[Bug 1972028] Re: [SRU] _get_pci_passthrough_devices prone to race condition

[Andreas Hasenack]

What's the status in noble, oracular, and plucky? The main devel task is
still "New".

About the test plan:

"* live migrate the vms between the hosts and check for the Traceback in
/var/log/nova/nova-compute.log"

Is the log the only noticeable defect? If yes, what's the real impact to
users, just log noise? I would expect the migration of the VM to either
fail, or to have the target VM not come up, or have lost some feature
during the migration.

** Changed in: nova (Ubuntu Jammy)
       Status: New => Incomplete

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Title:
  [SRU] _get_pci_passthrough_devices prone to race condition

Status in Ubuntu Cloud Archive:
  New
Status in Ubuntu Cloud Archive yoga series:
  New
Status in Ubuntu Cloud Archive zed series:
  Fix Released
Status in OpenStack Compute (nova):
  Fix Released
Status in nova package in Ubuntu:
  New
Status in nova source package in Jammy:
  Incomplete

Bug description:
  [Impact]

  Nova suffers from a race condition when it does live migrations of vms
  with SRIOV ports whereby a pre-check of available ports and their
  capabilities can error if one or more ports becomes unavailable during
  the check. The fix backported here simply ignores libvirt errors when
  checking device capabilities resulting in those that throw an error
  being ignored.

  [Test Plan]

  Since the bug is a race condition it can be hard to reproduce but a
  succession of live migrations between SRIOV capable nodes with a
  reasonably large quantity of VFs should be a reasonable test.

  * deploy OpenStack Yoga with SRIOV capable hardward
  * create 10 vms with e.g. 5 sriov ports
  * live migrate the vms between the hosts and check for the Traceback in /var/log/nova/nova-compute.log

  [Regression Potential]
  This patch is not anticipated to introduce any regressions.
  -------------------------------------------------

  At the moment, the `_get_pci_passthrough_devices` function is prone to
  race conditions.

  This specific code here calls `listCaps()`, however, it is possible
  that the device has disappeared by the time on method has been called:

  https://github.com/openstack/nova/blob/master/nova/virt/libvirt/driver.py#L7949-L7959

  Which would result in the following traceback:

  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager [req-51b7c1c4-2b4a-46cc-9baa-8bf61801c48d - - - - -] Error updating resources for node <snip>.: libvirt.libvirtError: Node device not found: no node device with matching name 'net_tap8b08ec90_e5_fe_16_3e_0f_0a_d4'
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager Traceback (most recent call last):
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager   File "/var/lib/openstack/lib/python3.8/site-packages/nova/compute/manager.py", line 9946, in _update_available_resource_for_node
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager     self.rt.update_available_resource(context, nodename,
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager   File "/var/lib/openstack/lib/python3.8/site-packages/nova/compute/resource_tracker.py", line 879, in update_available_resource
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager     resources = self.driver.get_available_resource(nodename)
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager   File "/var/lib/openstack/lib/python3.8/site-packages/nova/virt/libvirt/driver.py", line 8937, in get_available_resource
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager     data['pci_passthrough_devices'] = self._get_pci_passthrough_devices()
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager   File "/var/lib/openstack/lib/python3.8/site-packages/nova/virt/libvirt/driver.py", line 7663, in _get_pci_passthrough_devices
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager     vdpa_devs = [
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager   File "/var/lib/openstack/lib/python3.8/site-packages/nova/virt/libvirt/driver.py", line 7664, in <listcomp>
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager     dev for dev in devices.values() if "vdpa" in dev.listCaps()
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager   File "/var/lib/openstack/lib/python3.8/site-packages/libvirt.py", line 6276, in listCaps
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager     raise libvirtError('virNodeDeviceListCaps() failed')
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager libvirt.libvirtError: Node device not found: no node device with matching name 'net_tap8b08ec90_e5_fe_16_3e_0f_0a_d4'
  2022-05-06 20:16:16.110 4053032 ERROR nova.compute.manager

  I think the cleaner way is to loop over all the items and skip a
  device if it raises an error that the device is not found.

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