cgroup: Update comments about task exit path

		/*

		 * We are synchronized through cgroup_threadgroup_rwsem

		 * against PF_EXITING setting such that we can't race

		 * against cgroup_exit() changing the css_set to

		 * init_css_set and dropping the old one.

		 * against cgroup_exit()/cgroup_free() dropping the css_set.

		 */

		WARN_ON_ONCE(task->flags & PF_EXITING);

				     struct cgroup_root *root)

{

	/*

	 * No need to lock the task - since we hold cgroup_mutex the

	 * task can't change groups, so the only thing that can happen

	 * is that it exits and its css is set back to init_css_set.

	 * No need to lock the task - since we hold css_set_lock the

	 * task can't change groups.

	 */

	return cset_cgroup_from_root(task_css_set(task), root);

}

		struct css_set *cset;

		spin_lock_irq(&css_set_lock);

		cset = task_css_set(current);

		cset = task_css_set(current); /* current is @child's parent */

		if (list_empty(&child->cg_list)) {

			get_css_set(cset);

			cset->nr_tasks++;

 * cgroup_exit - detach cgroup from exiting task

 * @tsk: pointer to task_struct of exiting process

 *

 * Description: Detach cgroup from @tsk and release it.

 *

 * Note that cgroups marked notify_on_release force every task in

 * them to take the global cgroup_mutex mutex when exiting.

 * This could impact scaling on very large systems.  Be reluctant to

 * use notify_on_release cgroups where very high task exit scaling

 * is required on large systems.

 * Description: Detach cgroup from @tsk.

 *

 * We set the exiting tasks cgroup to the root cgroup (top_cgroup).  We

 * call cgroup_exit() while the task is still competent to handle

 * notify_on_release(), then leave the task attached to the root cgroup in

 * each hierarchy for the remainder of its exit.  No need to bother with

 * init_css_set refcnting.  init_css_set never goes away and we can't race

 * with migration path - PF_EXITING is visible to migration path.

 */

void cgroup_exit(struct task_struct *tsk)

{

	/*

	 * Unlink from @tsk from its css_set.  As migration path can't race

	 * with us, we can check css_set and cg_list without synchronization.

	 * with us (thanks to cgroup_threadgroup_rwsem), we can check css_set

	 * and cg_list without synchronization.

	 */

	cset = task_css_set(tsk);

		spin_unlock_irq(&css_set_lock);

	} else {

		/* Take reference to avoid freeing init_css_set in cgroup_free,

		 * see cgroup_fork(). */

		get_css_set(cset);

	}