cgroup: make css_next_child() skip missing csses

css_next_child() walks the children of the specified css.  It does
this by finding the next cgroup and then returning the requested css.
On the default unified hierarchy, a cgroup may not have a css
associated with it even if the hierarchy has the subsystem enabled.
This patch updates css_next_child() so that it skips children without
the requested css associated.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -2708,10 +2708,19 @@ css_next_child(struct cgroup_subsys_state *pos_css,
 				break;
 	}
 
-	if (&next->sibling == &cgrp->children)
-		return NULL;
+	/*
+	 * @next, if not pointing to the head, can be dereferenced and is
+	 * the next sibling; however, it might have @ss disabled.  If so,
+	 * fast-forward to the next enabled one.
+	 */
+	while (&next->sibling != &cgrp->children) {
+		struct cgroup_subsys_state *next_css = cgroup_css(next, parent_css->ss);
 
-	return cgroup_css(next, parent_css->ss);
+		if (next_css)
+			return next_css;
+		next = list_entry_rcu(next->sibling.next, struct cgroup, sibling);
+	}
+	return NULL;
 }
 
 /**