cgroup: make cgroup_destroy_locked() test cgroup_is_populated()

cgroup_destroy_locked() currently tests whether any css_sets are
associated to reject removal if the cgroup contains tasks.  This works
because a css_set's refcnt converges with the number of tasks linked
to it and thus there's no css_set linked to a cgroup if it doesn't
have any live tasks.

To help tracking resource usage of zombie tasks, putting the ref of
css_set will be separated from disassociating the task from the
css_set which means that a cgroup may have css_sets linked to it even
when it doesn't have any live tasks.

This patch updates cgroup_destroy_locked() so that it tests
cgroup_is_populated(), which counts the number of populated css_sets,
instead of whether cgrp->cset_links is empty to determine whether the
cgroup is populated or not.  This ensures that rmdirs won't be
incorrectly rejected for cgroups which only contain zombie tasks.
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -4998,16 +4998,15 @@ static int cgroup_destroy_locked(struct cgroup *cgrp)
 	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
 {
 	struct cgroup_subsys_state *css;
-	bool empty;
 	int ssid;
 
 	lockdep_assert_held(&cgroup_mutex);
 
-	/* css_set_rwsem synchronizes access to ->cset_links */
-	down_read(&css_set_rwsem);
-	empty = list_empty(&cgrp->cset_links);
-	up_read(&css_set_rwsem);
-	if (!empty)
+	/*
+	 * Only migration can raise populated from zero and we're already
+	 * holding cgroup_mutex.
+	 */
+	if (cgroup_is_populated(cgrp))
 		return -EBUSY;
 
 	/*