Comment 9 for bug 1631513

Reviewed: https://review.openstack.org/413240
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=95d93495d743115808b511740899d649cab6b5ae
Submitter: Jenkins
Branch: stable/newton

commit 95d93495d743115808b511740899d649cab6b5ae
Author: Swaminathan Vasudevan <email address hidden>
Date: Fri Oct 7 10:30:40 2016 -0700

    DVR: Fix race condition in creation of fip gateway

    In large-scale environments, we have seen a router update
    arrive for one tenant while we are still creating the
    router for a different tenant and initializing the shared
    floating IP gateway port. Sometimes these updates can
    get scheduled simultaneously, with the second running
    before we are done creating all the resources in the
    first, causing an exception when trying to set the
    default route since either the interface or IP address
    does not exist yet.

    Add a lock to better synchronize these functions so
    a create can finish before an update can be done.

    If it still fails, we will throw an exception so that
    the namespace will be cleaned-up and the update can be
    re-scheduled for the next iteration.

    Closes-Bug: #1631513
    Change-Id: Ia8c92cea2f8798582c39ad3450ab3b3c45a356f7
    (cherry picked from commit d40322c7d4aa1dd6d595dfe415278c9f252f4da2)