As a follow up to https://bugs.python.org/issue14976 which just introduced queue.SimpleQueue:

concurrent.futures.thread currently uses a queue.Queue() from weakref callbacks which could in theory lead to a deadlock when periodic gc triggers a cleanup invalidating some weakrefed objects at an inopportune time while Python code has the queue's lock held.

I don't have a test case proving this (deadlocks are hard).

Switching it to use a SimpleQueue should avoid the problem?

...

This and the C extension module based SimpleQueue would be good to backport to https://pypi.python.org/pypi/futures as well.