Apache and mod_wsgi, worker process stuck to W state, request never dies. How to diagnose?

I am running Apache 2.2 with mod_wsgi, Python 2.7 and mpm_worker. Occassionally, one of the worker processes gets stuck and all of it threads stop in writing state (as demostrated in the screenshot below).

This happens ~once in a day for one worker process.

I assume this is either because

• Some internal problem in Apache

• All of my Python threads inside the mod_wsgi worker process deadlock somehow

Thus far, the only remedy I have found is Apache full restart (not graceful).

I'd hope to find some pointers how to diagnose the issue what is causing it

• Why Apace Timeout does not kill the worker threads / processes. Time timeout is one minute, but looks like those threads and workers have been running happily several hours on one request.

• Is it possible to obtain thread dump from inside mod_wsgiand see if the Python thread themselves are somehow deadlocked

• Any idea what could be causing this and how to remedy the situation?

Below is an screenshot showing Apache server-status where one of the worker processes (1-0) is stuck.