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[Oleg Bondarev <obondarev@xxxxxxxxxxxx>]

Public bug reported:

In ml2 plugin when dvr seviceable port is deleted, we check if any dvr routers should be deleted from port's host. 
This is done prior to actual port deletion from db by checking if there are any more dvr serviceable ports on this host.
This is prone to races: if two last compute ports on the host are deleted concurrently, the check might not return any routers as in both cases it will see yet another dvr serviceable port on the host:

- p1 and p2 are last compute ports on compute host 'host1'
- p1 and p2 are on the same subnet connected to a dvr router 'r1'
- p1 and p2 are deleted concurrently
- on p1 deletion plugin checks if there are any more dvr serviceable ports on host1 - sees p2 -> no dvr routers should be deleted
- same on p2 deletion plugin checks if there are any more dvr serviceable ports on host1 - sees p1 -> no dvr routers should be deleted
- p1 is deleted from DB
- p2 is deleted from DB
- r1 is not deleted from host1 though there are no more ports on it

** Affects: neutron
     Importance: Undecided
     Assignee: Oleg Bondarev (obondarev)
         Status: New


** Tags: l3-dvr-backlog

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https://bugs.launchpad.net/bugs/1538163

Title:
  DVR: race in dvr serviceable port deletion

Status in neutron:
  New

Bug description:
  In ml2 plugin when dvr seviceable port is deleted, we check if any dvr routers should be deleted from port's host. 
  This is done prior to actual port deletion from db by checking if there are any more dvr serviceable ports on this host.
  This is prone to races: if two last compute ports on the host are deleted concurrently, the check might not return any routers as in both cases it will see yet another dvr serviceable port on the host:

  - p1 and p2 are last compute ports on compute host 'host1'
  - p1 and p2 are on the same subnet connected to a dvr router 'r1'
  - p1 and p2 are deleted concurrently
  - on p1 deletion plugin checks if there are any more dvr serviceable ports on host1 - sees p2 -> no dvr routers should be deleted
  - same on p2 deletion plugin checks if there are any more dvr serviceable ports on host1 - sees p1 -> no dvr routers should be deleted
  - p1 is deleted from DB
  - p2 is deleted from DB
  - r1 is not deleted from host1 though there are no more ports on it

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