spa_async_thread
done = (spa->spa_async_thread == NULL && !spa->spa_async_tasks);
spa->spa_async_thread = NULL;
while (spa->spa_async_thread != NULL)
spa->spa_async_thread == NULL &&
spa->spa_async_thread = thread_create(NULL, 0,
spa_async_thread, spa, 0, &p0, TS_RUN, maxclsyspri);
kthread_t *spa_async_thread; /* thread doing async task */