RE: [PATCH] PCI: hv: Fix a race condition when removing the device

[Dexuan Cui <decui@microsoft.com>]

> From: Michael Kelley <mikelley@microsoft.com>
> Sent: Wednesday, April 21, 2021 2:06 PM
>  ...
> > Yes I think put_hvpcibus() and get_hvpcibus() can be removed, as we have
> > changed to use
> > a dedicated workqueue for hbus since they were introduced.
> >
> > But we still need to call tasklet_disable/enable() the same way
> > hv_pci_suspend() does, the
> > reason is that we need to protect hbus->state. This value needs to be
> consistent for the
> > driver. For example, a CPU may decide to schedule a work on a work queue
> that we just
> > flushed or destroyed, by reading the wrong hbus->state.
> >
> 
> Yes, I would agree the tasklet disable/enable are needed, especially since
> tasklet_disable()
> is what ensures that the tasklet is not currently running.
> 
> If the hbus ref counting isn't needed any longer, I would strongly recommend
> adding
> a patch to the series that removes it.  This synchronization stuff is hard
> enough to
> understand and reason about; having a leftover mechanism that doesn't really
> do
> anything useful makes it nearly impossible. :-)
> 
> Dexuan -- I'm hoping you can take a look as well and see if you agree.
> 
> Michael

I also think we can remove the reference counting.

But it looks like there is still race in hv_pci_remove() even with Long's
patch: in hv_pci_remove(), we disable the tasklet, change hbus->state to
hv_pcibus_removing, re-enable the tasklet and flush hbus->wq, and set
hbus->state to hv_pcibus_removed -- what if the channel callback runs
again? -- now hbus->state is no longer hv_pcibus_removing, so
hv_pci_devices_present() -> hv_pci_start_relations_work() and
hv_pci_eject_device() can still add new work items to hbus->wq, and the new
work items may race with the vmbus_close().

It looks like we should remove the state hv_pcibus_removed?

Thanks,
-- Dexuan