Write barrier requires VM lock because it accesses VM global bitmap but RB_VM_LOCK_ENTER() can invoke GC because another ractor can wait to invoke GC and RB_VM_LOCK_ENTER() is barrier point. This means that before protecting by a write barrier, GC can invoke. To prevent such situation, RB_VM_LOCK_ENTER_NO_BARRIER() is introduced. This lock primitive does not become GC barrier points.