3.0.x HEAD hanging

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.