At the end of both mevent_add() and mevent_update(), mevent_notify() is called to wakeup the I/O thread, that will call kevent(changelist) to update the kernel.
A race condition is possible where the client calls mevent_add() and mevent_update(EV_ENABLE) before the I/O thread has the chance to wake up and call mevent_build()+kevent(changelist) in response to mevent_add(). The mevent_add() is therefore ignored by the I/O thread, and kevent(fd, EV_ENABLE) is called before kevent(fd, EV_ADD), resuliting in a failure of the kevent(fd, EV_ENABLE) call.