Ah good catch. That detail isn't apparent from the interview. See this paper for more: https://www.usenix.org/events/vee05/full_papers/p46-click.pdf https://www

Ah good catch. That detail isn't apparent from the interview. See this paper for more: https://www.usenix.org/events/vee05/full_papers/p46-click.pdf https://www.usenix.org/events/vee05/full_papers/p46-click.pd.... Basically, references contain flags that indicate whether the GC thread has already visited that reference. When the read-barrier traps, the handler checks the flag. If the reference has been visited already, fine. If not, it gets added to the GC thread's queue of objects to visit. Then the program marks the reference as visited, and continues. New objects are always marked visited, and aren't collected until the following GC cycle, so the current mark phase is guaranteed to complete.