Function vdev_open_children_impl

freebsd/contrib/openzfs/module/zfs/vdev.c:1675–1704

* Open the requested child vdevs. If any of the leaf vdevs are using * a ZFS volume then do the opens in a single thread. This avoids a * deadlock when the current thread is holding the spa_namespace_lock. */

 * deadlock when the current thread is holding the spa_namespace_lock.
 */
static void
vdev_open_children_impl(vdev_t *vd, vdev_open_children_func_t *open_func)
{
	int children = vd->vdev_children;

	taskq_t *tq = taskq_create("vdev_open", children, minclsyspri,
	    children, children, TASKQ_PREPOPULATE);
	vd->vdev_nonrot = B_TRUE;

	for (int c = 0; c < children; c++) {
		vdev_t *cvd = vd->vdev_child[c];

		if (open_func(cvd) == B_FALSE)
			continue;

		if (tq == NULL || vdev_uses_zvols(vd)) {
			cvd->vdev_open_error = vdev_open(cvd);
		} else {
			VERIFY(taskq_dispatch(tq, vdev_open_child,
			    cvd, TQ_SLEEP) != TASKQID_INVALID);
		}

		vd->vdev_nonrot &= cvd->vdev_nonrot;
	}

	if (tq != NULL) {
		taskq_wait(tq);
		taskq_destroy(tq);
	}
}

/*
 * Open all child vdevs.