Monitor#mon_enter

static VALUE
monitor_enter(VALUE monitor)
{
    struct rb_monitor *mc = monitor_ptr(monitor);
    if (!mc_owner_p(mc)) {
        rb_mutex_lock(mc->mutex);
        RB_OBJ_WRITE(monitor, &mc->owner, rb_fiber_current());
        mc->count = 0;
    }
    mc->count++;
    return Qnil;
}