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

[Sunil Muthuswamy]

> -----Original Message-----
> From: Dexuan Cui <decui@xxxxxxxxxxxxx>
> Sent: Thursday, May 16, 2019 10:17 AM
> To: Sunil Muthuswamy <sunilmut@xxxxxxxxxxxxx>; KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>;
> Stephen Hemminger <sthemmin@xxxxxxxxxxxxx>; Sasha Levin <sashal@xxxxxxxxxx>; David S. Miller <davem@xxxxxxxxxxxxx>;
> Michael Kelley <mikelley@xxxxxxxxxxxxx>
> Cc: netdev@xxxxxxxxxxxxxxx; linux-hyperv@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx
> Subject: RE: [PATCH v2] hv_sock: Add support for delayed close
> 
> > From: linux-hyperv-owner@xxxxxxxxxxxxxxx
> > <linux-hyperv-owner@xxxxxxxxxxxxxxx> On Behalf Of Dexuan Cui
> > Sent: Wednesday, May 15, 2019 9:34 PM
> > ...
> 
> 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.
> > 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
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.

> Thanks,
> -- Dexuan