sched: Fix/clarify set_task_cpu() locking rules

/*

 * Return the group to which this tasks belongs.

 *

 * We use task_subsys_state_check() and extend the RCU verification

 * with lockdep_is_held(&p->pi_lock) because cpu_cgroup_attach()

 * holds that lock for each task it moves into the cgroup. Therefore

 * by holding that lock, we pin the task to the current cgroup.

 * We use task_subsys_state_check() and extend the RCU verification with

 * pi->lock and rq->lock because cpu_cgroup_attach() holds those locks for each

 * task it moves into the cgroup. Therefore by holding either of those locks,

 * we pin the task to the current cgroup.

 */

static inline struct task_group *task_group(struct task_struct *p)

{

	struct cgroup_subsys_state *css;

	css = task_subsys_state_check(p, cpu_cgroup_subsys_id,

			lockdep_is_held(&p->pi_lock));

			lockdep_is_held(&p->pi_lock) ||

			lockdep_is_held(&task_rq(p)->lock));

	tg = container_of(css, struct task_group, css);

	return autogroup_task_group(p, tg);

			!(task_thread_info(p)->preempt_count & PREEMPT_ACTIVE));

#ifdef CONFIG_LOCKDEP

	/*

	 * The caller should hold either p->pi_lock or rq->lock, when changing

	 * a task's CPU. ->pi_lock for waking tasks, rq->lock for runnable tasks.

	 *

	 * sched_move_task() holds both and thus holding either pins the cgroup,

	 * see set_task_rq().

	 *

	 * Furthermore, all task_rq users should acquire both locks, see

	 * task_rq_lock().

	 */

	WARN_ON_ONCE(debug_locks && !(lockdep_is_held(&p->pi_lock) ||

				      lockdep_is_held(&task_rq(p)->lock)));

#endif