Based on the information at hand, I don't think this is a bug. The upstream Neutron team tests the linuxbridge agent with every patch that is submitted for the stable/xena branch: https://review.opendev.org/q/project:openstack%252Fneutron+branch:stable%252Fxena+status:open . One example is https://review.opendev.org/c/openstack/neutron/+/833857. Note that we have a job named neutron-tempest-plugin-scenario-linuxbridge-xena. Here's the successful execution of that job for the aforementioned patch https://zuul.opendev.org/t/openstack/build/1ee2736adf46480ea9d48f125b5cd229 and the corresponding linuxbridge agent log https://storage.bhs.cloud.ovh.net/v1/AUTH_dcaab5e32b234d56b626f72581e3644c/zuul_opendev_logs_1ee/833857/1/check/neutron-tempest-plugin-scenario-linuxbridge-xena/1ee2736/controller/logs/screen-q-agt.txt . As you can see, that agent started running today with no problems: "Logs begin at Wed 2022-03-16 10:05:56 UTC".

As far of thoughts on what may be causing your problem, I lean towards an installation / dependencies issue:

1) Is this a fresh new install or did it run successfully before and has started failing recently? If the latter, what changed?

2) Your logging seem a little odd. Note that the third line in the upstream log is: Mar 16 10:19:43.410735 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 neutron-linuxbridge-agent[71074]: INFO neutron.common.config [-] Logging enabled!. In your case, logging doesn't seem to be enabled at all. The second thing the agent does is setup logging: https://github.com/openstack/neutron/blob/2f4661c87681567bb08d7733c723c2b0c31ed6c8/neutron/plugins/ml2/drivers/linuxbridge/agent/linuxbridge_neutron_agent.py#L1017

3) Next, the agent sets up privsep (to be able to execute commands with privileges): https://github.com/openstack/neutron/blob/2f4661c87681567bb08d7733c723c2b0c31ed6c8/neutron/plugins/ml2/drivers/linuxbridge/agent/linuxbridge_neutron_agent.py#L1018. It seems this is where your agent fails, because we see in your log twice:

Mar 16 09:14:09 w3.int.lunarc sudo[37085]:  neutron : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/bin/neutron-rootwrap /etc/neutron/rootwrap.conf privsep-helper --config-file /usr/share/neutron/neutron-dist.conf --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/ml2/linuxbridge_agent.ini --config-dir /etc/neutron/conf.d/neutron-linuxbridge-agent --privsep_context neutron.privileged.default --privsep_sock_path /tmp/tmp93tzwqg3/privsep.sock

and then the traceback. Comparing with the upstream agent, we see that the privsep daemon starts running successfully: 

Mar 16 10:19:43.421615 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 sudo[71765]:    stack : TTY=unknown ; PWD=/ ; USER=root ; COMMAND=/usr/local/bin/neutron-rootwrap /etc/neutron/rootwrap.conf privsep-helper --config-file /etc/neutron/neutron.conf --config-file /etc/neutron/plugins/ml2/ml2_conf.ini --privsep_context neutron.privileged.default --privsep_sock_path /tmp/tmppedn6sui/privsep.sock
Mar 16 10:19:43.421948 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 sudo[71765]: pam_unix(sudo:session): session opened for user root by (uid=0)
Mar 16 10:19:43.898488 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 sudo[71765]: pam_unix(sudo:session): session closed for user root
Mar 16 10:19:43.908480 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 neutron-linuxbridge-agent[71074]: INFO oslo.privsep.daemon [-] Spawned new privsep daemon via rootwrap
Mar 16 10:19:43.909023 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 neutron-linuxbridge-agent[71074]: DEBUG oslo.privsep.daemon [-] Accepted privsep connection to /tmp/tmppedn6sui/privsep.sock {{(pid=71074) __init__ /usr/local/lib/python3.8/dist-packages/oslo_privsep/daemon.py:363}}
Mar 16 10:19:43.909722 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 neutron-linuxbridge-agent[71074]: INFO oslo.privsep.daemon [-] privsep daemon starting
Mar 16 10:19:43.909962 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 neutron-linuxbridge-agent[71074]: INFO oslo.privsep.daemon [-] privsep process running with uid/gid: 0/0
Mar 16 10:19:43.910155 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 neutron-linuxbridge-agent[71074]: INFO oslo.privsep.daemon [-] privsep process running with capabilities (eff/prm/inh): CAP_DAC_OVERRIDE|CAP_DAC_READ_SEARCH|CAP_NET_ADMIN|CAP_SYS_ADMIN|CAP_SYS_PTRACE/CAP_DAC_OVERRIDE|CAP_DAC_READ_SEARCH|CAP_NET_ADMIN|CAP_SYS_ADMIN|CAP_SYS_PTRACE/none
Mar 16 10:19:43.910337 nested-virt-ubuntu-focal-vexxhost-ca-ymq-1-0028853094 neutron-linuxbridge-agent[71074]: INFO oslo.privsep.daemon [-] privsep daemon running as pid 71807

I would compare the config files passed to the rootwrap commands above. What do you have for the root_helper option: https://storage.bhs.cloud.ovh.net/v1/AUTH_dcaab5e32b234d56b626f72581e3644c/zuul_opendev_logs_1ee/833857/1/check/neutron-tempest-plugin-scenario-linuxbridge-xena/1ee2736/controller/logs/etc/neutron/plugins/ml2/ml2_conf.ini

Here's a little explanation from the greenlet docs on the specific error you get at the bottom of the traceback: https://greenlet.readthedocs.io/en/stable/python_threads.html

For now, I'm going to mark this as a non bug, unless you provide more information to indicate otherwise