It is illegal to remove strong roots concurrently without clearing them first. A SATB collector with concurrent root scanning assumes that when strong roots disappear from the object graph, they are cleared first, which makes SATB notice the root. All global strong roots do this. Except CLD strong roots, which are turned into non-roots by decrementing the keep_alive counter to 0, when bootstrapping weak hidden class CLDs. This is not valid behaviour.