cgroup: avoid accessing modular cgroup subsys structure without locking

subsys[i] is set to NULL in cgroup_unload_subsys() at modular unload,
and that's protected by cgroup_mutex, and then the memory *subsys[i]
resides will be freed.

So this is unsafe without any locking:

  if (!ss || ss->module)
  ...

v2:
- add a comment for enum cgroup_subsys_id
- simplify the comment in cgroup_exit()

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -4940,17 +4940,17 @@ void cgroup_post_fork(struct task_struct *child)
 	 * and addition to css_set.
 	 */
 	if (need_forkexit_callback) {
-		for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+		/*
+		 * fork/exit callbacks are supported only for builtin
+		 * subsystems, and the builtin section of the subsys
+		 * array is immutable, so we don't need to lock the
+		 * subsys array here. On the other hand, modular section
+		 * of the array can be freed at module unload, so we
+		 * can't touch that.
+		 */
+		for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
 			struct cgroup_subsys *ss = subsys[i];
 
-			/*
-			 * fork/exit callbacks are supported only for
-			 * builtin subsystems and we don't need further
-			 * synchronization as they never go away.
-			 */
-			if (!ss || ss->module)
-				continue;
-
 			if (ss->fork)
 				ss->fork(child);
 		}
@@ -5015,13 +5015,13 @@ void cgroup_exit(struct task_struct *tsk, int run_callbacks)
 	tsk->cgroups = &init_css_set;
 
 	if (run_callbacks && need_forkexit_callback) {
-		for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
+		/*
+		 * fork/exit callbacks are supported only for builtin
+		 * subsystems, see cgroup_post_fork() for details.
+		 */
+		for (i = 0; i < CGROUP_BUILTIN_SUBSYS_COUNT; i++) {
 			struct cgroup_subsys *ss = subsys[i];
 
-			/* modular subsystems can't use callbacks */
-			if (!ss || ss->module)
-				continue;
-
 			if (ss->exit) {
 				struct cgroup *old_cgrp =
 					rcu_dereference_raw(cg->subsys[i])->cgroup;