cgroup: update cgroup_transfer_tasks() to either succeed or fail

 */

int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)

{

	LIST_HEAD(preloaded_csets);

	struct cgrp_cset_link *link;

	struct css_task_iter it;

	struct task_struct *task;

	int ret = 0;

	int ret;

	mutex_lock(&cgroup_mutex);

	/* all tasks in @from are being moved, all csets are source */

	down_read(&css_set_rwsem);

	list_for_each_entry(link, &from->cset_links, cset_link)

		cgroup_migrate_add_src(link->cset, to, &preloaded_csets);

	up_read(&css_set_rwsem);

	ret = cgroup_migrate_prepare_dst(to, &preloaded_csets);

	if (ret)

		goto out_err;

	/*

	 * Migrate tasks one-by-one until @form is empty.  This fails iff

	 * ->can_attach() fails.

	 */

	do {

		css_task_iter_start(&from->dummy_css, &it);

		task = css_task_iter_next(&it);

		css_task_iter_end(&it);

		if (task) {

			mutex_lock(&cgroup_mutex);

			ret = cgroup_attach_task(to, task, false);

			mutex_unlock(&cgroup_mutex);

			ret = cgroup_migrate(to, task, false);

			put_task_struct(task);

		}

	} while (task && !ret);

out_err:

	cgroup_migrate_finish(&preloaded_csets);

	mutex_unlock(&cgroup_mutex);

	return ret;

}