Comment 1 for bug 1410854

https://github.com/openstack/neutron/blob/3f44c9e27874511f05e2338c10e836361776ed88/neutron/plugins/ml2/drivers/helpers.py#L113 issued on the same database snapshot therefore it will select the same value (Repeatable Read) .

It is possible to create new session for each select outside to the current session/transaction.

(IMHO the VNI allocation should not be given up unless the select does not founds any VNI, I would suggest to do more than 10 try, otherwise allocation issues could be triggered with higher load and worker number.)