cgroup: update css iteration in cgroup_update_dfl_csses()

The existing sequences of operations ensure that the offlining csses
are drained before cgroup_update_dfl_csses(), so even though
cgroup_update_dfl_csses() uses css_for_each_descendant_pre() to walk
the target cgroups, it doesn't end up operating on dead cgroups.
Also, the function explicitly excludes the subtree root from
operation.

This is fragile and inconsistent with the rest of css update
operations.  This patch updates cgroup_update_dfl_csses() to use
cgroup_for_each_live_descendant_pre() instead and include the subtree
root.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Zefan Li <lizefan@huawei.com>

kernel/cgroup.c

@@ -2877,16 +2877,17 @@ static int cgroup_subtree_control_show(struct seq_file *seq, void *v)
  * 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;
 
@@ -2896,14 +2897,10 @@ static int cgroup_update_dfl_csses(struct cgroup *cgrp)
 
 	/* 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);
 	}