static struct css_set *find_css_set(struct css_set *old_cset,
				    struct cgroup *cgrp)
{
	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
	struct css_set *cset;
	struct list_head tmp_links;
	struct cgrp_cset_link *link;
	struct cgroup_subsys *ss;
	unsigned long key;
	int ssid;

	lockdep_assert_held(&cgroup_mutex);

	/* First see if we already have a cgroup group that matches
	 * the desired set */
	spin_lock_irq(&css_set_lock);
	cset = find_existing_css_set(old_cset, cgrp, template);
	if (cset)
		get_css_set(cset);
	spin_unlock_irq(&css_set_lock);

	if (cset)
		return cset;

	cset = kzalloc(sizeof(*cset), GFP_KERNEL);
	if (!cset)
		return NULL;

	/* Allocate all the cgrp_cset_link objects that we'll need */
	if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
		kfree(cset);
		return NULL;
	}

	refcount_set(&cset->refcount, 1);
	cset->dom_cset = cset;
	INIT_LIST_HEAD(&cset->tasks);
	INIT_LIST_HEAD(&cset->mg_tasks);
	INIT_LIST_HEAD(&cset->dying_tasks);
	INIT_LIST_HEAD(&cset->task_iters);
	INIT_LIST_HEAD(&cset->threaded_csets);
	INIT_HLIST_NODE(&cset->hlist);
	INIT_LIST_HEAD(&cset->cgrp_links);
	INIT_LIST_HEAD(&cset->mg_preload_node);
	INIT_LIST_HEAD(&cset->mg_node);

	/* Copy the set of subsystem state objects generated in
	 * find_existing_css_set() */
	memcpy(cset->subsys, template, sizeof(cset->subsys));

	spin_lock_irq(&css_set_lock);
	/* Add reference counts and links from the new css_set. */
	list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
		struct cgroup *c = link->cgrp;

		if (c->root == cgrp->root)
			c = cgrp;
		link_css_set(&tmp_links, cset, c);
	}

	BUG_ON(!list_empty(&tmp_links));

	css_set_count++;

	/* Add @cset to the hash table */
	key = css_set_hash(cset->subsys);
	hash_add(css_set_table, &cset->hlist, key);

	for_each_subsys(ss, ssid) {
		struct cgroup_subsys_state *css = cset->subsys[ssid];

		list_add_tail(&cset->e_cset_node[ssid],
			      &css->cgroup->e_csets[ssid]);
		css_get(css);
	}

	spin_unlock_irq(&css_set_lock);

	/*
	 * If @cset should be threaded, look up the matching dom_cset and
	 * link them up.  We first fully initialize @cset then look for the
	 * dom_cset.  It's simpler this way and safe as @cset is guaranteed
	 * to stay empty until we return.
	 */
	if (cgroup_is_threaded(cset->dfl_cgrp)) {
		struct css_set *dcset;

		dcset = find_css_set(cset, cset->dfl_cgrp->dom_cgrp);
		if (!dcset) {
			put_css_set(cset);
			return NULL;
		}

		spin_lock_irq(&css_set_lock);
		cset->dom_cset = dcset;
		list_add_tail(&cset->threaded_csets_node,
			      &dcset->threaded_csets);
		spin_unlock_irq(&css_set_lock);
	}

	return cset;
}
static struct css_set *find_css_set(struct css_set *old_cset,
				    struct cgroup *cgrp)
{
	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT] = { };
	struct css_set *cset;
	struct list_head tmp_links;
	struct cgrp_cset_link *link;
	struct cgroup_subsys *ss;
	unsigned long key;
	int ssid;

	lockdep_assert_held(&cgroup_mutex);

	 
	spin_lock_irq(&css_set_lock);
	cset = find_existing_css_set(old_cset, cgrp, template);
	if (cset)
		get_css_set(cset);
	spin_unlock_irq(&css_set_lock);

	if (cset)
		return cset;

	cset = kzalloc(sizeof(*cset), GFP_KERNEL);
	if (!cset)
		return NULL;

	 
	if (allocate_cgrp_cset_links(cgroup_root_count, &tmp_links) < 0) {
		kfree(cset);
		return NULL;
	}

	refcount_set(&cset->refcount, 1);
	cset->dom_cset = cset;
	INIT_LIST_HEAD(&cset->tasks);
	INIT_LIST_HEAD(&cset->mg_tasks);
	INIT_LIST_HEAD(&cset->dying_tasks);
	INIT_LIST_HEAD(&cset->task_iters);
	INIT_LIST_HEAD(&cset->threaded_csets);
	INIT_HLIST_NODE(&cset->hlist);
	INIT_LIST_HEAD(&cset->cgrp_links);
	INIT_LIST_HEAD(&cset->mg_preload_node);
	INIT_LIST_HEAD(&cset->mg_node);

	 
	memcpy(cset->subsys, template, sizeof(cset->subsys));

	spin_lock_irq(&css_set_lock);
	 
	list_for_each_entry(link, &old_cset->cgrp_links, cgrp_link) {
		struct cgroup *c = link->cgrp;

		if (c->root == cgrp->root)
			c = cgrp;
		link_css_set(&tmp_links, cset, c);
	}

	BUG_ON(!list_empty(&tmp_links));

	css_set_count++;

	 
	key = css_set_hash(cset->subsys);
	hash_add(css_set_table, &cset->hlist, key);

	for_each_subsys(ss, ssid) {
		struct cgroup_subsys_state *css = cset->subsys[ssid];

		list_add_tail(&cset->e_cset_node[ssid],
			      &css->cgroup->e_csets[ssid]);
		css_get(css);
	}

	spin_unlock_irq(&css_set_lock);

	 
	if (cgroup_is_threaded(cset->dfl_cgrp)) {
		struct css_set *dcset;

		dcset = find_css_set(cset, cset->dfl_cgrp->dom_cgrp);
		if (!dcset) {
			put_css_set(cset);
			return NULL;
		}

		spin_lock_irq(&css_set_lock);
		cset->dom_cset = dcset;
		list_add_tail(&cset->threaded_csets_node,
			      &dcset->threaded_csets);
		spin_unlock_irq(&css_set_lock);
	}

	return cset;
}