jboner/akka — comment on commit e60d3f386d0239c12c7dccdc794b484402c2b1cc
File: akka-actor/src/main/scala/akka/actor/ActorSystem.scala line 405
By rkuhn:

This runs the risk of a classical AB/BA deadlock. If we really do care about concurrent extension registrations (which I find a bit esoteric, but that’s probably just me), then we need something a bit more elaborate, like putting a Marker into the map, and it that fails wait for the marker to be replaced and return the other instance.