cgroup: fix harmless bugs in cgroup_load_subsys() fail path and cgroup_unload_subsys()

* If idr init fails, cgroup_load_subsys() cleared dummytop->subsys[]
  before calilng ->destroy() making CSS inaccessible to the callback,
  and didn't unlink ss->sibling.  As no modular controller uses
  ->use_id, this doesn't cause any actual problems.

* cgroup_unload_subsys() was forgetting to free idr, call
  ->pre_destroy() and clear ->active.  As there currently is no
  modular controller which uses ->use_id, ->pre_destroy() or ->active,
  this doesn't cause any actual problems.
Signed-off-by: NTejun Heo <tj@kernel.org>
Acked-by: NLi Zefan <lizefan@huawei.com>

kernel/cgroup.c

@@ -4420,9 +4420,10 @@ int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
 	if (ss->use_id) {
 		int ret = cgroup_init_idr(ss, css);
 		if (ret) {
-			dummytop->subsys[ss->subsys_id] = NULL;
 			ss->destroy(dummytop);
+			dummytop->subsys[ss->subsys_id] = NULL;
 			subsys[ss->subsys_id] = NULL;
+			list_del_init(&ss->sibling);
 			mutex_unlock(&cgroup_mutex);
 			return ret;
 		}
@@ -4490,7 +4491,19 @@ void cgroup_unload_subsys(struct cgroup_subsys *ss)
 	 */
 	BUG_ON(ss->root != &rootnode);
 
+	/* ->pre_destroy() should be called outside cgroup_mutex for now */
+	if (ss->pre_destroy)
+		ss->pre_destroy(dummytop);
+
 	mutex_lock(&cgroup_mutex);
+
+	ss->active = 0;
+
+	if (ss->use_id) {
+		idr_remove_all(&ss->idr);
+		idr_destroy(&ss->idr);
+	}
+
 	/* deassign the subsys_id */
 	subsys[ss->subsys_id] = NULL;