Comment 0 for bug 1792099

We are seeing deadlocks during hotplug of devices under vfio.

As per the Linux kernel source code, there is a deadlock situation between vfio_pci_remove() and vfio_pci_release() on PCIe hotplug events. This issue can be avoided either by skipping the PCIe reset functionality or do device_unlock() in vfio_pci_remove() beforfe calling the function vfio_del_group_dev()().

Code flow on PCIe hotplug event:

Execution flow 1:
  device_release_driver() ( ( https://elixir.bootlin.com/linux/latest/source/drivers/base/dd.c#L935 )
   device_release_driver_internal() ( https://elixir.bootlin.com/linux/latest/source/drivers/base/dd.c#L908 )
   device_lock(dev); ( https://elixir.bootlin.com/linux/latest/source/drivers/base/dd.c#L915 )
   vfio_pci_remove() ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/vfio/pci/vfio_pci.c#L392 )
     vfio_del_group_dev() https://elixir.bootlin.com/linux/v4.18.5/source/drivers/vfio/vfio.c#L923
       send event request to user and wait for VFIO_PCI_DEVICE release in vfio_pci_release() ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/vfio/vfio.c#L967 )

Execution flow 2 triggered by above step "send event request to user":
  vfio_pci_releas() ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/vfio/pci/vfio_pci.c#L392 )
    vfio_pci_disable() ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/vfio/pci/vfio_pci.c#L302 )
      vfio_pci_try_bus_reset() ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/vfio/pci/vfio_pci.c#L1346 )
        pci_try_reset_bus() ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/pci/pci.c#L4981 )
          pci_bus_save_and_disable() ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/pci/pci.c#L4760 )
            pci_dev_lock(dev); ( https://elixir.bootlin.com/linux/v4.18.5/source/drivers/pci/pci.c#L4765 )

             DEADLOCK here since PCI_DEIVCE_LOCK is held by PCI_DEVICE remove code path in DD.c