cgroup: update css iteration in cgroup_update_dfl_csses()

 * cgroup_update_dfl_csses - update css assoc of a subtree in default hierarchy

 * @cgrp: root of the subtree to update csses for

 *

 * @cgrp's subtree_ss_mask has changed and its subtree's (self excluded)

 * css associations need to be updated accordingly.  This function looks up

 * all css_sets which are attached to the subtree, creates the matching

 * updated css_sets and migrates the tasks to the new ones.

 * @cgrp's control masks have changed and its subtree's css associations

 * need to be updated accordingly.  This function looks up all css_sets

 * which are attached to the subtree, creates the matching updated css_sets

 * and migrates the tasks to the new ones.

 */

static int cgroup_update_dfl_csses(struct cgroup *cgrp)

{

	LIST_HEAD(preloaded_csets);

	struct cgroup_taskset tset = CGROUP_TASKSET_INIT(tset);

	struct cgroup_subsys_state *css;

	struct cgroup_subsys_state *d_css;

	struct cgroup *dsct;

	struct css_set *src_cset;

	int ret;

	/* look up all csses currently attached to @cgrp's subtree */

	spin_lock_bh(&css_set_lock);

	css_for_each_descendant_pre(css, cgroup_css(cgrp, NULL)) {

	cgroup_for_each_live_descendant_pre(dsct, d_css, cgrp) {

		struct cgrp_cset_link *link;

		/* self is not affected by subtree_ss_mask change */

		if (css->cgroup == cgrp)

			continue;

		list_for_each_entry(link, &css->cgroup->cset_links, cset_link)

		list_for_each_entry(link, &dsct->cset_links, cset_link)

			cgroup_migrate_add_src(link->cset, cgrp,

					       &preloaded_csets);

	}