which should no longer be referenced when
> > > hvs_close_timeout() finishes, i.e. "get_per_channel_state(chan)" is no
> > > longer safe. The problem is: currently there is no sync mechanism
> > > between vmbus_onoffer_rescind() and hvs_close_timeout().
> > >
> > > The race is a real issue only after we remove the "channel->rescind"
> > > in vmbus_hvsock_device_unregister().
> >
> > A correction: IMO the race is real even for the current code, i.e. without
> > your patch: in vmbus_onoffer_rescind(), between we set channel->rescind
> > and we call channel->chn_rescind_callback(), the channel may have been
> > freed by vmbus_hvsock_device_unregister().
> >
> > This race window is small and I guess that's why we never noticed it.
> >
> > > I guess we need to introduce a new single-threaded workqueue in the
> > > vmbus driver, and offload both vmbus_onoffer_rescind() and
> > > hvs_close_timeout() onto the new workqueue.
> >
> Something is a miss if the guest has to wait for the host to close the channel
> prior to cleaning it up from it's side. That's waste of resources, doesn't matter

I agree. 

> if you do it in a system thread, dedicated pool or anyway else. I think the right
> way to deal with this is to unregister the rescind callback routine, wait for any
> running rescind callback routine to finish and then drop the last reference to
> the socket, which should lead to all the cleanup. I understand that some of the
> facility of unregistering the rescind callback might not exist today.

Considering the concurrency, I'm not sure if it's easy or possible to safely
unregister the chn_rescind_callback. My hunch is: doing that may be more
difficult than adding a new single-thread workqueue.

Thanks,
-- Dexuan