How are you attempting to extend the volume(command line cinder client, Horizon GUI, or custom). Can you provide the exact steps to reproduce? What was the original size of the volume and the requested extended size of the volume? I have attempted this myself using the cinder client with stable/Havana devstack on ubuntu 12.04.4LTS and could not reproduce. My flexvol was 50GB in size, but allowed to automatically grow, and my cDOT is running 8.2P3.
The failure will appear after a pseudo-random number of extends against the same volume. The exact number of times it takes seems somewhat dependent on the original size of the volume and the size of the extensions. The following sequence appears to always cause the failure:
I have identified the bug and I have submitted a patch for review in the community. Initially this is going into master (icehouse) but I think we should be able to back port easily enough to Havana. In the mean time, you can view the patch and make the code changes manually if you would like to test it out: https://review.openstack.org/74524