vfio/pci: Use pci_try_reset_function() on initial open

Device lock bites again; if a device .remove() callback races a user
calling ioctl(VFIO_GROUP_GET_DEVICE_FD), the unbind request will hold
the device lock, but the user ioctl may have already taken a vfio_device
reference.  In the case of a PCI device, the initial open will attempt
to reset the device, which again attempts to get the device lock,
resulting in deadlock.  Use the trylock PCI reset interface and return
error on the open path if reset fails due to lock contention.

Link: https://lkml.org/lkml/2017/7/25/381Reported-by: Wen Congyang <wencongyang2@huawei.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/vfio_pci.c

@@ -226,7 +226,14 @@ static int vfio_pci_enable(struct vfio_pci_device *vdev)
 	if (ret)
 		return ret;
 
-	vdev->reset_works = (pci_reset_function(pdev) == 0);
+	/* If reset fails because of the device lock, fail this path entirely */
+	ret = pci_try_reset_function(pdev);
+	if (ret == -EAGAIN) {
+		pci_disable_device(pdev);
+		return ret;
+	}
+
+	vdev->reset_works = !ret;
 	pci_save_state(pdev);
 	vdev->pci_saved_state = pci_store_saved_state(pdev);
 	if (!vdev->pci_saved_state)