Function l2arc_dev_get_next

freebsd/contrib/openzfs/module/zfs/arc.c:8105–8162

* Cycle through L2ARC devices. This is how L2ARC load balances. * If a device is returned, this also returns holding the spa config lock. */

 * If a device is returned, this also returns holding the spa config lock.
 */
static l2arc_dev_t *
l2arc_dev_get_next(void)
{
	l2arc_dev_t *first, *next = NULL;

	/*
	 * Lock out the removal of spas (spa_namespace_lock), then removal
	 * of cache devices (l2arc_dev_mtx).  Once a device has been selected,
	 * both locks will be dropped and a spa config lock held instead.
	 */
	mutex_enter(&spa_namespace_lock);
	mutex_enter(&l2arc_dev_mtx);

	/* if there are no vdevs, there is nothing to do */
	if (l2arc_ndev == 0)
		goto out;

	first = NULL;
	next = l2arc_dev_last;
	do {
		/* loop around the list looking for a non-faulted vdev */
		if (next == NULL) {
			next = list_head(l2arc_dev_list);
		} else {
			next = list_next(l2arc_dev_list, next);
			if (next == NULL)
				next = list_head(l2arc_dev_list);
		}

		/* if we have come back to the start, bail out */
		if (first == NULL)
			first = next;
		else if (next == first)
			break;

	} while (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
	    next->l2ad_trim_all);

	/* if we were unable to find any usable vdevs, return NULL */
	if (vdev_is_dead(next->l2ad_vdev) || next->l2ad_rebuild ||
	    next->l2ad_trim_all)
		next = NULL;

	l2arc_dev_last = next;

out:
	mutex_exit(&l2arc_dev_mtx);

	/*
	 * Grab the config lock to prevent the 'next' device from being
	 * removed while we are writing to it.
	 */
	if (next != NULL)
		spa_config_enter(next->l2ad_spa, SCL_L2ARC, next, RW_READER);
	mutex_exit(&spa_namespace_lock);

	return (next);
}