3.0.x HEAD hanging

[Phil Mayers]

We've got a version of 3.0.x HEAD running on our test server. It's 
sporadically hanging - stopping answering requests.

I've caught a process doing it - backtrace here:

https://gist.github.com/philmayers/5582ca9fb89f373919b5

Looks like it's actually faulted, but the fault handler stuff is stuck 
on a mutex - inside talloc?

No idea what's triggering the fault.