RE: [PATCH 3/3] PCI: hv: Add pci_destroy_slot() in pci_devices_present_work(), if necessary

[Dexuan Cui <decui@microsoft.com>]

> From: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
> Sent: Tuesday, March 26, 2019 12:55 PM
> On Mon, Mar 04, 2019 at 09:34:49PM +0000, Dexuan Cui wrote:
> > When we hot-remove a device, usually the host sends us a PCI_EJECT
> message,
> > and a PCI_BUS_RELATIONS message with bus_rel->device_count == 0. But
> when
> > we do the quick hot-add/hot-remove test, the host may not send us the
> > PCI_EJECT message, if the guest has not fully finished the initialization
> > by sending the PCI_RESOURCES_ASSIGNED* message to the host, so it's
> > potentially unsafe to only depend on the pci_destroy_slot() in
> > hv_eject_device_work(), though create_root_hv_pci_bus() ->
> > hv_pci_assign_slots() is not called in this case. Note: in this case, the
> > host still sends the guest a PCI_BUS_RELATIONS message with
> > bus_rel->device_count == 0.
> >
> > And, in the quick hot-add/hot-remove test, we can have such a race: before
> > pci_devices_present_work() -> new_pcichild_device() adds the new device
> > into hbus->children, we may have already received the PCI_EJECT message,
> > and hence the taklet handler hv_pci_onchannelcallback() may fail to find
> > the "hpdev" by get_pcichild_wslot(hbus, dev_message->wslot.slot), so
> > hv_pci_eject_device() is NOT called; later create_root_hv_pci_bus() ->
> > hv_pci_assign_slots() creates the slot, and the PCI_BUS_RELATIONS message
> > with bus_rel->device_count == 0 removes the device from hbus->children,
> and
> > we end up being unable to remove the slot in hv_pci_remove() ->
> > hv_pci_remove_slots().
> >
> > The patch removes the slot in pci_devices_present_work() when the device
> > is removed. This can address the above race. Note 1:
> > pci_devices_present_work() and hv_eject_device_work() run in the
> > singled-threaded hbus->wq, so there is not a double-remove issue for the
> > slot. Note 2: we can't offload hv_pci_eject_device() from
> > hv_pci_onchannelcallback() to the workqueue, because we need
> > hv_pci_onchannelcallback() synchronously call hv_pci_eject_device() to
> > poll the channel's ringbuffer to work around the
> > "hangs in hv_compose_msi_msg()" issue: see
> > commit de0aa7b2f97d ("PCI: hv: Fix 2 hang issues in
> hv_compose_msi_msg()")
> 
> This commit log is unreadable, sorry. Indentation, punctuation and
> formatting are just a mess, try to read it, you will notice by
> yourself.
> 
> I basically reformatted it completely and pushed the series to
> pci/controller-fixes but that's the last time I do it since I am not an
> editor, next time I won't merge it.

Hi Lorenzo,
Thank you for helping improve my changelogs! I did learn a lot after
carefully comparing the improved version with my original version. :-)

I'll try my best to write a good changelog for my future patches.

> More importantly, these patches are marked for stable, given the series
> of fixes that triggered this series please ensure it was tested
> thoroughly because it is honestly complicate to understand and I do not
> want to backport further fixes to stable kernels on top of this.

I did the hot-add/hot-remove test in a loop for several thousand times,
and the patchset worked as expected and didn't show any issue.

> Please have a look and report back.
> 
> Thanks,
> Lorenzo

Thanks again!

Thanks,
-- Dexuan