cgroup.c

	 * already PF_EXITING could be freed from underneath us unless we
	 * take an rcu_read_lock.
	 */
	down_write(&css_set_rwsem);
	rcu_read_lock();
	task = leader;
	do {
		/* @task either already exited or can't exit until the end */
		if (task->flags & PF_EXITING)
			goto next;

		/* leave @task alone if post_fork() hasn't linked it yet */
		if (list_empty(&task->cg_list))
			goto next;

		cset = task_css_set(task);
		if (!cset->mg_src_cgrp)
			goto next;

		/*
		 * cgroup_taskset_first() must always return the leader.
		 * Take care to avoid disturbing the ordering.
		 */
		list_move_tail(&task->cg_list, &cset->mg_tasks);
		if (list_empty(&cset->mg_node))
			list_add_tail(&cset->mg_node, &tset.src_csets);
		if (list_empty(&cset->mg_dst_cset->mg_node))
			list_move_tail(&cset->mg_dst_cset->mg_node,
				       &tset.dst_csets);
	next:
		if (!threadgroup)
			break;
	} while_each_thread(leader, task);
	rcu_read_unlock();
	up_write(&css_set_rwsem);

	/* methods shouldn't be called if no task is actually migrating */
	if (list_empty(&tset.src_csets))
		return 0;

	/* check that we can legitimately attach to the cgroup */
	for_each_e_css(css, i, cgrp) {
		if (css->ss->can_attach) {
			ret = css->ss->can_attach(css, &tset);
			if (ret) {
				failed_css = css;
				goto out_cancel_attach;
			}
		}
	}

	/*
	 * Now that we're guaranteed success, proceed to move all tasks to
	 * the new cgroup.  There are no failure cases after here, so this
	 * is the commit point.
	 */
	down_write(&css_set_rwsem);
	list_for_each_entry(cset, &tset.src_csets, mg_node) {
		list_for_each_entry_safe(task, tmp_task, &cset->mg_tasks, cg_list)
			cgroup_task_migrate(cset->mg_src_cgrp, task,
					    cset->mg_dst_cset);
	}
	up_write(&css_set_rwsem);

	/*
	 * Migration is committed, all target tasks are now on dst_csets.
	 * Nothing is sensitive to fork() after this point.  Notify
	 * controllers that migration is complete.
	 */
	tset.csets = &tset.dst_csets;

	for_each_e_css(css, i, cgrp)
		if (css->ss->attach)
			css->ss->attach(css, &tset);

	ret = 0;
	goto out_release_tset;

out_cancel_attach:
	for_each_e_css(css, i, cgrp) {
		if (css == failed_css)
			break;
		if (css->ss->cancel_attach)
			css->ss->cancel_attach(css, &tset);
	}
out_release_tset:
	down_write(&css_set_rwsem);
	list_splice_init(&tset.dst_csets, &tset.src_csets);
	list_for_each_entry_safe(cset, tmp_cset, &tset.src_csets, mg_node) {
		list_splice_tail_init(&cset->mg_tasks, &cset->tasks);
		list_del_init(&cset->mg_node);
	}
	up_write(&css_set_rwsem);
	return ret;
}

/**
 * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
 * @dst_cgrp: the cgroup to attach to
 * @leader: the task or the leader of the threadgroup to be attached
 * @threadgroup: attach the whole threadgroup?
 *
 * Call holding cgroup_mutex and threadgroup_lock of @leader.
 */
static int cgroup_attach_task(struct cgroup *dst_cgrp,
			      struct task_struct *leader, bool threadgroup)
{
	LIST_HEAD(preloaded_csets);
	struct task_struct *task;
	int ret;

	/* look up all src csets */
	down_read(&css_set_rwsem);
	rcu_read_lock();
	task = leader;
	do {
		cgroup_migrate_add_src(task_css_set(task), dst_cgrp,
				       &preloaded_csets);
		if (!threadgroup)
			break;
	} while_each_thread(leader, task);
	rcu_read_unlock();
	up_read(&css_set_rwsem);

	/* prepare dst csets and commit */
	ret = cgroup_migrate_prepare_dst(dst_cgrp, &preloaded_csets);
	if (!ret)
		ret = cgroup_migrate(dst_cgrp, leader, threadgroup);

	cgroup_migrate_finish(&preloaded_csets);
	return ret;
}

/*
 * Find the task_struct of the task to attach by vpid and pass it along to the
 * function to attach either it or all tasks in its threadgroup. Will lock
 * cgroup_mutex and threadgroup.
 */
static int attach_task_by_pid(struct cgroup *cgrp, u64 pid, bool threadgroup)
{
	struct task_struct *tsk;
	const struct cred *cred = current_cred(), *tcred;
	int ret;

	if (!cgroup_lock_live_group(cgrp))
		return -ENODEV;

retry_find_task:
	rcu_read_lock();
	if (pid) {
		tsk = find_task_by_vpid(pid);
		if (!tsk) {
			rcu_read_unlock();
			ret = -ESRCH;
			goto out_unlock_cgroup;
		}
		/*
		 * even if we're attaching all tasks in the thread group, we
		 * only need to check permissions on one of them.
		 */
		tcred = __task_cred(tsk);
		if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
		    !uid_eq(cred->euid, tcred->uid) &&
		    !uid_eq(cred->euid, tcred->suid)) {
			rcu_read_unlock();
			ret = -EACCES;
			goto out_unlock_cgroup;
		}
	} else
		tsk = current;

	if (threadgroup)
		tsk = tsk->group_leader;

	/*
	 * Workqueue threads may acquire PF_NO_SETAFFINITY and become
	 * trapped in a cpuset, or RT worker may be born in a cgroup
	 * with no rt_runtime allocated.  Just say no.
	 */
	if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
		ret = -EINVAL;
		rcu_read_unlock();
		goto out_unlock_cgroup;
	}

	get_task_struct(tsk);
	rcu_read_unlock();

	threadgroup_lock(tsk);
	if (threadgroup) {
		if (!thread_group_leader(tsk)) {
			/*
			 * a race with de_thread from another thread's exec()
			 * may strip us of our leadership, if this happens,
			 * there is no choice but to throw this task away and
			 * try again; this is
			 * "double-double-toil-and-trouble-check locking".
			 */
			threadgroup_unlock(tsk);
			put_task_struct(tsk);
			goto retry_find_task;
		}
	}

	ret = cgroup_attach_task(cgrp, tsk, threadgroup);

	threadgroup_unlock(tsk);

	put_task_struct(tsk);
out_unlock_cgroup:
	mutex_unlock(&cgroup_mutex);
	return ret;
}

/**
 * cgroup_attach_task_all - attach task 'tsk' to all cgroups of task 'from'
 * @from: attach to all cgroups of a given task
 * @tsk: the task to be attached
 */
int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)
{
	struct cgroup_root *root;
	int retval = 0;

	mutex_lock(&cgroup_mutex);
	for_each_root(root) {
		struct cgroup *from_cgrp;

		if (root == &cgrp_dfl_root)
			continue;

		down_read(&css_set_rwsem);
		from_cgrp = task_cgroup_from_root(from, root);
		up_read(&css_set_rwsem);

		retval = cgroup_attach_task(from_cgrp, tsk, false);
		if (retval)
			break;
	}
	mutex_unlock(&cgroup_mutex);

	return retval;
}
EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

static int cgroup_tasks_write(struct cgroup_subsys_state *css,
			      struct cftype *cft, u64 pid)
{
	return attach_task_by_pid(css->cgroup, pid, false);
}

static int cgroup_procs_write(struct cgroup_subsys_state *css,
			      struct cftype *cft, u64 tgid)
{
	return attach_task_by_pid(css->cgroup, tgid, true);
}

static int cgroup_release_agent_write(struct cgroup_subsys_state *css,
				      struct cftype *cft, char *buffer)
{
	struct cgroup_root *root = css->cgroup->root;

	BUILD_BUG_ON(sizeof(root->release_agent_path) < PATH_MAX);
	if (!cgroup_lock_live_group(css->cgroup))
		return -ENODEV;
	spin_lock(&release_agent_path_lock);
	strlcpy(root->release_agent_path, buffer,
		sizeof(root->release_agent_path));
	spin_unlock(&release_agent_path_lock);
	mutex_unlock(&cgroup_mutex);
	return 0;
}

static int cgroup_release_agent_show(struct seq_file *seq, void *v)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	if (!cgroup_lock_live_group(cgrp))
		return -ENODEV;
	seq_puts(seq, cgrp->root->release_agent_path);
	seq_putc(seq, '\n');
	mutex_unlock(&cgroup_mutex);
	return 0;
}

static int cgroup_sane_behavior_show(struct seq_file *seq, void *v)
{
	struct cgroup *cgrp = seq_css(seq)->cgroup;

	seq_printf(seq, "%d\n", cgroup_sane_behavior(cgrp));
	return 0;
}

static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
				 size_t nbytes, loff_t off)
{
	struct cgroup *cgrp = of->kn->parent->priv;
	struct cftype *cft = of->kn->priv;
	struct cgroup_subsys_state *css;
	int ret;

	/*
	 * kernfs guarantees that a file isn't deleted with operations in
	 * flight, which means that the matching css is and stays alive and
	 * doesn't need to be pinned.  The RCU locking is not necessary
	 * either.  It's just for the convenience of using cgroup_css().
	 */
	rcu_read_lock();
	css = cgroup_css(cgrp, cft->ss);
	rcu_read_unlock();

	if (cft->write_string) {
		ret = cft->write_string(css, cft, strstrip(buf));
	} else if (cft->write_u64) {
		unsigned long long v;
		ret = kstrtoull(buf, 0, &v);
		if (!ret)
			ret = cft->write_u64(css, cft, v);
	} else if (cft->write_s64) {
		long long v;
		ret = kstrtoll(buf, 0, &v);
		if (!ret)
			ret = cft->write_s64(css, cft, v);
	} else if (cft->trigger) {
		ret = cft->trigger(css, (unsigned int)cft->private);
	} else {
		ret = -EINVAL;
	}

	return ret ?: nbytes;
}

static void *cgroup_seqfile_start(struct seq_file *seq, loff_t *ppos)
{
	return seq_cft(seq)->seq_start(seq, ppos);
}

static void *cgroup_seqfile_next(struct seq_file *seq, void *v, loff_t *ppos)
{
	return seq_cft(seq)->seq_next(seq, v, ppos);
}

static void cgroup_seqfile_stop(struct seq_file *seq, void *v)
{
	seq_cft(seq)->seq_stop(seq, v);
}

static int cgroup_seqfile_show(struct seq_file *m, void *arg)
{
	struct cftype *cft = seq_cft(m);
	struct cgroup_subsys_state *css = seq_css(m);

	if (cft->seq_show)
		return cft->seq_show(m, arg);

	if (cft->read_u64)
		seq_printf(m, "%llu\n", cft->read_u64(css, cft));
	else if (cft->read_s64)
		seq_printf(m, "%lld\n", cft->read_s64(css, cft));
	else
		return -EINVAL;
	return 0;
}

static struct kernfs_ops cgroup_kf_single_ops = {
	.atomic_write_len	= PAGE_SIZE,
	.write			= cgroup_file_write,
	.seq_show		= cgroup_seqfile_show,
};

static struct kernfs_ops cgroup_kf_ops = {
	.atomic_write_len	= PAGE_SIZE,
	.write			= cgroup_file_write,
	.seq_start		= cgroup_seqfile_start,
	.seq_next		= cgroup_seqfile_next,
	.seq_stop		= cgroup_seqfile_stop,
	.seq_show		= cgroup_seqfile_show,
};

/*
 * cgroup_rename - Only allow simple rename of directories in place.
 */
static int cgroup_rename(struct kernfs_node *kn, struct kernfs_node *new_parent,
			 const char *new_name_str)
{
	struct cgroup *cgrp = kn->priv;
	int ret;

	if (kernfs_type(kn) != KERNFS_DIR)
		return -ENOTDIR;
	if (kn->parent != new_parent)
		return -EIO;

	/*
	 * This isn't a proper migration and its usefulness is very
	 * limited.  Disallow if sane_behavior.
	 */
	if (cgroup_sane_behavior(cgrp))
		return -EPERM;

	/*
	 * We're gonna grab cgroup_tree_mutex which nests outside kernfs
	 * active_ref.  kernfs_rename() doesn't require active_ref
	 * protection.  Break them before grabbing cgroup_tree_mutex.
	 */
	kernfs_break_active_protection(new_parent);
	kernfs_break_active_protection(kn);

	mutex_lock(&cgroup_tree_mutex);
	mutex_lock(&cgroup_mutex);

	ret = kernfs_rename(kn, new_parent, new_name_str);

	mutex_unlock(&cgroup_mutex);
	mutex_unlock(&cgroup_tree_mutex);

	kernfs_unbreak_active_protection(kn);
	kernfs_unbreak_active_protection(new_parent);
	return ret;
}

/* set uid and gid of cgroup dirs and files to that of the creator */
static int cgroup_kn_set_ugid(struct kernfs_node *kn)
{
	struct iattr iattr = { .ia_valid = ATTR_UID | ATTR_GID,
			       .ia_uid = current_fsuid(),
			       .ia_gid = current_fsgid(), };

	if (uid_eq(iattr.ia_uid, GLOBAL_ROOT_UID) &&
	    gid_eq(iattr.ia_gid, GLOBAL_ROOT_GID))
		return 0;

	return kernfs_setattr(kn, &iattr);
}

static int cgroup_add_file(struct cgroup *cgrp, struct cftype *cft)
{
	char name[CGROUP_FILE_NAME_MAX];
	struct kernfs_node *kn;
	struct lock_class_key *key = NULL;
	int ret;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
	key = &cft->lockdep_key;
#endif
	kn = __kernfs_create_file(cgrp->kn, cgroup_file_name(cgrp, cft, name),
				  cgroup_file_mode(cft), 0, cft->kf_ops, cft,
				  NULL, false, key);
	if (IS_ERR(kn))
		return PTR_ERR(kn);

	ret = cgroup_kn_set_ugid(kn);
	if (ret)
		kernfs_remove(kn);
	return ret;
}

/**
 * cgroup_addrm_files - add or remove files to a cgroup directory
 * @cgrp: the target cgroup
 * @cfts: array of cftypes to be added
 * @is_add: whether to add or remove
 *
 * Depending on @is_add, add or remove files defined by @cfts on @cgrp.
 * For removals, this function never fails.  If addition fails, this
 * function doesn't remove files already added.  The caller is responsible
 * for cleaning up.
 */
static int cgroup_addrm_files(struct cgroup *cgrp, struct cftype cfts[],
			      bool is_add)
{
	struct cftype *cft;
	int ret;

	lockdep_assert_held(&cgroup_tree_mutex);

	for (cft = cfts; cft->name[0] != '\0'; cft++) {
		/* does cft->flags tell us to skip this file on @cgrp? */
		if ((cft->flags & CFTYPE_ONLY_ON_DFL) && !cgroup_on_dfl(cgrp))
			continue;
		if ((cft->flags & CFTYPE_INSANE) && cgroup_sane_behavior(cgrp))
			continue;
		if ((cft->flags & CFTYPE_NOT_ON_ROOT) && !cgrp->parent)
			continue;
		if ((cft->flags & CFTYPE_ONLY_ON_ROOT) && cgrp->parent)
			continue;

		if (is_add) {
			ret = cgroup_add_file(cgrp, cft);
			if (ret) {
				pr_warn("cgroup_addrm_files: failed to add %s, err=%d\n",
					cft->name, ret);
				return ret;
			}
		} else {
			cgroup_rm_file(cgrp, cft);
		}
	}
	return 0;
}

static int cgroup_apply_cftypes(struct cftype *cfts, bool is_add)
{
	LIST_HEAD(pending);
	struct cgroup_subsys *ss = cfts[0].ss;
	struct cgroup *root = &ss->root->cgrp;
	struct cgroup_subsys_state *css;
	int ret = 0;

	lockdep_assert_held(&cgroup_tree_mutex);

	/* add/rm files for all cgroups created before */
	css_for_each_descendant_pre(css, cgroup_css(root, ss)) {
		struct cgroup *cgrp = css->cgroup;

		if (cgroup_is_dead(cgrp))
			continue;

		ret = cgroup_addrm_files(cgrp, cfts, is_add);
		if (ret)
			break;
	}

	if (is_add && !ret)
		kernfs_activate(root->kn);
	return ret;
}

static void cgroup_exit_cftypes(struct cftype *cfts)
{
	struct cftype *cft;

	for (cft = cfts; cft->name[0] != '\0'; cft++) {
		/* free copy for custom atomic_write_len, see init_cftypes() */
		if (cft->max_write_len && cft->max_write_len != PAGE_SIZE)
			kfree(cft->kf_ops);
		cft->kf_ops = NULL;
		cft->ss = NULL;
	}
}

static int cgroup_init_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
	struct cftype *cft;

	for (cft = cfts; cft->name[0] != '\0'; cft++) {
		struct kernfs_ops *kf_ops;

		WARN_ON(cft->ss || cft->kf_ops);

		if (cft->seq_start)
			kf_ops = &cgroup_kf_ops;
		else
			kf_ops = &cgroup_kf_single_ops;

		/*
		 * Ugh... if @cft wants a custom max_write_len, we need to
		 * make a copy of kf_ops to set its atomic_write_len.
		 */
		if (cft->max_write_len && cft->max_write_len != PAGE_SIZE) {
			kf_ops = kmemdup(kf_ops, sizeof(*kf_ops), GFP_KERNEL);
			if (!kf_ops) {
				cgroup_exit_cftypes(cfts);
				return -ENOMEM;
			}
			kf_ops->atomic_write_len = cft->max_write_len;
		}

		cft->kf_ops = kf_ops;
		cft->ss = ss;
	}

	return 0;
}

static int cgroup_rm_cftypes_locked(struct cftype *cfts)
{
	lockdep_assert_held(&cgroup_tree_mutex);

	if (!cfts || !cfts[0].ss)
		return -ENOENT;

	list_del(&cfts->node);
	cgroup_apply_cftypes(cfts, false);
	cgroup_exit_cftypes(cfts);
	return 0;
}

/**
 * cgroup_rm_cftypes - remove an array of cftypes from a subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Unregister @cfts.  Files described by @cfts are removed from all
 * existing cgroups and all future cgroups won't have them either.  This
 * function can be called anytime whether @cfts' subsys is attached or not.
 *
 * Returns 0 on successful unregistration, -ENOENT if @cfts is not
 * registered.
 */
int cgroup_rm_cftypes(struct cftype *cfts)
{
	int ret;

	mutex_lock(&cgroup_tree_mutex);
	ret = cgroup_rm_cftypes_locked(cfts);
	mutex_unlock(&cgroup_tree_mutex);
	return ret;
}

/**
 * cgroup_add_cftypes - add an array of cftypes to a subsystem
 * @ss: target cgroup subsystem
 * @cfts: zero-length name terminated array of cftypes
 *
 * Register @cfts to @ss.  Files described by @cfts are created for all
 * existing cgroups to which @ss is attached and all future cgroups will
 * have them too.  This function can be called anytime whether @ss is
 * attached or not.
 *
 * Returns 0 on successful registration, -errno on failure.  Note that this
 * function currently returns 0 as long as @cfts registration is successful
 * even if some file creation attempts on existing cgroups fail.
 */
int cgroup_add_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
{
	int ret;

	if (!cfts || cfts[0].name[0] == '\0')
		return 0;

	ret = cgroup_init_cftypes(ss, cfts);
	if (ret)
		return ret;

	mutex_lock(&cgroup_tree_mutex);

	list_add_tail(&cfts->node, &ss->cfts);
	ret = cgroup_apply_cftypes(cfts, true);
	if (ret)
		cgroup_rm_cftypes_locked(cfts);

	mutex_unlock(&cgroup_tree_mutex);
	return ret;
}

/**
 * cgroup_task_count - count the number of tasks in a cgroup.
 * @cgrp: the cgroup in question
 *
 * Return the number of tasks in the cgroup.
 */
static int cgroup_task_count(const struct cgroup *cgrp)
{
	int count = 0;
	struct cgrp_cset_link *link;

	down_read(&css_set_rwsem);
	list_for_each_entry(link, &cgrp->cset_links, cset_link)
		count += atomic_read(&link->cset->refcount);
	up_read(&css_set_rwsem);
	return count;
}

/**
 * css_next_child - find the next child of a given css
 * @pos_css: the current position (%NULL to initiate traversal)
 * @parent_css: css whose children to walk
 *
 * This function returns the next child of @parent_css and should be called
 * under either cgroup_mutex or RCU read lock.  The only requirement is
 * that @parent_css and @pos_css are accessible.  The next sibling is
 * guaranteed to be returned regardless of their states.
 */
struct cgroup_subsys_state *
css_next_child(struct cgroup_subsys_state *pos_css,
	       struct cgroup_subsys_state *parent_css)
{
	struct cgroup *pos = pos_css ? pos_css->cgroup : NULL;
	struct cgroup *cgrp = parent_css->cgroup;
	struct cgroup *next;

	cgroup_assert_mutexes_or_rcu_locked();

	/*
	 * @pos could already have been removed.  Once a cgroup is removed,
	 * its ->sibling.next is no longer updated when its next sibling
	 * changes.  As CGRP_DEAD assertion is serialized and happens
	 * before the cgroup is taken off the ->sibling list, if we see it
	 * unasserted, it's guaranteed that the next sibling hasn't
	 * finished its grace period even if it's already removed, and thus
	 * safe to dereference from this RCU critical section.  If
	 * ->sibling.next is inaccessible, cgroup_is_dead() is guaranteed
	 * to be visible as %true here.
	 *
	 * If @pos is dead, its next pointer can't be dereferenced;
	 * however, as each cgroup is given a monotonically increasing
	 * unique serial number and always appended to the sibling list,
	 * the next one can be found by walking the parent's children until
	 * we see a cgroup with higher serial number than @pos's.  While
	 * this path can be slower, it's taken only when either the current
	 * cgroup is removed or iteration and removal race.
	 */
	if (!pos) {
		next = list_entry_rcu(cgrp->children.next, struct cgroup, sibling);
	} else if (likely(!cgroup_is_dead(pos))) {
		next = list_entry_rcu(pos->sibling.next, struct cgroup, sibling);
	} else {
		list_for_each_entry_rcu(next, &cgrp->children, sibling)
			if (next->serial_nr > pos->serial_nr)
				break;
	}

	/*
	 * @next, if not pointing to the head, can be dereferenced and is
	 * the next sibling; however, it might have @ss disabled.  If so,
	 * fast-forward to the next enabled one.
	 */
	while (&next->sibling != &cgrp->children) {
		struct cgroup_subsys_state *next_css = cgroup_css(next, parent_css->ss);

		if (next_css)
			return next_css;
		next = list_entry_rcu(next->sibling.next, struct cgroup, sibling);
	}
	return NULL;
}

/**
 * css_next_descendant_pre - find the next descendant for pre-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @root: css whose descendants to walk
 *
 * To be used by css_for_each_descendant_pre().  Find the next descendant
 * to visit for pre-order traversal of @root's descendants.  @root is
 * included in the iteration and the first node to be visited.
 *
 * While this function requires cgroup_mutex or RCU read locking, it
 * doesn't require the whole traversal to be contained in a single critical
 * section.  This function will return the correct next descendant as long
 * as both @pos and @root are accessible and @pos is a descendant of @root.
 */
struct cgroup_subsys_state *
css_next_descendant_pre(struct cgroup_subsys_state *pos,
			struct cgroup_subsys_state *root)
{
	struct cgroup_subsys_state *next;

	cgroup_assert_mutexes_or_rcu_locked();

	/* if first iteration, visit @root */
	if (!pos)
		return root;

	/* visit the first child if exists */
	next = css_next_child(NULL, pos);
	if (next)
		return next;

	/* no child, visit my or the closest ancestor's next sibling */
	while (pos != root) {
		next = css_next_child(pos, css_parent(pos));
		if (next)
			return next;
		pos = css_parent(pos);
	}

	return NULL;
}

/**
 * css_rightmost_descendant - return the rightmost descendant of a css
 * @pos: css of interest
 *
 * Return the rightmost descendant of @pos.  If there's no descendant, @pos
 * is returned.  This can be used during pre-order traversal to skip
 * subtree of @pos.
 *
 * While this function requires cgroup_mutex or RCU read locking, it
 * doesn't require the whole traversal to be contained in a single critical
 * section.  This function will return the correct rightmost descendant as
 * long as @pos is accessible.
 */
struct cgroup_subsys_state *
css_rightmost_descendant(struct cgroup_subsys_state *pos)
{
	struct cgroup_subsys_state *last, *tmp;

	cgroup_assert_mutexes_or_rcu_locked();

	do {
		last = pos;
		/* ->prev isn't RCU safe, walk ->next till the end */
		pos = NULL;
		css_for_each_child(tmp, last)
			pos = tmp;
	} while (pos);

	return last;
}

static struct cgroup_subsys_state *
css_leftmost_descendant(struct cgroup_subsys_state *pos)
{
	struct cgroup_subsys_state *last;

	do {
		last = pos;
		pos = css_next_child(NULL, pos);
	} while (pos);

	return last;
}

/**
 * css_next_descendant_post - find the next descendant for post-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @root: css whose descendants to walk
 *
 * To be used by css_for_each_descendant_post().  Find the next descendant
 * to visit for post-order traversal of @root's descendants.  @root is
 * included in the iteration and the last node to be visited.
 *
 * While this function requires cgroup_mutex or RCU read locking, it
 * doesn't require the whole traversal to be contained in a single critical
 * section.  This function will return the correct next descendant as long
 * as both @pos and @cgroup are accessible and @pos is a descendant of
 * @cgroup.
 */
struct cgroup_subsys_state *
css_next_descendant_post(struct cgroup_subsys_state *pos,
			 struct cgroup_subsys_state *root)
{
	struct cgroup_subsys_state *next;

	cgroup_assert_mutexes_or_rcu_locked();

	/* if first iteration, visit leftmost descendant which may be @root */
	if (!pos)
		return css_leftmost_descendant(root);

	/* if we visited @root, we're done */
	if (pos == root)
		return NULL;

	/* if there's an unvisited sibling, visit its leftmost descendant */
	next = css_next_child(pos, css_parent(pos));
	if (next)
		return css_leftmost_descendant(next);

	/* no sibling left, visit parent */
	return css_parent(pos);
}

/**
 * css_advance_task_iter - advance a task itererator to the next css_set
 * @it: the iterator to advance
 *
 * Advance @it to the next css_set to walk.
 */
static void css_advance_task_iter(struct css_task_iter *it)
{
	struct list_head *l = it->cset_pos;
	struct cgrp_cset_link *link;
	struct css_set *cset;

	/* Advance to the next non-empty css_set */
	do {
		l = l->next;
		if (l == it->cset_head) {
			it->cset_pos = NULL;
			return;
		}

		if (it->ss) {
			cset = container_of(l, struct css_set,
					    e_cset_node[it->ss->id]);
		} else {
			link = list_entry(l, struct cgrp_cset_link, cset_link);
			cset = link->cset;
		}
	} while (list_empty(&cset->tasks) && list_empty(&cset->mg_tasks));

	it->cset_pos = l;

	if (!list_empty(&cset->tasks))
		it->task_pos = cset->tasks.next;
	else
		it->task_pos = cset->mg_tasks.next;

	it->tasks_head = &cset->tasks;
	it->mg_tasks_head = &cset->mg_tasks;
}

/**
 * css_task_iter_start - initiate task iteration
 * @css: the css to walk tasks of
 * @it: the task iterator to use
 *
 * Initiate iteration through the tasks of @css.  The caller can call
 * css_task_iter_next() to walk through the tasks until the function
 * returns NULL.  On completion of iteration, css_task_iter_end() must be
 * called.
 *
 * Note that this function acquires a lock which is released when the
 * iteration finishes.  The caller can't sleep while iteration is in
 * progress.
 */
void css_task_iter_start(struct cgroup_subsys_state *css,
			 struct css_task_iter *it)
	__acquires(css_set_rwsem)
{
	/* no one should try to iterate before mounting cgroups */
	WARN_ON_ONCE(!use_task_css_set_links);

	down_read(&css_set_rwsem);

	it->ss = css->ss;

	if (it->ss)
		it->cset_pos = &css->cgroup->e_csets[css->ss->id];
	else
		it->cset_pos = &css->cgroup->cset_links;

	it->cset_head = it->cset_pos;

	css_advance_task_iter(it);
}

/**
 * css_task_iter_next - return the next task for the iterator
 * @it: the task iterator being iterated
 *
 * The "next" function for task iteration.  @it should have been
 * initialized via css_task_iter_start().  Returns NULL when the iteration
 * reaches the end.
 */
struct task_struct *css_task_iter_next(struct css_task_iter *it)
{
	struct task_struct *res;
	struct list_head *l = it->task_pos;

	/* If the iterator cg is NULL, we have no tasks */
	if (!it->cset_pos)
		return NULL;
	res = list_entry(l, struct task_struct, cg_list);

	/*
	 * Advance iterator to find next entry.  cset->tasks is consumed
	 * first and then ->mg_tasks.  After ->mg_tasks, we move onto the
	 * next cset.
	 */
	l = l->next;

	if (l == it->tasks_head)
		l = it->mg_tasks_head->next;

	if (l == it->mg_tasks_head)
		css_advance_task_iter(it);
	else
		it->task_pos = l;

	return res;
}

/**
 * css_task_iter_end - finish task iteration
 * @it: the task iterator to finish
 *
 * Finish task iteration started by css_task_iter_start().
 */
void css_task_iter_end(struct css_task_iter *it)
	__releases(css_set_rwsem)
{
	up_read(&css_set_rwsem);
}

/**
 * cgroup_trasnsfer_tasks - move tasks from one cgroup to another
 * @to: cgroup to which the tasks will be moved
 * @from: cgroup in which the tasks currently reside
 *
 * Locking rules between cgroup_post_fork() and the migration path
 * guarantee that, if a task is forking while being migrated, the new child
 * is guaranteed to be either visible in the source cgroup after the
 * parent's migration is complete or put into the target cgroup.  No task
 * can slip out of migration through forking.
 */
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;

	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)