Threadsafe configuration can cause deadlocks

[375gnu]

Hi.

I working on debian package for berkshelf-api, in debian we have rspec-* 3.3.0 and I have problem to run testsuite because it hangs (on this line: https://github.com/berkshelf/berkshelf-api/blob/master/spec/unit/berkshelf/api/cache_builder/worker/github_spec.rb#L51, https://github.com/berkshelf/berkshelf-api/blob/master/spec/support/human_reaable.rb#L3), While it works perfectly with 2.99.0.

I started to bisect and found that hangs are caused by rspec-core commit ffe00a1d4.