If you have such callback that means your code can block a GC thread, which then affects the GC pacing params. I think it will be quite difficult to design such

If you have such callback that means your code can block a GC thread, which then affects the GC pacing params. I think it will be quite difficult to design such feature in a robust manner. You will definitely need some new internal flags on objects that are pending the callback and possibly extra locks around them. New threadpool just for that callback, blocking timeouts, lock coordination with all GC algorithms.