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

[Dexuan Cui]

> 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".

> 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.
 
Thanks,
-- Dexuan