* freezer_create(), freezer_destroy():
* cgroup_mutex [ by cgroup core ]
*
* freezer_create(), freezer_destroy():
* cgroup_mutex [ by cgroup core ]
*
* task->alloc_lock (to get task's cgroup)
*
* freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
* task->alloc_lock (to get task's cgroup)
*
* freezer_fork() (preserving fork() performance means can't take cgroup_mutex):