How does Ruby GC maintain correctness during incremental marking?

Incremental GC uses write barriers and tri-color marking to preserve correctness during partial marking.

Ruby GC uses tri-color marking (white, gray, black). Objects are gradually marked instead of all at once. Write barriers ensure that mutations during marking do not violate invariants. If a black object references a white object, the barrier promotes it to gray. This prevents premature collection and ensures correctness even while GC runs concurrently with program execution.