In vmThread.cpp we have the following code:

bool ok = _vm_queue->add(op);
op->set_timestamp(os::javaTimeMillis());
VMOperationQueue_lock->notify();
VMOperationQueue_lock->unlock();
// VM_Operation got skipped
if (!ok) {
        assert(concurrent, "can only skip concurrent tasks");
        if (op->is_cheap_allocated()) delete op;
        return;
}

but VMOperationQueue::add(VM_Operation *op) always returns true:

bool VMOperationQueue::add(VM_Operation *op) {

  HOTSPOT_VMOPS_REQUEST(
                   (char *) op->name(), strlen(op->name()),
                   op->evaluation_mode());

  // Encapsulates VM queue policy. Currently, that
  // only involves putting them on the right list
  if (op->evaluate_at_safepoint()) {
    queue_add_back(SafepointPriority, op);
    return true;
  }

  queue_add_back(MediumPriority, op);
  return true;
}