cgroup.c

	css_get(css);

	/*
	 * cgroup core guarantees that, by the time ->css_offline() is
	 * invoked, no new css reference will be given out via
	 * css_tryget().  We can't simply call percpu_ref_kill() and
	 * proceed to offlining css's because percpu_ref_kill() doesn't
	 * guarantee that the ref is seen as killed on all CPUs on return.
	 *
	 * Use percpu_ref_kill_and_confirm() to get notifications as each
	 * css is confirmed to be seen as killed on all CPUs.
	 */
	percpu_ref_kill_and_confirm(&css->refcnt, css_killed_ref_fn);
}

/**
 * cgroup_destroy_locked - the first stage of cgroup destruction
 * @cgrp: cgroup to be destroyed
 *
 * css's make use of percpu refcnts whose killing latency shouldn't be
 * exposed to userland and are RCU protected.  Also, cgroup core needs to
 * guarantee that css_tryget() won't succeed by the time ->css_offline() is
 * invoked.  To satisfy all the requirements, destruction is implemented in
 * the following two steps.
 *
 * s1. Verify @cgrp can be destroyed and mark it dying.  Remove all
 *     userland visible parts and start killing the percpu refcnts of
 *     css's.  Set up so that the next stage will be kicked off once all
 *     the percpu refcnts are confirmed to be killed.
 *
 * s2. Invoke ->css_offline(), mark the cgroup dead and proceed with the
 *     rest of destruction.  Once all cgroup references are gone, the
 *     cgroup is RCU-freed.
 *
 * This function implements s1.  After this step, @cgrp is gone as far as
 * the userland is concerned and a new cgroup with the same name may be
 * created.  As cgroup doesn't care about the names internally, this
 * doesn't cause any problem.
 */
static int cgroup_destroy_locked(struct cgroup *cgrp)
	__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
{
	struct cgroup *child;
	struct cgroup_subsys_state *css;
	bool empty;
	int ssid;

	lockdep_assert_held(&cgroup_tree_mutex);
	lockdep_assert_held(&cgroup_mutex);

	/*
	 * css_set_rwsem synchronizes access to ->cset_links and prevents
	 * @cgrp from being removed while put_css_set() is in progress.
	 */
	down_read(&css_set_rwsem);
	empty = list_empty(&cgrp->cset_links);
	up_read(&css_set_rwsem);
	if (!empty)
		return -EBUSY;

	/*
	 * Make sure there's no live children.  We can't test ->children
	 * emptiness as dead children linger on it while being destroyed;
	 * otherwise, "rmdir parent/child parent" may fail with -EBUSY.
	 */
	empty = true;
	rcu_read_lock();
	list_for_each_entry_rcu(child, &cgrp->children, sibling) {
		empty = cgroup_is_dead(child);
		if (!empty)
			break;
	}
	rcu_read_unlock();
	if (!empty)
		return -EBUSY;

	/*
	 * Mark @cgrp dead.  This prevents further task migration and child
	 * creation by disabling cgroup_lock_live_group().  Note that
	 * CGRP_DEAD assertion is depended upon by css_next_child() to
	 * resume iteration after dropping RCU read lock.  See
	 * css_next_child() for details.
	 */
	set_bit(CGRP_DEAD, &cgrp->flags);

	/*
	 * Initiate massacre of all css's.  cgroup_destroy_css_killed()
	 * will be invoked to perform the rest of destruction once the
	 * percpu refs of all css's are confirmed to be killed.  This
	 * involves removing the subsystem's files, drop cgroup_mutex.
	 */
	mutex_unlock(&cgroup_mutex);
	for_each_css(css, ssid, cgrp)
		kill_css(css);
	mutex_lock(&cgroup_mutex);

	/* CGRP_DEAD is set, remove from ->release_list for the last time */
	raw_spin_lock(&release_list_lock);
	if (!list_empty(&cgrp->release_list))
		list_del_init(&cgrp->release_list);
	raw_spin_unlock(&release_list_lock);

	/*
	 * If @cgrp has css's attached, the second stage of cgroup
	 * destruction is kicked off from css_killed_work_fn() after the
	 * refs of all attached css's are killed.  If @cgrp doesn't have
	 * any css, we kick it off here.
	 */
	if (!cgrp->nr_css)
		cgroup_destroy_css_killed(cgrp);

	/* remove @cgrp directory along with the base files */
	mutex_unlock(&cgroup_mutex);

	/*
	 * There are two control paths which try to determine cgroup from
	 * dentry without going through kernfs - cgroupstats_build() and
	 * css_tryget_from_dir().  Those are supported by RCU protecting
	 * clearing of cgrp->kn->priv backpointer, which should happen
	 * after all files under it have been removed.
	 */
	kernfs_remove(cgrp->kn);	/* @cgrp has an extra ref on its kn */
	RCU_INIT_POINTER(*(void __rcu __force **)&cgrp->kn->priv, NULL);

	mutex_lock(&cgroup_mutex);

	return 0;
};

/**
 * cgroup_destroy_css_killed - the second step of cgroup destruction
 * @work: cgroup->destroy_free_work
 *
 * This function is invoked from a work item for a cgroup which is being
 * destroyed after all css's are offlined and performs the rest of
 * destruction.  This is the second step of destruction described in the
 * comment above cgroup_destroy_locked().
 */
static void cgroup_destroy_css_killed(struct cgroup *cgrp)
{
	struct cgroup *parent = cgrp->parent;

	lockdep_assert_held(&cgroup_tree_mutex);
	lockdep_assert_held(&cgroup_mutex);

	/* delete this cgroup from parent->children */
	list_del_rcu(&cgrp->sibling);

	cgroup_put(cgrp);

	set_bit(CGRP_RELEASABLE, &parent->flags);
	check_for_release(parent);
}

static int cgroup_rmdir(struct kernfs_node *kn)
{
	struct cgroup *cgrp = kn->priv;
	int ret = 0;

	/*
	 * This is self-destruction but @kn can't be removed while this
	 * callback is in progress.  Let's break active protection.  Once
	 * the protection is broken, @cgrp can be destroyed at any point.
	 * Pin it so that it stays accessible.
	 */
	cgroup_get(cgrp);
	kernfs_break_active_protection(kn);

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

	/*
	 * @cgrp might already have been destroyed while we're trying to
	 * grab the mutexes.
	 */
	if (!cgroup_is_dead(cgrp))
		ret = cgroup_destroy_locked(cgrp);

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

	kernfs_unbreak_active_protection(kn);
	cgroup_put(cgrp);
	return ret;
}

static struct kernfs_syscall_ops cgroup_kf_syscall_ops = {
	.remount_fs		= cgroup_remount,
	.show_options		= cgroup_show_options,
	.mkdir			= cgroup_mkdir,
	.rmdir			= cgroup_rmdir,
	.rename			= cgroup_rename,
};

static void __init cgroup_init_subsys(struct cgroup_subsys *ss)
{
	struct cgroup_subsys_state *css;

	printk(KERN_INFO "Initializing cgroup subsys %s\n", ss->name);

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

	INIT_LIST_HEAD(&ss->cfts);

	/* Create the root cgroup state for this subsystem */
	ss->root = &cgrp_dfl_root;
	css = ss->css_alloc(cgroup_css(&cgrp_dfl_root.cgrp, ss));
	/* We don't handle early failures gracefully */
	BUG_ON(IS_ERR(css));
	init_css(css, ss, &cgrp_dfl_root.cgrp);

	/* Update the init_css_set to contain a subsys
	 * pointer to this state - since the subsystem is
	 * newly registered, all tasks and hence the
	 * init_css_set is in the subsystem's root cgroup. */
	init_css_set.subsys[ss->id] = css;

	need_forkexit_callback |= ss->fork || ss->exit;

	/* At system boot, before all subsystems have been
	 * registered, no tasks have been forked, so we don't
	 * need to invoke fork callbacks here. */
	BUG_ON(!list_empty(&init_task.tasks));

	BUG_ON(online_css(css));

	cgrp_dfl_root.subsys_mask |= 1 << ss->id;

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

/**
 * cgroup_init_early - cgroup initialization at system boot
 *
 * Initialize cgroups at system boot, and initialize any
 * subsystems that request early init.
 */
int __init cgroup_init_early(void)
{
	static struct cgroup_sb_opts __initdata opts =
		{ .flags = CGRP_ROOT_SANE_BEHAVIOR };
	struct cgroup_subsys *ss;
	int i;

	init_cgroup_root(&cgrp_dfl_root, &opts);
	RCU_INIT_POINTER(init_task.cgroups, &init_css_set);

	for_each_subsys(ss, i) {
		WARN(!ss->css_alloc || !ss->css_free || ss->name || ss->id,
		     "invalid cgroup_subsys %d:%s css_alloc=%p css_free=%p name:id=%d:%s\n",
		     i, cgroup_subsys_name[i], ss->css_alloc, ss->css_free,
		     ss->id, ss->name);
		WARN(strlen(cgroup_subsys_name[i]) > MAX_CGROUP_TYPE_NAMELEN,
		     "cgroup_subsys_name %s too long\n", cgroup_subsys_name[i]);

		ss->id = i;
		ss->name = cgroup_subsys_name[i];

		if (ss->early_init)
			cgroup_init_subsys(ss);
	}
	return 0;
}

/**
 * cgroup_init - cgroup initialization
 *
 * Register cgroup filesystem and /proc file, and initialize
 * any subsystems that didn't request early init.
 */
int __init cgroup_init(void)
{
	struct cgroup_subsys *ss;
	unsigned long key;
	int ssid, err;

	BUG_ON(cgroup_init_cftypes(NULL, cgroup_base_files));

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

	/* Add init_css_set to the hash table */
	key = css_set_hash(init_css_set.subsys);
	hash_add(css_set_table, &init_css_set.hlist, key);

	BUG_ON(cgroup_setup_root(&cgrp_dfl_root, 0));

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

	for_each_subsys(ss, ssid) {
		if (!ss->early_init)
			cgroup_init_subsys(ss);

		list_add_tail(&init_css_set.e_cset_node[ssid],
			      &cgrp_dfl_root.cgrp.e_csets[ssid]);

		/*
		 * cftype registration needs kmalloc and can't be done
		 * during early_init.  Register base cftypes separately.
		 */
		if (ss->base_cftypes)
			WARN_ON(cgroup_add_cftypes(ss, ss->base_cftypes));
	}

	cgroup_kobj = kobject_create_and_add("cgroup", fs_kobj);
	if (!cgroup_kobj)
		return -ENOMEM;

	err = register_filesystem(&cgroup_fs_type);
	if (err < 0) {
		kobject_put(cgroup_kobj);
		return err;
	}

	proc_create("cgroups", 0, NULL, &proc_cgroupstats_operations);
	return 0;
}

static int __init cgroup_wq_init(void)
{
	/*
	 * There isn't much point in executing destruction path in
	 * parallel.  Good chunk is serialized with cgroup_mutex anyway.
	 * Use 1 for @max_active.
	 *
	 * We would prefer to do this in cgroup_init() above, but that
	 * is called before init_workqueues(): so leave this until after.
	 */
	cgroup_destroy_wq = alloc_workqueue("cgroup_destroy", 0, 1);
	BUG_ON(!cgroup_destroy_wq);

	/*
	 * Used to destroy pidlists and separate to serve as flush domain.
	 * Cap @max_active to 1 too.
	 */
	cgroup_pidlist_destroy_wq = alloc_workqueue("cgroup_pidlist_destroy",
						    0, 1);
	BUG_ON(!cgroup_pidlist_destroy_wq);

	return 0;
}
core_initcall(cgroup_wq_init);

/*
 * proc_cgroup_show()
 *  - Print task's cgroup paths into seq_file, one line for each hierarchy
 *  - Used for /proc/<pid>/cgroup.
 */

/* TODO: Use a proper seq_file iterator */
int proc_cgroup_show(struct seq_file *m, void *v)
{
	struct pid *pid;
	struct task_struct *tsk;
	char *buf, *path;
	int retval;
	struct cgroup_root *root;

	retval = -ENOMEM;
	buf = kmalloc(PATH_MAX, GFP_KERNEL);
	if (!buf)
		goto out;

	retval = -ESRCH;
	pid = m->private;
	tsk = get_pid_task(pid, PIDTYPE_PID);
	if (!tsk)
		goto out_free;

	retval = 0;

	mutex_lock(&cgroup_mutex);
	down_read(&css_set_rwsem);

	for_each_root(root) {
		struct cgroup_subsys *ss;
		struct cgroup *cgrp;
		int ssid, count = 0;

		if (root == &cgrp_dfl_root && !cgrp_dfl_root_visible)
			continue;

		seq_printf(m, "%d:", root->hierarchy_id);
		for_each_subsys(ss, ssid)
			if (root->subsys_mask & (1 << ssid))
				seq_printf(m, "%s%s", count++ ? "," : "", ss->name);
		if (strlen(root->name))
			seq_printf(m, "%sname=%s", count ? "," : "",
				   root->name);
		seq_putc(m, ':');
		cgrp = task_cgroup_from_root(tsk, root);
		path = cgroup_path(cgrp, buf, PATH_MAX);
		if (!path) {
			retval = -ENAMETOOLONG;
			goto out_unlock;
		}
		seq_puts(m, path);
		seq_putc(m, '\n');
	}

out_unlock:
	up_read(&css_set_rwsem);
	mutex_unlock(&cgroup_mutex);
	put_task_struct(tsk);
out_free:
	kfree(buf);
out:
	return retval;
}

/* Display information about each subsystem and each hierarchy */
static int proc_cgroupstats_show(struct seq_file *m, void *v)
{
	struct cgroup_subsys *ss;
	int i;

	seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
	/*
	 * ideally we don't want subsystems moving around while we do this.
	 * cgroup_mutex is also necessary to guarantee an atomic snapshot of
	 * subsys/hierarchy state.
	 */
	mutex_lock(&cgroup_mutex);

	for_each_subsys(ss, i)
		seq_printf(m, "%s\t%d\t%d\t%d\n",
			   ss->name, ss->root->hierarchy_id,
			   atomic_read(&ss->root->nr_cgrps), !ss->disabled);

	mutex_unlock(&cgroup_mutex);
	return 0;
}

static int cgroupstats_open(struct inode *inode, struct file *file)
{
	return single_open(file, proc_cgroupstats_show, NULL);
}

static const struct file_operations proc_cgroupstats_operations = {
	.open = cgroupstats_open,
	.read = seq_read,
	.llseek = seq_lseek,
	.release = single_release,
};

/**
 * cgroup_fork - initialize cgroup related fields during copy_process()
 * @child: pointer to task_struct of forking parent process.
 *
 * A task is associated with the init_css_set until cgroup_post_fork()
 * attaches it to the parent's css_set.  Empty cg_list indicates that
 * @child isn't holding reference to its css_set.
 */
void cgroup_fork(struct task_struct *child)
{
	RCU_INIT_POINTER(child->cgroups, &init_css_set);
	INIT_LIST_HEAD(&child->cg_list);
}

/**
 * cgroup_post_fork - called on a new task after adding it to the task list
 * @child: the task in question
 *
 * Adds the task to the list running through its css_set if necessary and
 * call the subsystem fork() callbacks.  Has to be after the task is
 * visible on the task list in case we race with the first call to
 * cgroup_task_iter_start() - to guarantee that the new task ends up on its
 * list.
 */
void cgroup_post_fork(struct task_struct *child)
{
	struct cgroup_subsys *ss;
	int i;

	/*
	 * This may race against cgroup_enable_task_cg_links().  As that
	 * function sets use_task_css_set_links before grabbing
	 * tasklist_lock and we just went through tasklist_lock to add
	 * @child, it's guaranteed that either we see the set
	 * use_task_css_set_links or cgroup_enable_task_cg_lists() sees
	 * @child during its iteration.
	 *
	 * If we won the race, @child is associated with %current's
	 * css_set.  Grabbing css_set_rwsem guarantees both that the
	 * association is stable, and, on completion of the parent's
	 * migration, @child is visible in the source of migration or
	 * already in the destination cgroup.  This guarantee is necessary
	 * when implementing operations which need to migrate all tasks of
	 * a cgroup to another.
	 *
	 * Note that if we lose to cgroup_enable_task_cg_links(), @child
	 * will remain in init_css_set.  This is safe because all tasks are
	 * in the init_css_set before cg_links is enabled and there's no
	 * operation which transfers all tasks out of init_css_set.
	 */
	if (use_task_css_set_links) {
		struct css_set *cset;

		down_write(&css_set_rwsem);
		cset = task_css_set(current);
		if (list_empty(&child->cg_list)) {
			rcu_assign_pointer(child->cgroups, cset);
			list_add(&child->cg_list, &cset->tasks);
			get_css_set(cset);
		}
		up_write(&css_set_rwsem);
	}

	/*
	 * Call ss->fork().  This must happen after @child is linked on
	 * css_set; otherwise, @child might change state between ->fork()
	 * and addition to css_set.
	 */
	if (need_forkexit_callback) {
		for_each_subsys(ss, i)
			if (ss->fork)
				ss->fork(child);
	}
}

/**
 * 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.
 *
 * 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)
{
	struct cgroup_subsys *ss;
	struct css_set *cset;
	bool put_cset = false;
	int i;

	/*
	 * Unlink from @tsk from its css_set.  As migration path can't race
	 * with us, we can check cg_list without grabbing css_set_rwsem.
	 */
	if (!list_empty(&tsk->cg_list)) {
		down_write(&css_set_rwsem);
		list_del_init(&tsk->cg_list);
		up_write(&css_set_rwsem);
		put_cset = true;
	}

	/* Reassign the task to the init_css_set. */
	cset = task_css_set(tsk);
	RCU_INIT_POINTER(tsk->cgroups, &init_css_set);

	if (need_forkexit_callback) {
		/* see cgroup_post_fork() for details */
		for_each_subsys(ss, i) {
			if (ss->exit) {
				struct cgroup_subsys_state *old_css = cset->subsys[i];
				struct cgroup_subsys_state *css = task_css(tsk, i);

				ss->exit(css, old_css, tsk);
			}
		}
	}

	if (put_cset)
		put_css_set(cset, true);
}

static void check_for_release(struct cgroup *cgrp)
{
	if (cgroup_is_releasable(cgrp) &&
	    list_empty(&cgrp->cset_links) && list_empty(&cgrp->children)) {
		/*
		 * Control Group is currently removeable. If it's not
		 * already queued for a userspace notification, queue
		 * it now
		 */
		int need_schedule_work = 0;

		raw_spin_lock(&release_list_lock);
		if (!cgroup_is_dead(cgrp) &&
		    list_empty(&cgrp->release_list)) {
			list_add(&cgrp->release_list, &release_list);
			need_schedule_work = 1;
		}
		raw_spin_unlock(&release_list_lock);
		if (need_schedule_work)
			schedule_work(&release_agent_work);
	}
}

/*
 * Notify userspace when a cgroup is released, by running the
 * configured release agent with the name of the cgroup (path
 * relative to the root of cgroup file system) as the argument.
 *
 * Most likely, this user command will try to rmdir this cgroup.
 *
 * This races with the possibility that some other task will be
 * attached to this cgroup before it is removed, or that some other
 * user task will 'mkdir' a child cgroup of this cgroup.  That's ok.
 * The presumed 'rmdir' will fail quietly if this cgroup is no longer
 * unused, and this cgroup will be reprieved from its death sentence,
 * to continue to serve a useful existence.  Next time it's released,
 * we will get notified again, if it still has 'notify_on_release' set.
 *
 * The final arg to call_usermodehelper() is UMH_WAIT_EXEC, which
 * means only wait until the task is successfully execve()'d.  The
 * separate release agent task is forked by call_usermodehelper(),
 * then control in this thread returns here, without waiting for the
 * release agent task.  We don't bother to wait because the caller of
 * this routine has no use for the exit status of the release agent
 * task, so no sense holding our caller up for that.
 */
static void cgroup_release_agent(struct work_struct *work)
{
	BUG_ON(work != &release_agent_work);
	mutex_lock(&cgroup_mutex);
	raw_spin_lock(&release_list_lock);
	while (!list_empty(&release_list)) {
		char *argv[3], *envp[3];
		int i;
		char *pathbuf = NULL, *agentbuf = NULL, *path;
		struct cgroup *cgrp = list_entry(release_list.next,
						    struct cgroup,
						    release_list);
		list_del_init(&cgrp->release_list);
		raw_spin_unlock(&release_list_lock);
		pathbuf = kmalloc(PATH_MAX, GFP_KERNEL);
		if (!pathbuf)
			goto continue_free;
		path = cgroup_path(cgrp, pathbuf, PATH_MAX);
		if (!path)
			goto continue_free;
		agentbuf = kstrdup(cgrp->root->release_agent_path, GFP_KERNEL);
		if (!agentbuf)
			goto continue_free;

		i = 0;
		argv[i++] = agentbuf;
		argv[i++] = path;
		argv[i] = NULL;

		i = 0;
		/* minimal command environment */
		envp[i++] = "HOME=/";
		envp[i++] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
		envp[i] = NULL;

		/* Drop the lock while we invoke the usermode helper,
		 * since the exec could involve hitting disk and hence
		 * be a slow process */
		mutex_unlock(&cgroup_mutex);
		call_usermodehelper(argv[0], argv, envp, UMH_WAIT_EXEC);
		mutex_lock(&cgroup_mutex);
 continue_free:
		kfree(pathbuf);
		kfree(agentbuf);
		raw_spin_lock(&release_list_lock);
	}
	raw_spin_unlock(&release_list_lock);
	mutex_unlock(&cgroup_mutex);
}

static int __init cgroup_disable(char *str)
{
	struct cgroup_subsys *ss;
	char *token;
	int i;

	while ((token = strsep(&str, ",")) != NULL) {
		if (!*token)
			continue;

		for_each_subsys(ss, i) {
			if (!strcmp(token, ss->name)) {
				ss->disabled = 1;
				printk(KERN_INFO "Disabling %s control group"
					" subsystem\n", ss->name);
				break;
			}
		}
	}
	return 1;
}
__setup("cgroup_disable=", cgroup_disable);

/**
 * css_tryget_from_dir - get corresponding css from the dentry of a cgroup dir
 * @dentry: directory dentry of interest
 * @ss: subsystem of interest
 *
 * If @dentry is a directory for a cgroup which has @ss enabled on it, try
 * to get the corresponding css and return it.  If such css doesn't exist
 * or can't be pinned, an ERR_PTR value is returned.
 */
struct cgroup_subsys_state *css_tryget_from_dir(struct dentry *dentry,
						struct cgroup_subsys *ss)
{
	struct kernfs_node *kn = kernfs_node_from_dentry(dentry);
	struct cgroup_subsys_state *css = NULL;
	struct cgroup *cgrp;

	/* is @dentry a cgroup dir? */
	if (dentry->d_sb->s_type != &cgroup_fs_type || !kn ||
	    kernfs_type(kn) != KERNFS_DIR)
		return ERR_PTR(-EBADF);

	rcu_read_lock();

	/*
	 * This path doesn't originate from kernfs and @kn could already
	 * have been or be removed at any point.  @kn->priv is RCU
	 * protected for this access.  See destroy_locked() for details.
	 */
	cgrp = rcu_dereference(kn->priv);
	if (cgrp)
		css = cgroup_css(cgrp, ss);

	if (!css || !css_tryget(css))
		css = ERR_PTR(-ENOENT);

	rcu_read_unlock();
	return css;
}

/**
 * css_from_id - lookup css by id
 * @id: the cgroup id
 * @ss: cgroup subsys to be looked into
 *
 * Returns the css if there's valid one with @id, otherwise returns NULL.
 * Should be called under rcu_read_lock().
 */
struct cgroup_subsys_state *css_from_id(int id, struct cgroup_subsys *ss)
{
	struct cgroup *cgrp;

	cgroup_assert_mutexes_or_rcu_locked();

	cgrp = idr_find(&ss->root->cgroup_idr, id);
	if (cgrp)
		return cgroup_css(cgrp, ss);
	return NULL;
}

#ifdef CONFIG_CGROUP_DEBUG
static struct cgroup_subsys_state *
debug_css_alloc(struct cgroup_subsys_state *parent_css)
{
	struct cgroup_subsys_state *css = kzalloc(sizeof(*css), GFP_KERNEL);

	if (!css)
		return ERR_PTR(-ENOMEM);

	return css;
}

static void debug_css_free(struct cgroup_subsys_state *css)
{
	kfree(css);
}

static u64 debug_taskcount_read(struct cgroup_subsys_state *css,
				struct cftype *cft)
{
	return cgroup_task_count(css->cgroup);
}

static u64 current_css_set_read(struct cgroup_subsys_state *css,
				struct cftype *cft)
{
	return (u64)(unsigned long)current->cgroups;
}

static u64 current_css_set_refcount_read(struct cgroup_subsys_state *css,
					 struct cftype *cft)
{
	u64 count;

	rcu_read_lock();
	count = atomic_read(&task_css_set(current)->refcount);
	rcu_read_unlock();
	return count;
}

static int current_css_set_cg_links_read(struct seq_file *seq, void *v)
{
	struct cgrp_cset_link *link;
	struct css_set *cset;
	char *name_buf;

	name_buf = kmalloc(NAME_MAX + 1, GFP_KERNEL);
	if (!name_buf)
		return -ENOMEM;

	down_read(&css_set_rwsem);
	rcu_read_lock();
	cset = rcu_dereference(current->cgroups);
	list_for_each_entry(link, &cset->cgrp_links, cgrp_link) {
		struct cgroup *c = link->cgrp;

		cgroup_name(c, name_buf, NAME_MAX + 1);
		seq_printf(seq, "Root %d group %s\n",
			   c->root->hierarchy_id, name_buf);
	}
	rcu_read_unlock();
	up_read(&css_set_rwsem);
	kfree(name_buf);
	return 0;
}

#define MAX_TASKS_SHOWN_PER_CSS 25
static int cgroup_css_links_read(struct seq_file *seq, void *v)
{
	struct cgroup_subsys_state *css = seq_css(seq);
	struct cgrp_cset_link *link;

	down_read(&css_set_rwsem);
	list_for_each_entry(link, &css->cgroup->cset_links, cset_link) {
		struct css_set *cset = link->cset;
		struct task_struct *task;
		int count = 0;

		seq_printf(seq, "css_set %p\n", cset);

		list_for_each_entry(task, &cset->tasks, cg_list) {
			if (count++ > MAX_TASKS_SHOWN_PER_CSS)
				goto overflow;
			seq_printf(seq, "  task %d\n", task_pid_vnr(task));
		}

		list_for_each_entry(task, &cset->mg_tasks, cg_list) {
			if (count++ > MAX_TASKS_SHOWN_PER_CSS)
				goto overflow;
			seq_printf(seq, "  task %d\n", task_pid_vnr(task));
		}
		continue;
	overflow:
		seq_puts(seq, "  ...\n");
	}
	up_read(&css_set_rwsem);
	return 0;
}

static u64 releasable_read(struct cgroup_subsys_state *css, struct cftype *cft)
{
	return test_bit(CGRP_RELEASABLE, &css->cgroup->flags);
}

static struct cftype debug_files[] =  {
	{
		.name = "taskcount",
		.read_u64 = debug_taskcount_read,
	},

	{
		.name = "current_css_set",
		.read_u64 = current_css_set_read,
	},

	{
		.name = "current_css_set_refcount",
		.read_u64 = current_css_set_refcount_read,
	},

	{
		.name = "current_css_set_cg_links",
		.seq_show = current_css_set_cg_links_read,
	},

	{
		.name = "cgroup_css_links",
		.seq_show = cgroup_css_links_read,
	},

	{
		.name = "releasable",
		.read_u64 = releasable_read,
	},

	{ }	/* terminate */
};

struct cgroup_subsys debug_cgrp_subsys = {
	.css_alloc = debug_css_alloc,
	.css_free = debug_css_free,
	.base_cftypes = debug_files,
};
#endif /* CONFIG_CGROUP_DEBUG */