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

[Michael Kelley]

From: longli@xxxxxxxxxxxxxxxxx <longli@xxxxxxxxxxxxxxxxx>  Sent: Monday, April 19, 2021 12:21 PM
> 
> On removing the device, any work item (hv_pci_devices_present() or
> hv_pci_eject_device()) scheduled on workqueue hbus->wq may still be running
> and race with hv_pci_remove().
> 
> This can happen because the host may send PCI_EJECT or PCI_BUS_RELATIONS(2)
> and decide to rescind the channel immediately after that.
> 
> Fix this by flushing/stopping the workqueue of hbus before doing hbus remove.

I can see that this change follows the same pattern as in hv_pci_suspend().   The
comments there give a full explanation of the issue and the solution.  But
interestingly, the current code also has a reference count mechanism on
the hbus.  And code near the end of hv_pci_remove() decrements the reference
count and then waits for all users to finish before destroying the workqueue.
With this change, is this reference counting mechanism still needed?   If the
workqueue has already been emptied, it seems like the wait_for_completion()
near the end of hv_pci_remove() would never be waiting for anything.  It makes
me wonder if moving the reference count checking code from near the end of
hv_pci_remove() up to near the beginning would solve the problem as well
(and maybe in hv_pci_suspend also?).

Michael 

> 
> Signed-off-by: Long Li <longli@xxxxxxxxxxxxx>
> ---
>  drivers/pci/controller/pci-hyperv.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/drivers/pci/controller/pci-hyperv.c b/drivers/pci/controller/pci-hyperv.c
> index 27a17a1e4a7c..116815404313 100644
> --- a/drivers/pci/controller/pci-hyperv.c
> +++ b/drivers/pci/controller/pci-hyperv.c
> @@ -3305,6 +3305,17 @@ static int hv_pci_remove(struct hv_device *hdev)
> 
>  	hbus = hv_get_drvdata(hdev);
>  	if (hbus->state == hv_pcibus_installed) {
> +		tasklet_disable(&hdev->channel->callback_event);
> +		hbus->state = hv_pcibus_removing;
> +		tasklet_enable(&hdev->channel->callback_event);
> +
> +		flush_workqueue(hbus->wq);
> +		/*
> +		 * At this point, no work is running or can be scheduled
> +		 * on hbus-wq. We can't race with hv_pci_devices_present()
> +		 * or hv_pci_eject_device(), it's safe to proceed.
> +		 */
> +
>  		/* Remove the bus from PCI's point of view. */
>  		pci_lock_rescan_remove();
>  		pci_stop_root_bus(hbus->pci_bus);
> --
> 2.27.0