crash174.cpp

// from linux-3.11.2/kernel/cgroup.c



/*
 * cgroup_mutex is the master lock.  Any modification to cgroup or its
 * hierarchy must be performed while holding it.
 *
 * cgroup_root_mutex nests inside cgroup_mutex and should be held to modify
 * cgroupfs_root of any cgroup hierarchy - subsys list, flags,
 * release_agent_path and so on.  Modifying requires both cgroup_mutex and
 * cgroup_root_mutex.  Readers can acquire either of the two.  This is to
 * break the following locking order cycle.
 *
 *  A. cgroup_mutex -> cred_guard_mutex -> s_type->i_mutex_key -> namespace_sem
 *  B. namespace_sem -> cgroup_mutex
 *
 * B happens only through cgroup_show_options() and using cgroup_root_mutex
 * breaks it.
 */
#ifdef CONFIG_PROVE_RCU
DEFINE_MUTEX(cgroup_mutex);
EXPORT_SYMBOL_GPL(cgroup_mutex);	/* only for task_subsys_state_check() */
#else
static DEFINE_MUTEX(cgroup_mutex);
#endif

static DEFINE_MUTEX(cgroup_root_mutex);

/*
 * Generate an array of cgroup subsystem pointers. At boot time, this is
 * populated with the built in subsystems, and modular subsystems are
 * registered after that. The mutable section of this array is protected by
 * cgroup_mutex.
 */
#define SUBSYS(_x) [_x ## _subsys_id] = &_x ## _subsys,
#define IS_SUBSYS_ENABLED(option) IS_BUILTIN(option)
static struct cgroup_subsys *cgroup_subsys[CGROUP_SUBSYS_COUNT] =
{
#include <linux/cgroup_subsys.h>
};

/*
 * The dummy hierarchy, reserved for the subsystems that are otherwise
 * unattached - it never has more than a single cgroup, and all tasks are
 * part of that cgroup.
 */
static struct cgroupfs_root cgroup_dummy_root;

/* dummy_top is a shorthand for the dummy hierarchy's top cgroup */
static struct cgroup * const cgroup_dummy_top = &cgroup_dummy_root.top_cgroup;

/*
 * cgroupfs file entry, pointed to from leaf dentry->d_fsdata.
 */
struct cfent
{
    struct list_head		node;
    struct dentry			*dentry;
    struct cftype			*type;

    /* file xattrs */
    struct simple_xattrs		xattrs;
};

/*
 * CSS ID -- ID per subsys's Cgroup Subsys State(CSS). used only when
 * cgroup_subsys->use_id != 0.
 */
#define CSS_ID_MAX	(65535)
struct css_id
{
    /*
     * The css to which this ID points. This pointer is set to valid value
     * after cgroup is populated. If cgroup is removed, this will be NULL.
     * This pointer is expected to be RCU-safe because destroy()
     * is called after synchronize_rcu(). But for safe use, css_tryget()
     * should be used for avoiding race.
     */
    struct cgroup_subsys_state __rcu *css;
    /*
     * ID of this css.
     */
    unsigned short id;
    /*
     * Depth in hierarchy which this ID belongs to.
     */
    unsigned short depth;
    /*
     * ID is freed by RCU. (and lookup routine is RCU safe.)
     */
    struct rcu_head rcu_head;
    /*
     * Hierarchy of CSS ID belongs to.
     */
    unsigned short stack[0]; /* Array of Length (depth+1) */
};

/*
 * cgroup_event represents events which userspace want to receive.
 */
struct cgroup_event
{
    /*
     * Cgroup which the event belongs to.
     */
    struct cgroup *cgrp;
    /*
     * Control file which the event associated.
     */
    struct cftype *cft;
    /*
     * eventfd to signal userspace about the event.
     */
    struct eventfd_ctx *eventfd;
    /*
     * Each of these stored in a list by the cgroup.
     */
    struct list_head list;
    /*
     * All fields below needed to unregister event when
     * userspace closes eventfd.
     */
    poll_table pt;
    wait_queue_head_t *wqh;
    wait_queue_t wait;
    struct work_struct remove;
};

/* The list of hierarchy roots */

static LIST_HEAD(cgroup_roots);
static int cgroup_root_count;

/*
 * Hierarchy ID allocation and mapping.  It follows the same exclusion
 * rules as other root ops - both cgroup_mutex and cgroup_root_mutex for
 * writes, either for reads.
 */
static DEFINE_IDR(cgroup_hierarchy_idr);

static struct cgroup_name root_cgroup_name = { .name = "/" };

/*
 * Assign a monotonically increasing serial number to cgroups.  It
 * guarantees cgroups with bigger numbers are newer than those with smaller
 * numbers.  Also, as cgroups are always appended to the parent's
 * ->children list, it guarantees that sibling cgroups are always sorted in
 * the ascending serial number order on the list.  Protected by
 * cgroup_mutex.
 */
static u64 cgroup_serial_nr_next = 1;

/* This flag indicates whether tasks in the fork and exit paths should
 * check for fork/exit handlers to call. This avoids us having to do
 * extra work in the fork/exit path if none of the subsystems need to
 * be called.
 */
static int need_forkexit_callback __read_mostly;

static void cgroup_offline_fn(struct work_struct *work);
static int cgroup_destroy_locked(struct cgroup *cgrp);
static int cgroup_addrm_files(struct cgroup *cgrp, struct cgroup_subsys *subsys,
                              struct cftype cfts[], bool is_add);

/* convenient tests for these bits */
static inline bool cgroup_is_dead(const struct cgroup *cgrp)
{
    return test_bit(CGRP_DEAD, &cgrp->flags);
}

/**
 * cgroup_is_descendant - test ancestry
 * @cgrp: the cgroup to be tested
 * @ancestor: possible ancestor of @cgrp
 *
 * Test whether @cgrp is a descendant of @ancestor.  It also returns %true
 * if @cgrp == @ancestor.  This function is safe to call as long as @cgrp
 * and @ancestor are accessible.
 */
bool cgroup_is_descendant(struct cgroup *cgrp, struct cgroup *ancestor)
{
    while (cgrp)
    {
        if (cgrp == ancestor)
            return true;
        cgrp = cgrp->parent;
    }
    return false;
}
EXPORT_SYMBOL_GPL(cgroup_is_descendant);

static int cgroup_is_releasable(const struct cgroup *cgrp)
{
    const int bits =
        (1 << CGRP_RELEASABLE) |
        (1 << CGRP_NOTIFY_ON_RELEASE);
    return (cgrp->flags & bits) == bits;
}

static int notify_on_release(const struct cgroup *cgrp)
{
    return test_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
}

/**
 * for_each_subsys - iterate all loaded cgroup subsystems
 * @ss: the iteration cursor
 * @i: the index of @ss, CGROUP_SUBSYS_COUNT after reaching the end
 *
 * Should be called under cgroup_mutex.
 */
#define for_each_subsys(ss, i)						\
	for ((i) = 0; (i) < CGROUP_SUBSYS_COUNT; (i)++)			\
		if (({ lockdep_assert_held(&cgroup_mutex);		\
		       !((ss) = cgroup_subsys[i]); })) { }		\
		else

/**
 * for_each_builtin_subsys - iterate all built-in cgroup subsystems
 * @ss: the iteration cursor
 * @i: the index of @ss, CGROUP_BUILTIN_SUBSYS_COUNT after reaching the end
 *
 * Bulit-in subsystems are always present and iteration itself doesn't
 * require any synchronization.
 */
#define for_each_builtin_subsys(ss, i)					\
	for ((i) = 0; (i) < CGROUP_BUILTIN_SUBSYS_COUNT &&		\
	     (((ss) = cgroup_subsys[i]) || true); (i)++)

/* iterate each subsystem attached to a hierarchy */
#define for_each_root_subsys(root, ss)					\
	list_for_each_entry((ss), &(root)->subsys_list, sibling)

/* iterate across the active hierarchies */
#define for_each_active_root(root)					\
	list_for_each_entry((root), &cgroup_roots, root_list)

static inline struct cgroup *__d_cgrp(struct dentry *dentry)
{
    return dentry->d_fsdata;
}

static inline struct cfent *__d_cfe(struct dentry *dentry)
{
    return dentry->d_fsdata;
}

static inline struct cftype *__d_cft(struct dentry *dentry)
{
    return __d_cfe(dentry)->type;
}

/**
 * cgroup_lock_live_group - take cgroup_mutex and check that cgrp is alive.
 * @cgrp: the cgroup to be checked for liveness
 *
 * On success, returns true; the mutex should be later unlocked.  On
 * failure returns false with no lock held.
 */
static bool cgroup_lock_live_group(struct cgroup *cgrp)
{
    mutex_lock(&cgroup_mutex);
    if (cgroup_is_dead(cgrp))
    {
        mutex_unlock(&cgroup_mutex);
        return false;
    }
    return true;
}

/* the list of cgroups eligible for automatic release. Protected by
 * release_list_lock */
static LIST_HEAD(release_list);
static DEFINE_RAW_SPINLOCK(release_list_lock);
static void cgroup_release_agent(struct work_struct *work);
static DECLARE_WORK(release_agent_work, cgroup_release_agent);
static void check_for_release(struct cgroup *cgrp);

/*
 * A cgroup can be associated with multiple css_sets as different tasks may
 * belong to different cgroups on different hierarchies.  In the other
 * direction, a css_set is naturally associated with multiple cgroups.
 * This M:N relationship is represented by the following link structure
 * which exists for each association and allows traversing the associations
 * from both sides.
 */
struct cgrp_cset_link
{
    /* the cgroup and css_set this link associates */
    struct cgroup		*cgrp;
    struct css_set		*cset;

    /* list of cgrp_cset_links anchored at cgrp->cset_links */
    struct list_head	cset_link;

    /* list of cgrp_cset_links anchored at css_set->cgrp_links */
    struct list_head	cgrp_link;
};

/* The default css_set - used by init and its children prior to any
 * hierarchies being mounted. It contains a pointer to the root state
 * for each subsystem. Also used to anchor the list of css_sets. Not
 * reference-counted, to improve performance when child cgroups
 * haven't been created.
 */

static struct css_set init_css_set;
static struct cgrp_cset_link init_cgrp_cset_link;

static int cgroup_init_idr(struct cgroup_subsys *ss,
                           struct cgroup_subsys_state *css);

/* css_set_lock protects the list of css_set objects, and the
 * chain of tasks off each css_set.  Nests outside task->alloc_lock
 * due to cgroup_iter_start() */
static DEFINE_RWLOCK(css_set_lock);
static int css_set_count;

/*
 * hash table for cgroup groups. This improves the performance to find
 * an existing css_set. This hash doesn't (currently) take into
 * account cgroups in empty hierarchies.
 */
#define CSS_SET_HASH_BITS	7
static DEFINE_HASHTABLE(css_set_table, CSS_SET_HASH_BITS);

static unsigned long css_set_hash(struct cgroup_subsys_state *css[])
{
    unsigned long key = 0UL;
    struct cgroup_subsys *ss;
    int i;

    for_each_subsys(ss, i)
    key += (unsigned long)css[i];
    key = (key >> 16) ^ key;

    return key;
}

/* We don't maintain the lists running through each css_set to its
 * task until after the first call to cgroup_iter_start(). This
 * reduces the fork()/exit() overhead for people who have cgroups
 * compiled into their kernel but not actually in use */
static int use_task_css_set_links __read_mostly;

static void __put_css_set(struct css_set *cset, int taskexit)
{
    struct cgrp_cset_link *link, *tmp_link;

    /*
     * Ensure that the refcount doesn't hit zero while any readers
     * can see it. Similar to atomic_dec_and_lock(), but for an
     * rwlock
     */
    if (atomic_add_unless(&cset->refcount, -1, 1))
        return;
    write_lock(&css_set_lock);
    if (!atomic_dec_and_test(&cset->refcount))
    {
        write_unlock(&css_set_lock);
        return;
    }

    /* This css_set is dead. unlink it and release cgroup refcounts */
    hash_del(&cset->hlist);
    css_set_count--;

    list_for_each_entry_safe(link, tmp_link, &cset->cgrp_links, cgrp_link)
    {
        struct cgroup *cgrp = link->cgrp;

        list_del(&link->cset_link);
        list_del(&link->cgrp_link);

        /* @cgrp can't go away while we're holding css_set_lock */
        if (list_empty(&cgrp->cset_links) && notify_on_release(cgrp))
        {
            if (taskexit)
                set_bit(CGRP_RELEASABLE, &cgrp->flags);
            check_for_release(cgrp);
        }

        kfree(link);
    }

    write_unlock(&css_set_lock);
    kfree_rcu(cset, rcu_head);
}

/*
 * refcounted get/put for css_set objects
 */
static inline void get_css_set(struct css_set *cset)
{
    atomic_inc(&cset->refcount);
}

static inline void put_css_set(struct css_set *cset)
{
    __put_css_set(cset, 0);
}

static inline void put_css_set_taskexit(struct css_set *cset)
{
    __put_css_set(cset, 1);
}

/**
 * compare_css_sets - helper function for find_existing_css_set().
 * @cset: candidate css_set being tested
 * @old_cset: existing css_set for a task
 * @new_cgrp: cgroup that's being entered by the task
 * @template: desired set of css pointers in css_set (pre-calculated)
 *
 * Returns true if "cg" matches "old_cg" except for the hierarchy
 * which "new_cgrp" belongs to, for which it should match "new_cgrp".
 */
static bool compare_css_sets(struct css_set *cset,
                             struct css_set *old_cset,
                             struct cgroup *new_cgrp,
                             struct cgroup_subsys_state *template[])
{
    struct list_head *l1, *l2;

    if (memcmp(template, cset->subsys, sizeof(cset->subsys)))
    {
        /* Not all subsystems matched */
        return false;
    }

    /*
     * Compare cgroup pointers in order to distinguish between
     * different cgroups in heirarchies with no subsystems. We
     * could get by with just this check alone (and skip the
     * memcmp above) but on most setups the memcmp check will
     * avoid the need for this more expensive check on almost all
     * candidates.
     */

    l1 = &cset->cgrp_links;
    l2 = &old_cset->cgrp_links;
    while (1)
    {
        struct cgrp_cset_link *link1, *link2;
        struct cgroup *cgrp1, *cgrp2;

        l1 = l1->next;
        l2 = l2->next;
        /* See if we reached the end - both lists are equal length. */
        if (l1 == &cset->cgrp_links)
        {
            BUG_ON(l2 != &old_cset->cgrp_links);
            break;
        }
        else
        {
            BUG_ON(l2 == &old_cset->cgrp_links);
        }
        /* Locate the cgroups associated with these links. */
        link1 = list_entry(l1, struct cgrp_cset_link, cgrp_link);
        link2 = list_entry(l2, struct cgrp_cset_link, cgrp_link);
        cgrp1 = link1->cgrp;
        cgrp2 = link2->cgrp;
        /* Hierarchies should be linked in the same order. */
        BUG_ON(cgrp1->root != cgrp2->root);

        /*
         * If this hierarchy is the hierarchy of the cgroup
         * that's changing, then we need to check that this
         * css_set points to the new cgroup; if it's any other
         * hierarchy, then this css_set should point to the
         * same cgroup as the old css_set.
         */
        if (cgrp1->root == new_cgrp->root)
        {
            if (cgrp1 != new_cgrp)
                return false;
        }
        else
        {
            if (cgrp1 != cgrp2)
                return false;
        }
    }
    return true;
}

/**
 * find_existing_css_set - init css array and find the matching css_set
 * @old_cset: the css_set that we're using before the cgroup transition
 * @cgrp: the cgroup that we're moving into
 * @template: out param for the new set of csses, should be clear on entry
 */
static struct css_set *find_existing_css_set(struct css_set *old_cset,
        struct cgroup *cgrp,
        struct cgroup_subsys_state *template[])
{
    struct cgroupfs_root *root = cgrp->root;
    struct cgroup_subsys *ss;
    struct css_set *cset;
    unsigned long key;
    int i;

    /*
     * Build the set of subsystem state objects that we want to see in the
     * new css_set. while subsystems can change globally, the entries here
     * won't change, so no need for locking.
     */
    for_each_subsys(ss, i)
    {
        if (root->subsys_mask & (1UL << i))
        {
            /* Subsystem is in this hierarchy. So we want
             * the subsystem state from the new
             * cgroup */
            template[i] = cgrp->subsys[i];
        }
        else
        {
            /* Subsystem is not in this hierarchy, so we
             * don't want to change the subsystem state */
            template[i] = old_cset->subsys[i];
        }
    }

    key = css_set_hash(template);
    hash_for_each_possible(css_set_table, cset, hlist, key)
    {
        if (!compare_css_sets(cset, old_cset, cgrp, template))
            continue;

        /* This css_set matches what we need */
        return cset;
    }

    /* No existing cgroup group matched */
    return NULL;
}

static void free_cgrp_cset_links(struct list_head *links_to_free)
{
    struct cgrp_cset_link *link, *tmp_link;

    list_for_each_entry_safe(link, tmp_link, links_to_free, cset_link)
    {
        list_del(&link->cset_link);
        kfree(link);
    }
}

/**
 * allocate_cgrp_cset_links - allocate cgrp_cset_links
 * @count: the number of links to allocate
 * @tmp_links: list_head the allocated links are put on
 *
 * Allocate @count cgrp_cset_link structures and chain them on @tmp_links
 * through ->cset_link.  Returns 0 on success or -errno.
 */
static int allocate_cgrp_cset_links(int count, struct list_head *tmp_links)
{
    struct cgrp_cset_link *link;
    int i;

    INIT_LIST_HEAD(tmp_links);

    for (i = 0; i < count; i++)
    {
        link = kzalloc(sizeof(*link), GFP_KERNEL);
        if (!link)
        {
            free_cgrp_cset_links(tmp_links);
            return -ENOMEM;
        }
        list_add(&link->cset_link, tmp_links);
    }
    return 0;
}

/**
 * link_css_set - a helper function to link a css_set to a cgroup
 * @tmp_links: cgrp_cset_link objects allocated by allocate_cgrp_cset_links()
 * @cset: the css_set to be linked
 * @cgrp: the destination cgroup
 */
static void link_css_set(struct list_head *tmp_links, struct css_set *cset,
                         struct cgroup *cgrp)
{
    struct cgrp_cset_link *link;

    BUG_ON(list_empty(tmp_links));
    link = list_first_entry(tmp_links, struct cgrp_cset_link, cset_link);
    link->cset = cset;
    link->cgrp = cgrp;
    list_move(&link->cset_link, &cgrp->cset_links);
    /*
     * Always add links to the tail of the list so that the list
     * is sorted by order of hierarchy creation
     */
    list_add_tail(&link->cgrp_link, &cset->cgrp_links);
}

/**
 * find_css_set - return a new css_set with one cgroup updated
 * @old_cset: the baseline css_set
 * @cgrp: the cgroup to be updated
 *
 * Return a new css_set that's equivalent to @old_cset, but with @cgrp
 * substituted into the appropriate hierarchy.
 */
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;
    unsigned long key;

    lockdep_assert_held(&cgroup_mutex);

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

    atomic_set(&cset->refcount, 1);
    INIT_LIST_HEAD(&cset->cgrp_links);
    INIT_LIST_HEAD(&cset->tasks);
    INIT_HLIST_NODE(&cset->hlist);

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

    write_lock(&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 this cgroup group to the hash table */
    key = css_set_hash(cset->subsys);
    hash_add(css_set_table, &cset->hlist, key);

    write_unlock(&css_set_lock);

    return cset;
}

/*
 * Return the cgroup for "task" from the given hierarchy. Must be
 * called with cgroup_mutex held.
 */
static struct cgroup *task_cgroup_from_root(struct task_struct *task,
        struct cgroupfs_root *root)
{
    struct css_set *cset;
    struct cgroup *res = NULL;

    BUG_ON(!mutex_is_locked(&cgroup_mutex));
    read_lock(&css_set_lock);
    /*
     * 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.
     */
    cset = task_css_set(task);
    if (cset == &init_css_set)
    {
        res = &root->top_cgroup;
    }
    else
    {
        struct cgrp_cset_link *link;

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

            if (c->root == root)
            {
                res = c;
                break;
            }
        }
    }
    read_unlock(&css_set_lock);
    BUG_ON(!res);
    return res;
}

/*
 * There is one global cgroup mutex. We also require taking
 * task_lock() when dereferencing a task's cgroup subsys pointers.
 * See "The task_lock() exception", at the end of this comment.
 *
 * A task must hold cgroup_mutex to modify cgroups.
 *
 * Any task can increment and decrement the count field without lock.
 * So in general, code holding cgroup_mutex can't rely on the count
 * field not changing.  However, if the count goes to zero, then only
 * cgroup_attach_task() can increment it again.  Because a count of zero
 * means that no tasks are currently attached, therefore there is no
 * way a task attached to that cgroup can fork (the other way to
 * increment the count).  So code holding cgroup_mutex can safely
 * assume that if the count is zero, it will stay zero. Similarly, if
 * a task holds cgroup_mutex on a cgroup with zero count, it
 * knows that the cgroup won't be removed, as cgroup_rmdir()
 * needs that mutex.
 *
 * The fork and exit callbacks cgroup_fork() and cgroup_exit(), don't
 * (usually) take cgroup_mutex.  These are the two most performance
 * critical pieces of code here.  The exception occurs on cgroup_exit(),
 * when a task in a notify_on_release cgroup exits.  Then cgroup_mutex
 * is taken, and if the cgroup count is zero, a usermode call made
 * to the release agent with the name of the cgroup (path relative to
 * the root of cgroup file system) as the argument.
 *
 * A cgroup can only be deleted if both its 'count' of using tasks
 * is zero, and its list of 'children' cgroups is empty.  Since all
 * tasks in the system use _some_ cgroup, and since there is always at
 * least one task in the system (init, pid == 1), therefore, top_cgroup
 * always has either children cgroups and/or using tasks.  So we don't
 * need a special hack to ensure that top_cgroup cannot be deleted.
 *
 *	The task_lock() exception
 *
 * The need for this exception arises from the action of
 * cgroup_attach_task(), which overwrites one task's cgroup pointer with
 * another.  It does so using cgroup_mutex, however there are
 * several performance critical places that need to reference
 * task->cgroup without the expense of grabbing a system global
 * mutex.  Therefore except as noted below, when dereferencing or, as
 * in cgroup_attach_task(), modifying a task's cgroup pointer we use
 * task_lock(), which acts on a spinlock (task->alloc_lock) already in
 * the task_struct routinely used for such matters.
 *
 * P.S.  One more locking exception.  RCU is used to guard the
 * update of a tasks cgroup pointer by cgroup_attach_task()
 */

/*
 * A couple of forward declarations required, due to cyclic reference loop:
 * cgroup_mkdir -> cgroup_create -> cgroup_populate_dir ->
 * cgroup_add_file -> cgroup_create_file -> cgroup_dir_inode_operations
 * -> cgroup_mkdir.
 */

static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode);
static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry);
static int cgroup_populate_dir(struct cgroup *cgrp, bool base_files,
                               unsigned long subsys_mask);
static const struct inode_operations cgroup_dir_inode_operations;
static const struct file_operations proc_cgroupstats_operations;

static struct backing_dev_info cgroup_backing_dev_info =
{
    .name		= "cgroup",
    .capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK,
};

static int alloc_css_id(struct cgroup_subsys *ss,
                        struct cgroup *parent, struct cgroup *child);

static struct inode *cgroup_new_inode(umode_t mode, struct super_block *sb)
{
    struct inode *inode = new_inode(sb);

    if (inode)
    {
        inode->i_ino = get_next_ino();
        inode->i_mode = mode;
        inode->i_uid = current_fsuid();
        inode->i_gid = current_fsgid();
        inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
        inode->i_mapping->backing_dev_info = &cgroup_backing_dev_info;
    }
    return inode;
}

static struct cgroup_name *cgroup_alloc_name(struct dentry *dentry)
{
    struct cgroup_name *name;

    name = kmalloc(sizeof(*name) + dentry->d_name.len + 1, GFP_KERNEL);
    if (!name)
        return NULL;
    strcpy(name->name, dentry->d_name.name);
    return name;
}

static void cgroup_free_fn(struct work_struct *work)
{
    struct cgroup *cgrp = container_of(work, struct cgroup, destroy_work);
    struct cgroup_subsys *ss;

    mutex_lock(&cgroup_mutex);
    /*
     * Release the subsystem state objects.
     */
    for_each_root_subsys(cgrp->root, ss)
    ss->css_free(cgrp);

    cgrp->root->number_of_cgroups--;
    mutex_unlock(&cgroup_mutex);

    /*
     * We get a ref to the parent's dentry, and put the ref when
     * this cgroup is being freed, so it's guaranteed that the
     * parent won't be destroyed before its children.
     */
    dput(cgrp->parent->dentry);

    ida_simple_remove(&cgrp->root->cgroup_ida, cgrp->id);

    /*
     * Drop the active superblock reference that we took when we
     * created the cgroup. This will free cgrp->root, if we are
     * holding the last reference to @sb.
     */
    deactivate_super(cgrp->root->sb);

    /*
     * if we're getting rid of the cgroup, refcount should ensure
     * that there are no pidlists left.
     */
    BUG_ON(!list_empty(&cgrp->pidlists));

    simple_xattrs_free(&cgrp->xattrs);

    kfree(rcu_dereference_raw(cgrp->name));
    kfree(cgrp);
}

static void cgroup_free_rcu(struct rcu_head *head)
{
    struct cgroup *cgrp = container_of(head, struct cgroup, rcu_head);

    INIT_WORK(&cgrp->destroy_work, cgroup_free_fn);
    schedule_work(&cgrp->destroy_work);
}

static void cgroup_diput(struct dentry *dentry, struct inode *inode)
{
    /* is dentry a directory ? if so, kfree() associated cgroup */
    if (S_ISDIR(inode->i_mode))
    {
        struct cgroup *cgrp = dentry->d_fsdata;

        BUG_ON(!(cgroup_is_dead(cgrp)));
        call_rcu(&cgrp->rcu_head, cgroup_free_rcu);
    }
    else
    {
        struct cfent *cfe = __d_cfe(dentry);
        struct cgroup *cgrp = dentry->d_parent->d_fsdata;

        WARN_ONCE(!list_empty(&cfe->node) &&
                  cgrp != &cgrp->root->top_cgroup,
                  "cfe still linked for %s\n", cfe->type->name);
        simple_xattrs_free(&cfe->xattrs);
        kfree(cfe);
    }
    iput(inode);
}

static int cgroup_delete(const struct dentry *d)
{
    return 1;
}

static void remove_dir(struct dentry *d)
{
    struct dentry *parent = dget(d->d_parent);

    d_delete(d);
    simple_rmdir(parent->d_inode, d);
    dput(parent);
}

static void cgroup_rm_file(struct cgroup *cgrp, const struct cftype *cft)
{
    struct cfent *cfe;

    lockdep_assert_held(&cgrp->dentry->d_inode->i_mutex);
    lockdep_assert_held(&cgroup_mutex);

    /*
     * If we're doing cleanup due to failure of cgroup_create(),
     * the corresponding @cfe may not exist.
     */
    list_for_each_entry(cfe, &cgrp->files, node)
    {
        struct dentry *d = cfe->dentry;

        if (cft && cfe->type != cft)
            continue;

        dget(d);
        d_delete(d);
        simple_unlink(cgrp->dentry->d_inode, d);
        list_del_init(&cfe->node);
        dput(d);

        break;
    }
}

/**
 * cgroup_clear_directory - selective removal of base and subsystem files
 * @dir: directory containing the files
 * @base_files: true if the base files should be removed
 * @subsys_mask: mask of the subsystem ids whose files should be removed
 */
static void cgroup_clear_directory(struct dentry *dir, bool base_files,
                                   unsigned long subsys_mask)
{
    struct cgroup *cgrp = __d_cgrp(dir);
    struct cgroup_subsys *ss;

    for_each_root_subsys(cgrp->root, ss)
    {
        struct cftype_set *set;
        if (!test_bit(ss->subsys_id, &subsys_mask))
            continue;
        list_for_each_entry(set, &ss->cftsets, node)
        cgroup_addrm_files(cgrp, NULL, set->cfts, false);
    }
    if (base_files)
    {
        while (!list_empty(&cgrp->files))
            cgroup_rm_file(cgrp, NULL);
    }
}

/*
 * NOTE : the dentry must have been dget()'ed
 */
static void cgroup_d_remove_dir(struct dentry *dentry)
{
    struct dentry *parent;
    struct cgroupfs_root *root = dentry->d_sb->s_fs_info;

    cgroup_clear_directory(dentry, true, root->subsys_mask);

    parent = dentry->d_parent;
    spin_lock(&parent->d_lock);
    spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
    list_del_init(&dentry->d_u.d_child);
    spin_unlock(&dentry->d_lock);
    spin_unlock(&parent->d_lock);
    remove_dir(dentry);
}

/*
 * Call with cgroup_mutex held. Drops reference counts on modules, including
 * any duplicate ones that parse_cgroupfs_options took. If this function
 * returns an error, no reference counts are touched.
 */
static int rebind_subsystems(struct cgroupfs_root *root,
                             unsigned long added_mask, unsigned removed_mask)
{
    struct cgroup *cgrp = &root->top_cgroup;
    struct cgroup_subsys *ss;
    int i;

    BUG_ON(!mutex_is_locked(&cgroup_mutex));
    BUG_ON(!mutex_is_locked(&cgroup_root_mutex));

    /* Check that any added subsystems are currently free */
    for_each_subsys(ss, i)
    {
        unsigned long bit = 1UL << i;

        if (!(bit & added_mask))
            continue;

        if (ss->root != &cgroup_dummy_root)
        {
            /* Subsystem isn't free */
            return -EBUSY;
        }
    }

    /* Currently we don't handle adding/removing subsystems when
     * any child cgroups exist. This is theoretically supportable
     * but involves complex error handling, so it's being left until
     * later */
    if (root->number_of_cgroups > 1)
        return -EBUSY;

    /* Process each subsystem */
    for_each_subsys(ss, i)
    {
        unsigned long bit = 1UL << i;

        if (bit & added_mask)
        {
            /* We're binding this subsystem to this hierarchy */
            BUG_ON(cgrp->subsys[i]);
            BUG_ON(!cgroup_dummy_top->subsys[i]);
            BUG_ON(cgroup_dummy_top->subsys[i]->cgroup != cgroup_dummy_top);

            cgrp->subsys[i] = cgroup_dummy_top->subsys[i];
            cgrp->subsys[i]->cgroup = cgrp;
            list_move(&ss->sibling, &root->subsys_list);
            ss->root = root;
            if (ss->bind)
                ss->bind(cgrp);

            /* refcount was already taken, and we're keeping it */
            root->subsys_mask |= bit;
        }
        else if (bit & removed_mask)
        {
            /* We're removing this subsystem */
            BUG_ON(cgrp->subsys[i] != cgroup_dummy_top->subsys[i]);
            BUG_ON(cgrp->subsys[i]->cgroup != cgrp);

            if (ss->bind)
                ss->bind(cgroup_dummy_top);
            cgroup_dummy_top->subsys[i]->cgroup = cgroup_dummy_top;
            cgrp->subsys[i] = NULL;
            cgroup_subsys[i]->root = &cgroup_dummy_root;
            list_move(&ss->sibling, &cgroup_dummy_root.subsys_list);

            /* subsystem is now free - drop reference on module */
            module_put(ss->module);
            root->subsys_mask &= ~bit;
        }
        else if (bit & root->subsys_mask)
        {
            /* Subsystem state should already exist */
            BUG_ON(!cgrp->subsys[i]);
            /*
             * a refcount was taken, but we already had one, so
             * drop the extra reference.
             */
            module_put(ss->module);
#ifdef CONFIG_MODULE_UNLOAD
            BUG_ON(ss->module && !module_refcount(ss->module));
#endif
        }
        else
        {
            /* Subsystem state shouldn't exist */
            BUG_ON(cgrp->subsys[i]);
        }
    }

    /*
     * Mark @root has finished binding subsystems.  @root->subsys_mask
     * now matches the bound subsystems.
     */
    root->flags |= CGRP_ROOT_SUBSYS_BOUND;

    return 0;
}

static int cgroup_show_options(struct seq_file *seq, struct dentry *dentry)
{
    struct cgroupfs_root *root = dentry->d_sb->s_fs_info;
    struct cgroup_subsys *ss;

    mutex_lock(&cgroup_root_mutex);
    for_each_root_subsys(root, ss)
    seq_printf(seq, ",%s", ss->name);
    if (root->flags & CGRP_ROOT_SANE_BEHAVIOR)
        seq_puts(seq, ",sane_behavior");
    if (root->flags & CGRP_ROOT_NOPREFIX)
        seq_puts(seq, ",noprefix");
    if (root->flags & CGRP_ROOT_XATTR)
        seq_puts(seq, ",xattr");
    if (strlen(root->release_agent_path))
        seq_printf(seq, ",release_agent=%s", root->release_agent_path);
    if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->top_cgroup.flags))
        seq_puts(seq, ",clone_children");
    if (strlen(root->name))
        seq_printf(seq, ",name=%s", root->name);
    mutex_unlock(&cgroup_root_mutex);
    return 0;
}

struct cgroup_sb_opts
{
    unsigned long subsys_mask;
    unsigned long flags;
    char *release_agent;
    bool cpuset_clone_children;
    char *name;
    /* User explicitly requested empty subsystem */
    bool none;

    struct cgroupfs_root *new_root;

};

/*
 * Convert a hierarchy specifier into a bitmask of subsystems and
 * flags. Call with cgroup_mutex held to protect the cgroup_subsys[]
 * array. This function takes refcounts on subsystems to be used, unless it
 * returns error, in which case no refcounts are taken.
 */
static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)
{
    char *token, *o = data;
    bool all_ss = false, one_ss = false;
    unsigned long mask = (unsigned long)-1;
    bool module_pin_failed = false;
    struct cgroup_subsys *ss;
    int i;

    BUG_ON(!mutex_is_locked(&cgroup_mutex));

#ifdef CONFIG_CPUSETS
    mask = ~(1UL << cpuset_subsys_id);
#endif

    memset(opts, 0, sizeof(*opts));

    while ((token = strsep(&o, ",")) != NULL)
    {
        if (!*token)
            return -EINVAL;
        if (!strcmp(token, "none"))
        {
            /* Explicitly have no subsystems */
            opts->none = true;
            continue;
        }
        if (!strcmp(token, "all"))
        {
            /* Mutually exclusive option 'all' + subsystem name */
            if (one_ss)
                return -EINVAL;
            all_ss = true;
            continue;
        }
        if (!strcmp(token, "__DEVEL__sane_behavior"))
        {
            opts->flags |= CGRP_ROOT_SANE_BEHAVIOR;
            continue;
        }
        if (!strcmp(token, "noprefix"))
        {
            opts->flags |= CGRP_ROOT_NOPREFIX;
            continue;
        }
        if (!strcmp(token, "clone_children"))
        {
            opts->cpuset_clone_children = true;
            continue;
        }
        if (!strcmp(token, "xattr"))
        {
            opts->flags |= CGRP_ROOT_XATTR;
            continue;
        }
        if (!strncmp(token, "release_agent=", 14))
        {
            /* Specifying two release agents is forbidden */
            if (opts->release_agent)
                return -EINVAL;
            opts->release_agent =
            kstrndup(token + 14, PATH_MAX - 1, GFP_KERNEL);
            if (!opts->release_agent)
                return -ENOMEM;
            continue;
        }
        if (!strncmp(token, "name=", 5))
        {
            const char *name = token + 5;
            /* Can't specify an empty name */
            if (!strlen(name))
                return -EINVAL;
            /* Must match [\w.-]+ */
            for (i = 0; i < strlen(name); i++)
            {
                char c = name[i];
                if (isalnum(c))
                    continue;
                if ((c == '.') || (c == '-') || (c == '_'))
                    continue;
                return -EINVAL;
            }
            /* Specifying two names is forbidden */
            if (opts->name)
                return -EINVAL;
            opts->name = kstrndup(name,
                                  MAX_CGROUP_ROOT_NAMELEN - 1,
                                  GFP_KERNEL);
            if (!opts->name)
                return -ENOMEM;

            continue;
        }

        for_each_subsys(ss, i)
        {
            if (strcmp(token, ss->name))
                continue;
            if (ss->disabled)
                continue;

            /* Mutually exclusive option 'all' + subsystem name */
            if (all_ss)
                return -EINVAL;
            set_bit(i, &opts->subsys_mask);
            one_ss = true;

            break;
        }
        if (i == CGROUP_SUBSYS_COUNT)
            return -ENOENT;
    }

    /*
     * If the 'all' option was specified select all the subsystems,
     * otherwise if 'none', 'name=' and a subsystem name options
     * were not specified, let's default to 'all'
     */
    if (all_ss || (!one_ss && !opts->none && !opts->name))
        for_each_subsys(ss, i)
        if (!ss->disabled)
            set_bit(i, &opts->subsys_mask);

    /* Consistency checks */

    if (opts->flags & CGRP_ROOT_SANE_BEHAVIOR)
    {
        pr_warning("cgroup: sane_behavior: this is still under development and its behaviors will change, proceed at your own risk\n");

        if (opts->flags & CGRP_ROOT_NOPREFIX)
        {
            pr_err("cgroup: sane_behavior: noprefix is not allowed\n");
            return -EINVAL;
        }

        if (opts->cpuset_clone_children)
        {
            pr_err("cgroup: sane_behavior: clone_children is not allowed\n");
            return -EINVAL;
        }
    }

    /*
     * Option noprefix was introduced just for backward compatibility
     * with the old cpuset, so we allow noprefix only if mounting just
     * the cpuset subsystem.
     */
    if ((opts->flags & CGRP_ROOT_NOPREFIX) && (opts->subsys_mask & mask))
        return -EINVAL;


    /* Can't specify "none" and some subsystems */
    if (opts->subsys_mask && opts->none)
        return -EINVAL;

    /*
     * We either have to specify by name or by subsystems. (So all
     * empty hierarchies must have a name).
     */
    if (!opts->subsys_mask && !opts->name)
        return -EINVAL;

    /*
     * Grab references on all the modules we'll need, so the subsystems
     * don't dance around before rebind_subsystems attaches them. This may
     * take duplicate reference counts on a subsystem that's already used,
     * but rebind_subsystems handles this case.
     */
    for_each_subsys(ss, i)
    {
        if (!(opts->subsys_mask & (1UL << i)))
            continue;
        if (!try_module_get(cgroup_subsys[i]->module))
        {
            module_pin_failed = true;
            break;
        }
    }
    if (module_pin_failed)
    {
        /*
         * oops, one of the modules was going away. this means that we
         * raced with a module_delete call, and to the user this is
         * essentially a "subsystem doesn't exist" case.
         */
        for (i--; i >= 0; i--)
        {
            /* drop refcounts only on the ones we took */
            unsigned long bit = 1UL << i;

            if (!(bit & opts->subsys_mask))
                continue;
            module_put(cgroup_subsys[i]->module);
        }
        return -ENOENT;
    }

    return 0;
}

static void drop_parsed_module_refcounts(unsigned long subsys_mask)
{
    struct cgroup_subsys *ss;
    int i;

    mutex_lock(&cgroup_mutex);
    for_each_subsys(ss, i)
    if (subsys_mask & (1UL << i))
        module_put(cgroup_subsys[i]->module);
    mutex_unlock(&cgroup_mutex);
}

static int cgroup_remount(struct super_block *sb, int *flags, char *data)
{
    int ret = 0;
    struct cgroupfs_root *root = sb->s_fs_info;
    struct cgroup *cgrp = &root->top_cgroup;
    struct cgroup_sb_opts opts;
    unsigned long added_mask, removed_mask;

    if (root->flags & CGRP_ROOT_SANE_BEHAVIOR)
    {
        pr_err("cgroup: sane_behavior: remount is not allowed\n");
        return -EINVAL;
    }

    mutex_lock(&cgrp->dentry->d_inode->i_mutex);
    mutex_lock(&cgroup_mutex);
    mutex_lock(&cgroup_root_mutex);

    /* See what subsystems are wanted */
    ret = parse_cgroupfs_options(data, &opts);
    if (ret)
        goto out_unlock;

    if (opts.subsys_mask != root->subsys_mask || opts.release_agent)
        pr_warning("cgroup: option changes via remount are deprecated (pid=%d comm=%s)\n",
                   task_tgid_nr(current), current->comm);

    added_mask = opts.subsys_mask & ~root->subsys_mask;
    removed_mask = root->subsys_mask & ~opts.subsys_mask;

    /* Don't allow flags or name to change at remount */
    if (((opts.flags ^ root->flags) & CGRP_ROOT_OPTION_MASK) ||
            (opts.name && strcmp(opts.name, root->name)))
    {
        pr_err("cgroup: option or name mismatch, new: 0x%lx \"%s\", old: 0x%lx \"%s\"\n",
               opts.flags & CGRP_ROOT_OPTION_MASK, opts.name ?: "",
               root->flags & CGRP_ROOT_OPTION_MASK, root->name);
        ret = -EINVAL;
        goto out_unlock;
    }

    /*
     * Clear out the files of subsystems that should be removed, do
     * this before rebind_subsystems, since rebind_subsystems may
     * change this hierarchy's subsys_list.
     */
    cgroup_clear_directory(cgrp->dentry, false, removed_mask);

    ret = rebind_subsystems(root, added_mask, removed_mask);
    if (ret)
    {
        /* rebind_subsystems failed, re-populate the removed files */
        cgroup_populate_dir(cgrp, false, removed_mask);
        goto out_unlock;
    }

    /* re-populate subsystem files */
    cgroup_populate_dir(cgrp, false, added_mask);

    if (opts.release_agent)
        strcpy(root->release_agent_path, opts.release_agent);
out_unlock:
    kfree(opts.release_agent);
    kfree(opts.name);
    mutex_unlock(&cgroup_root_mutex);
    mutex_unlock(&cgroup_mutex);
    mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
    if (ret)
        drop_parsed_module_refcounts(opts.subsys_mask);
    return ret;
}

static const struct super_operations cgroup_ops =
{
    .statfs = simple_statfs,
    .drop_inode = generic_delete_inode,
    .show_options = cgroup_show_options,
    .remount_fs = cgroup_remount,
};

static void init_cgroup_housekeeping(struct cgroup *cgrp)
{
    INIT_LIST_HEAD(&cgrp->sibling);
    INIT_LIST_HEAD(&cgrp->children);
    INIT_LIST_HEAD(&cgrp->files);
    INIT_LIST_HEAD(&cgrp->cset_links);
    INIT_LIST_HEAD(&cgrp->release_list);
    INIT_LIST_HEAD(&cgrp->pidlists);
    mutex_init(&cgrp->pidlist_mutex);
    INIT_LIST_HEAD(&cgrp->event_list);
    spin_lock_init(&cgrp->event_list_lock);
    simple_xattrs_init(&cgrp->xattrs);
}

static void init_cgroup_root(struct cgroupfs_root *root)
{
    struct cgroup *cgrp = &root->top_cgroup;

    INIT_LIST_HEAD(&root->subsys_list);
    INIT_LIST_HEAD(&root->root_list);
    root->number_of_cgroups = 1;
    cgrp->root = root;
    RCU_INIT_POINTER(cgrp->name, &root_cgroup_name);
    init_cgroup_housekeeping(cgrp);
}

static int cgroup_init_root_id(struct cgroupfs_root *root, int start, int end)
{
    int id;

    lockdep_assert_held(&cgroup_mutex);
    lockdep_assert_held(&cgroup_root_mutex);

    id = idr_alloc_cyclic(&cgroup_hierarchy_idr, root, start, end,
                          GFP_KERNEL);
    if (id < 0)
        return id;

    root->hierarchy_id = id;
    return 0;
}

static void cgroup_exit_root_id(struct cgroupfs_root *root)
{
    lockdep_assert_held(&cgroup_mutex);
    lockdep_assert_held(&cgroup_root_mutex);

    if (root->hierarchy_id)
    {
        idr_remove(&cgroup_hierarchy_idr, root->hierarchy_id);
        root->hierarchy_id = 0;
    }
}

static int cgroup_test_super(struct super_block *sb, void *data)
{
    struct cgroup_sb_opts *opts = data;
    struct cgroupfs_root *root = sb->s_fs_info;

    /* If we asked for a name then it must match */
    if (opts->name && strcmp(opts->name, root->name))
        return 0;

    /*
     * If we asked for subsystems (or explicitly for no
     * subsystems) then they must match
     */
    if ((opts->subsys_mask || opts->none)
            && (opts->subsys_mask != root->subsys_mask))
        return 0;

    return 1;
}

static struct cgroupfs_root *cgroup_root_from_opts(struct cgroup_sb_opts *opts)
{
    struct cgroupfs_root *root;

    if (!opts->subsys_mask && !opts->none)
        return NULL;

    root = kzalloc(sizeof(*root), GFP_KERNEL);
    if (!root)
        return ERR_PTR(-ENOMEM);

    init_cgroup_root(root);

    /*
     * We need to set @root->subsys_mask now so that @root can be
     * matched by cgroup_test_super() before it finishes
     * initialization; otherwise, competing mounts with the same
     * options may try to bind the same subsystems instead of waiting
     * for the first one leading to unexpected mount errors.
     * SUBSYS_BOUND will be set once actual binding is complete.
     */
    root->subsys_mask = opts->subsys_mask;
    root->flags = opts->flags;
    ida_init(&root->cgroup_ida);
    if (opts->release_agent)
        strcpy(root->release_agent_path, opts->release_agent);
    if (opts->name)
        strcpy(root->name, opts->name);
    if (opts->cpuset_clone_children)
        set_bit(CGRP_CPUSET_CLONE_CHILDREN, &root->top_cgroup.flags);
    return root;
}

static void cgroup_free_root(struct cgroupfs_root *root)
{
    if (root)
    {
        /* hierarhcy ID shoulid already have been released */
        WARN_ON_ONCE(root->hierarchy_id);

        ida_destroy(&root->cgroup_ida);
        kfree(root);
    }
}

static int cgroup_set_super(struct super_block *sb, void *data)
{
    int ret;
    struct cgroup_sb_opts *opts = data;

    /* If we don't have a new root, we can't set up a new sb */
    if (!opts->new_root)
        return -EINVAL;

    BUG_ON(!opts->subsys_mask && !opts->none);

    ret = set_anon_super(sb, NULL);
    if (ret)
        return ret;

    sb->s_fs_info = opts->new_root;
    opts->new_root->sb = sb;

    sb->s_blocksize = PAGE_CACHE_SIZE;
    sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
    sb->s_magic = CGROUP_SUPER_MAGIC;
    sb->s_op = &cgroup_ops;

    return 0;
}

static int cgroup_get_rootdir(struct super_block *sb)
{
    static const struct dentry_operations cgroup_dops =
    {
        .d_iput = cgroup_diput,
        .d_delete = cgroup_delete,
    };

    struct inode *inode =
    cgroup_new_inode(S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR, sb);

    if (!inode)
        return -ENOMEM;

    inode->i_fop = &simple_dir_operations;
    inode->i_op = &cgroup_dir_inode_operations;
    /* directories start off with i_nlink == 2 (for "." entry) */
    inc_nlink(inode);
    sb->s_root = d_make_root(inode);
    if (!sb->s_root)
        return -ENOMEM;
    /* for everything else we want ->d_op set */
    sb->s_d_op = &cgroup_dops;
    return 0;
}

static struct dentry *cgroup_mount(struct file_system_type *fs_type,
                                   int flags, const char *unused_dev_name,
                                   void *data)
{
    struct cgroup_sb_opts opts;
    struct cgroupfs_root *root;
    int ret = 0;
    struct super_block *sb;
    struct cgroupfs_root *new_root;
    struct inode *inode;

    /* First find the desired set of subsystems */
    mutex_lock(&cgroup_mutex);
    ret = parse_cgroupfs_options(data, &opts);
    mutex_unlock(&cgroup_mutex);
    if (ret)
        goto out_err;

    /*
     * Allocate a new cgroup root. We may not need it if we're
     * reusing an existing hierarchy.
     */
    new_root = cgroup_root_from_opts(&opts);
    if (IS_ERR(new_root))
    {
        ret = PTR_ERR(new_root);
        goto drop_modules;
    }
    opts.new_root = new_root;

    /* Locate an existing or new sb for this hierarchy */
    sb = sget(fs_type, cgroup_test_super, cgroup_set_super, 0, &opts);
    if (IS_ERR(sb))
    {
        ret = PTR_ERR(sb);
        cgroup_free_root(opts.new_root);
        goto drop_modules;
    }

    root = sb->s_fs_info;
    BUG_ON(!root);
    if (root == opts.new_root)
    {
        /* We used the new root structure, so this is a new hierarchy */
        struct list_head tmp_links;
        struct cgroup *root_cgrp = &root->top_cgroup;
        struct cgroupfs_root *existing_root;
        const struct cred *cred;
        int i;
        struct css_set *cset;

        BUG_ON(sb->s_root != NULL);

        ret = cgroup_get_rootdir(sb);
        if (ret)
            goto drop_new_super;
        inode = sb->s_root->d_inode;

        mutex_lock(&inode->i_mutex);
        mutex_lock(&cgroup_mutex);
        mutex_lock(&cgroup_root_mutex);

        /* Check for name clashes with existing mounts */
        ret = -EBUSY;
        if (strlen(root->name))
            for_each_active_root(existing_root)
            if (!strcmp(existing_root->name, root->name))
                goto unlock_drop;

        /*
         * We're accessing css_set_count without locking
         * css_set_lock here, but that's OK - it can only be
         * increased by someone holding cgroup_lock, and
         * that's us. The worst that can happen is that we
         * have some link structures left over
         */
        ret = allocate_cgrp_cset_links(css_set_count, &tmp_links);
        if (ret)
            goto unlock_drop;

        /* ID 0 is reserved for dummy root, 1 for unified hierarchy */
        ret = cgroup_init_root_id(root, 2, 0);
        if (ret)
            goto unlock_drop;

        ret = rebind_subsystems(root, root->subsys_mask, 0);
        if (ret == -EBUSY)
        {
            free_cgrp_cset_links(&tmp_links);
            goto unlock_drop;
        }
        /*
         * There must be no failure case after here, since rebinding
         * takes care of subsystems' refcounts, which are explicitly
         * dropped in the failure exit path.
         */

        /* EBUSY should be the only error here */
        BUG_ON(ret);

        list_add(&root->root_list, &cgroup_roots);
        cgroup_root_count++;

        sb->s_root->d_fsdata = root_cgrp;
        root->top_cgroup.dentry = sb->s_root;

        /* Link the top cgroup in this hierarchy into all
         * the css_set objects */
        write_lock(&css_set_lock);
        hash_for_each(css_set_table, i, cset, hlist)
        link_css_set(&tmp_links, cset, root_cgrp);
        write_unlock(&css_set_lock);

        free_cgrp_cset_links(&tmp_links);

        BUG_ON(!list_empty(&root_cgrp->children));
        BUG_ON(root->number_of_cgroups != 1);

        cred = override_creds(&init_cred);
        cgroup_populate_dir(root_cgrp, true, root->subsys_mask);
        revert_creds(cred);
        mutex_unlock(&cgroup_root_mutex);
        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&inode->i_mutex);
    }
    else
    {
        /*
         * We re-used an existing hierarchy - the new root (if
         * any) is not needed
         */
        cgroup_free_root(opts.new_root);

        if ((root->flags ^ opts.flags) & CGRP_ROOT_OPTION_MASK)
        {
            if ((root->flags | opts.flags) & CGRP_ROOT_SANE_BEHAVIOR)
            {
                pr_err("cgroup: sane_behavior: new mount options should match the existing superblock\n");
                ret = -EINVAL;
                goto drop_new_super;
            }
            else
            {
                pr_warning("cgroup: new mount options do not match the existing superblock, will be ignored\n");
            }
        }

        /* no subsys rebinding, so refcounts don't change */
        drop_parsed_module_refcounts(opts.subsys_mask);
    }

    kfree(opts.release_agent);
    kfree(opts.name);
    return dget(sb->s_root);

unlock_drop:
    cgroup_exit_root_id(root);
    mutex_unlock(&cgroup_root_mutex);
    mutex_unlock(&cgroup_mutex);
    mutex_unlock(&inode->i_mutex);
drop_new_super:
    deactivate_locked_super(sb);
drop_modules:
    drop_parsed_module_refcounts(opts.subsys_mask);
out_err:
    kfree(opts.release_agent);
    kfree(opts.name);
    return ERR_PTR(ret);
}

static void cgroup_kill_sb(struct super_block *sb)
{
    struct cgroupfs_root *root = sb->s_fs_info;
    struct cgroup *cgrp = &root->top_cgroup;
    struct cgrp_cset_link *link, *tmp_link;
    int ret;

    BUG_ON(!root);

    BUG_ON(root->number_of_cgroups != 1);
    BUG_ON(!list_empty(&cgrp->children));

    mutex_lock(&cgroup_mutex);
    mutex_lock(&cgroup_root_mutex);

    /* Rebind all subsystems back to the default hierarchy */
    if (root->flags & CGRP_ROOT_SUBSYS_BOUND)
    {
        ret = rebind_subsystems(root, 0, root->subsys_mask);
        /* Shouldn't be able to fail ... */
        BUG_ON(ret);
    }

    /*
     * Release all the links from cset_links to this hierarchy's
     * root cgroup
     */
    write_lock(&css_set_lock);

    list_for_each_entry_safe(link, tmp_link, &cgrp->cset_links, cset_link)
    {
        list_del(&link->cset_link);
        list_del(&link->cgrp_link);
        kfree(link);
    }
    write_unlock(&css_set_lock);

    if (!list_empty(&root->root_list))
    {
        list_del(&root->root_list);
        cgroup_root_count--;
    }

    cgroup_exit_root_id(root);

    mutex_unlock(&cgroup_root_mutex);
    mutex_unlock(&cgroup_mutex);

    simple_xattrs_free(&cgrp->xattrs);

    kill_litter_super(sb);
    cgroup_free_root(root);
}

static struct file_system_type cgroup_fs_type =
{
    .name = "cgroup",
    .mount = cgroup_mount,
    .kill_sb = cgroup_kill_sb,
};

static struct kobject *cgroup_kobj;

/**
 * cgroup_path - generate the path of a cgroup
 * @cgrp: the cgroup in question
 * @buf: the buffer to write the path into
 * @buflen: the length of the buffer
 *
 * Writes path of cgroup into buf.  Returns 0 on success, -errno on error.
 *
 * We can't generate cgroup path using dentry->d_name, as accessing
 * dentry->name must be protected by irq-unsafe dentry->d_lock or parent
 * inode's i_mutex, while on the other hand cgroup_path() can be called
 * with some irq-safe spinlocks held.
 */
int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
{
    int ret = -ENAMETOOLONG;
    char *start;

    if (!cgrp->parent)
    {
        if (strlcpy(buf, "/", buflen) >= buflen)
            return -ENAMETOOLONG;
        return 0;
    }

    start = buf + buflen - 1;
    *start = '\0';

    rcu_read_lock();
    do
    {
        const char *name = cgroup_name(cgrp);
        int len;

        len = strlen(name);
        if ((start -= len) < buf)
            goto out;
        memcpy(start, name, len);

        if (--start < buf)
            goto out;
        *start = '/';

        cgrp = cgrp->parent;
    }
    while (cgrp->parent);
    ret = 0;
    memmove(buf, start, buf + buflen - start);
out:
    rcu_read_unlock();
    return ret;
}
EXPORT_SYMBOL_GPL(cgroup_path);

/**
 * task_cgroup_path - cgroup path of a task in the first cgroup hierarchy
 * @task: target task
 * @buf: the buffer to write the path into
 * @buflen: the length of the buffer
 *
 * Determine @task's cgroup on the first (the one with the lowest non-zero
 * hierarchy_id) cgroup hierarchy and copy its path into @buf.  This
 * function grabs cgroup_mutex and shouldn't be used inside locks used by
 * cgroup controller callbacks.
 *
 * Returns 0 on success, fails with -%ENAMETOOLONG if @buflen is too short.
 */
int task_cgroup_path(struct task_struct *task, char *buf, size_t buflen)
{
    struct cgroupfs_root *root;
    struct cgroup *cgrp;
    int hierarchy_id = 1, ret = 0;

    if (buflen < 2)
        return -ENAMETOOLONG;

    mutex_lock(&cgroup_mutex);

    root = idr_get_next(&cgroup_hierarchy_idr, &hierarchy_id);

    if (root)
    {
        cgrp = task_cgroup_from_root(task, root);
        ret = cgroup_path(cgrp, buf, buflen);
    }
    else
    {
        /* if no hierarchy exists, everyone is in "/" */
        memcpy(buf, "/", 2);
    }

    mutex_unlock(&cgroup_mutex);
    return ret;
}
EXPORT_SYMBOL_GPL(task_cgroup_path);

/*
 * Control Group taskset
 */
struct task_and_cgroup
{
    struct task_struct	*task;
    struct cgroup		*cgrp;
    struct css_set		*cg;
};

struct cgroup_taskset
{
    struct task_and_cgroup	single;
    struct flex_array	*tc_array;
    int			tc_array_len;
    int			idx;
    struct cgroup		*cur_cgrp;
};

/**
 * cgroup_taskset_first - reset taskset and return the first task
 * @tset: taskset of interest
 *
 * @tset iteration is initialized and the first task is returned.
 */
struct task_struct *cgroup_taskset_first(struct cgroup_taskset *tset)
{
    if (tset->tc_array)
    {
        tset->idx = 0;
        return cgroup_taskset_next(tset);
    }
    else
    {
        tset->cur_cgrp = tset->single.cgrp;
        return tset->single.task;
    }
}
EXPORT_SYMBOL_GPL(cgroup_taskset_first);

/**
 * cgroup_taskset_next - iterate to the next task in taskset
 * @tset: taskset of interest
 *
 * Return the next task in @tset.  Iteration must have been initialized
 * with cgroup_taskset_first().
 */
struct task_struct *cgroup_taskset_next(struct cgroup_taskset *tset)
{
    struct task_and_cgroup *tc;

    if (!tset->tc_array || tset->idx >= tset->tc_array_len)
        return NULL;

    tc = flex_array_get(tset->tc_array, tset->idx++);
    tset->cur_cgrp = tc->cgrp;
    return tc->task;
}
EXPORT_SYMBOL_GPL(cgroup_taskset_next);

/**
 * cgroup_taskset_cur_cgroup - return the matching cgroup for the current task
 * @tset: taskset of interest
 *
 * Return the cgroup for the current (last returned) task of @tset.  This
 * function must be preceded by either cgroup_taskset_first() or
 * cgroup_taskset_next().
 */
struct cgroup *cgroup_taskset_cur_cgroup(struct cgroup_taskset *tset)
{
    return tset->cur_cgrp;
}
EXPORT_SYMBOL_GPL(cgroup_taskset_cur_cgroup);

/**
 * cgroup_taskset_size - return the number of tasks in taskset
 * @tset: taskset of interest
 */
int cgroup_taskset_size(struct cgroup_taskset *tset)
{
    return tset->tc_array ? tset->tc_array_len : 1;
}
EXPORT_SYMBOL_GPL(cgroup_taskset_size);


/*
 * cgroup_task_migrate - move a task from one cgroup to another.
 *
 * Must be called with cgroup_mutex and threadgroup locked.
 */
static void cgroup_task_migrate(struct cgroup *old_cgrp,
                                struct task_struct *tsk,
                                struct css_set *new_cset)
{
    struct css_set *old_cset;

    /*
     * We are synchronized through threadgroup_lock() 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.
     */
    WARN_ON_ONCE(tsk->flags & PF_EXITING);
    old_cset = task_css_set(tsk);

    task_lock(tsk);
    rcu_assign_pointer(tsk->cgroups, new_cset);
    task_unlock(tsk);

    /* Update the css_set linked lists if we're using them */
    write_lock(&css_set_lock);
    if (!list_empty(&tsk->cg_list))
        list_move(&tsk->cg_list, &new_cset->tasks);
    write_unlock(&css_set_lock);

    /*
     * We just gained a reference on old_cset by taking it from the
     * task. As trading it for new_cset is protected by cgroup_mutex,
     * we're safe to drop it here; it will be freed under RCU.
     */
    set_bit(CGRP_RELEASABLE, &old_cgrp->flags);
    put_css_set(old_cset);
}

/**
 * cgroup_attach_task - attach a task or a whole threadgroup to a cgroup
 * @cgrp: the cgroup to attach to
 * @tsk: the task or the leader of the threadgroup to be attached
 * @threadgroup: attach the whole threadgroup?
 *
 * Call holding cgroup_mutex and the group_rwsem of the leader. Will take
 * task_lock of @tsk or each thread in the threadgroup individually in turn.
 */
static int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk,
                              bool threadgroup)
{
    int retval, i, group_size;
    struct cgroup_subsys *ss, *failed_ss = NULL;
    struct cgroupfs_root *root = cgrp->root;
    /* threadgroup list cursor and array */
    struct task_struct *leader = tsk;
    struct task_and_cgroup *tc;
    struct flex_array *group;
    struct cgroup_taskset tset = { };

    /*
     * step 0: in order to do expensive, possibly blocking operations for
     * every thread, we cannot iterate the thread group list, since it needs
     * rcu or tasklist locked. instead, build an array of all threads in the
     * group - group_rwsem prevents new threads from appearing, and if
     * threads exit, this will just be an over-estimate.
     */
    if (threadgroup)
        group_size = get_nr_threads(tsk);
    else
        group_size = 1;
    /* flex_array supports very large thread-groups better than kmalloc. */
    group = flex_array_alloc(sizeof(*tc), group_size, GFP_KERNEL);
    if (!group)
        return -ENOMEM;
    /* pre-allocate to guarantee space while iterating in rcu read-side. */
    retval = flex_array_prealloc(group, 0, group_size, GFP_KERNEL);
    if (retval)
        goto out_free_group_list;

    i = 0;
    /*
     * Prevent freeing of tasks while we take a snapshot. Tasks that are
     * already PF_EXITING could be freed from underneath us unless we
     * take an rcu_read_lock.
     */
    rcu_read_lock();
    do
    {
        struct task_and_cgroup ent;

        /* @tsk either already exited or can't exit until the end */
        if (tsk->flags & PF_EXITING)
            continue;

        /* as per above, nr_threads may decrease, but not increase. */
        BUG_ON(i >= group_size);
        ent.task = tsk;
        ent.cgrp = task_cgroup_from_root(tsk, root);
        /* nothing to do if this task is already in the cgroup */
        if (ent.cgrp == cgrp)
            continue;
        /*
         * saying GFP_ATOMIC has no effect here because we did prealloc
         * earlier, but it's good form to communicate our expectations.
         */
        retval = flex_array_put(group, i, &ent, GFP_ATOMIC);
        BUG_ON(retval != 0);
        i++;

        if (!threadgroup)
            break;
    }
    while_each_thread(leader, tsk);
    rcu_read_unlock();
    /* remember the number of threads in the array for later. */
    group_size = i;
    tset.tc_array = group;
    tset.tc_array_len = group_size;

    /* methods shouldn't be called if no task is actually migrating */
    retval = 0;
    if (!group_size)
        goto out_free_group_list;

    /*
     * step 1: check that we can legitimately attach to the cgroup.
     */
    for_each_root_subsys(root, ss)
    {
        if (ss->can_attach)
        {
            retval = ss->can_attach(cgrp, &tset);
            if (retval)
            {
                failed_ss = ss;
                goto out_cancel_attach;
            }
        }
    }

    /*
     * step 2: make sure css_sets exist for all threads to be migrated.
     * we use find_css_set, which allocates a new one if necessary.
     */
    for (i = 0; i < group_size; i++)
    {
        struct css_set *old_cset;

        tc = flex_array_get(group, i);
        old_cset = task_css_set(tc->task);
        tc->cg = find_css_set(old_cset, cgrp);
        if (!tc->cg)
        {
            retval = -ENOMEM;
            goto out_put_css_set_refs;
        }
    }

    /*
     * step 3: now that we're guaranteed success wrt the css_sets,
     * proceed to move all tasks to the new cgroup.  There are no
     * failure cases after here, so this is the commit point.
     */
    for (i = 0; i < group_size; i++)
    {
        tc = flex_array_get(group, i);
        cgroup_task_migrate(tc->cgrp, tc->task, tc->cg);
    }
    /* nothing is sensitive to fork() after this point. */

    /*
     * step 4: do subsystem attach callbacks.
     */
    for_each_root_subsys(root, ss)
    {
        if (ss->attach)
            ss->attach(cgrp, &tset);
    }

    /*
     * step 5: success! and cleanup
     */
    retval = 0;
out_put_css_set_refs:
    if (retval)
    {
        for (i = 0; i < group_size; i++)
        {
            tc = flex_array_get(group, i);
            if (!tc->cg)
                break;
            put_css_set(tc->cg);
        }
    }
out_cancel_attach:
    if (retval)
    {
        for_each_root_subsys(root, ss)
        {
            if (ss == failed_ss)
                break;
            if (ss->cancel_attach)
                ss->cancel_attach(cgrp, &tset);
        }
    }
out_free_group_list:
    flex_array_free(group);
    return retval;
}

/*
 * 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; may take task_lock of task.
 */
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 cgroupfs_root *root;
    int retval = 0;

    mutex_lock(&cgroup_mutex);
    for_each_active_root(root)
    {
        struct cgroup *from_cg = task_cgroup_from_root(from, root);

        retval = cgroup_attach_task(from_cg, 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 *cgrp, struct cftype *cft, u64 pid)
{
    return attach_task_by_pid(cgrp, pid, false);
}

static int cgroup_procs_write(struct cgroup *cgrp, struct cftype *cft, u64 tgid)
{
    return attach_task_by_pid(cgrp, tgid, true);
}

static int cgroup_release_agent_write(struct cgroup *cgrp, struct cftype *cft,
                                      const char *buffer)
{
    BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
    if (strlen(buffer) >= PATH_MAX)
        return -EINVAL;
    if (!cgroup_lock_live_group(cgrp))
        return -ENODEV;
    mutex_lock(&cgroup_root_mutex);
    strcpy(cgrp->root->release_agent_path, buffer);
    mutex_unlock(&cgroup_root_mutex);
    mutex_unlock(&cgroup_mutex);
    return 0;
}

static int cgroup_release_agent_show(struct cgroup *cgrp, struct cftype *cft,
                                     struct seq_file *seq)
{
    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 cgroup *cgrp, struct cftype *cft,
                                     struct seq_file *seq)
{
    seq_printf(seq, "%d\n", cgroup_sane_behavior(cgrp));
    return 0;
}

/* A buffer size big enough for numbers or short strings */
#define CGROUP_LOCAL_BUFFER_SIZE 64

static ssize_t cgroup_write_X64(struct cgroup *cgrp, struct cftype *cft,
                                struct file *file,
                                const char __user *userbuf,
                                size_t nbytes, loff_t *unused_ppos)
{
    char buffer[CGROUP_LOCAL_BUFFER_SIZE];
    int retval = 0;
    char *end;

    if (!nbytes)
        return -EINVAL;
    if (nbytes >= sizeof(buffer))
        return -E2BIG;
    if (copy_from_user(buffer, userbuf, nbytes))
        return -EFAULT;

    buffer[nbytes] = 0;     /* nul-terminate */
    if (cft->write_u64)
    {
        u64 val = simple_strtoull(strstrip(buffer), &end, 0);
        if (*end)
            return -EINVAL;
        retval = cft->write_u64(cgrp, cft, val);
    }
    else
    {
        s64 val = simple_strtoll(strstrip(buffer), &end, 0);
        if (*end)
            return -EINVAL;
        retval = cft->write_s64(cgrp, cft, val);
    }
    if (!retval)
        retval = nbytes;
    return retval;
}

static ssize_t cgroup_write_string(struct cgroup *cgrp, struct cftype *cft,
                                   struct file *file,
                                   const char __user *userbuf,
                                   size_t nbytes, loff_t *unused_ppos)
{
    char local_buffer[CGROUP_LOCAL_BUFFER_SIZE];
    int retval = 0;
    size_t max_bytes = cft->max_write_len;
    char *buffer = local_buffer;

    if (!max_bytes)
        max_bytes = sizeof(local_buffer) - 1;
    if (nbytes >= max_bytes)
        return -E2BIG;
    /* Allocate a dynamic buffer if we need one */
    if (nbytes >= sizeof(local_buffer))
    {
        buffer = kmalloc(nbytes + 1, GFP_KERNEL);
        if (buffer == NULL)
            return -ENOMEM;
    }
    if (nbytes && copy_from_user(buffer, userbuf, nbytes))
    {
        retval = -EFAULT;
        goto out;
    }

    buffer[nbytes] = 0;     /* nul-terminate */
    retval = cft->write_string(cgrp, cft, strstrip(buffer));
    if (!retval)
        retval = nbytes;
out:
    if (buffer != local_buffer)
        kfree(buffer);
    return retval;
}

static ssize_t cgroup_file_write(struct file *file, const char __user *buf,
                                 size_t nbytes, loff_t *ppos)
{
    struct cftype *cft = __d_cft(file->f_dentry);
    struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);

    if (cgroup_is_dead(cgrp))
        return -ENODEV;
    if (cft->write)
        return cft->write(cgrp, cft, file, buf, nbytes, ppos);
    if (cft->write_u64 || cft->write_s64)
        return cgroup_write_X64(cgrp, cft, file, buf, nbytes, ppos);
    if (cft->write_string)
        return cgroup_write_string(cgrp, cft, file, buf, nbytes, ppos);
    if (cft->trigger)
    {
        int ret = cft->trigger(cgrp, (unsigned int)cft->private);
        return ret ? ret : nbytes;
    }
    return -EINVAL;
}

static ssize_t cgroup_read_u64(struct cgroup *cgrp, struct cftype *cft,
                               struct file *file,
                               char __user *buf, size_t nbytes,
                               loff_t *ppos)
{
    char tmp[CGROUP_LOCAL_BUFFER_SIZE];
    u64 val = cft->read_u64(cgrp, cft);
    int len = sprintf(tmp, "%llu\n", (unsigned long long) val);

    return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
}

static ssize_t cgroup_read_s64(struct cgroup *cgrp, struct cftype *cft,
                               struct file *file,
                               char __user *buf, size_t nbytes,
                               loff_t *ppos)
{
    char tmp[CGROUP_LOCAL_BUFFER_SIZE];
    s64 val = cft->read_s64(cgrp, cft);
    int len = sprintf(tmp, "%lld\n", (long long) val);

    return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
}

static ssize_t cgroup_file_read(struct file *file, char __user *buf,
                                size_t nbytes, loff_t *ppos)
{
    struct cftype *cft = __d_cft(file->f_dentry);
    struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);

    if (cgroup_is_dead(cgrp))
        return -ENODEV;

    if (cft->read)
        return cft->read(cgrp, cft, file, buf, nbytes, ppos);
    if (cft->read_u64)
        return cgroup_read_u64(cgrp, cft, file, buf, nbytes, ppos);
    if (cft->read_s64)
        return cgroup_read_s64(cgrp, cft, file, buf, nbytes, ppos);
    return -EINVAL;
}

/*
 * seqfile ops/methods for returning structured data. Currently just
 * supports string->u64 maps, but can be extended in future.
 */

struct cgroup_seqfile_state
{
    struct cftype *cft;
    struct cgroup *cgroup;
};

static int cgroup_map_add(struct cgroup_map_cb *cb, const char *key, u64 value)
{
    struct seq_file *sf = cb->state;
    return seq_printf(sf, "%s %llu\n", key, (unsigned long long)value);
}

static int cgroup_seqfile_show(struct seq_file *m, void *arg)
{
    struct cgroup_seqfile_state *state = m->private;
    struct cftype *cft = state->cft;
    if (cft->read_map)
    {
        struct cgroup_map_cb cb =
        {
            .fill = cgroup_map_add,
            .state = m,
        };
        return cft->read_map(state->cgroup, cft, &cb);
    }
    return cft->read_seq_string(state->cgroup, cft, m);
}

static int cgroup_seqfile_release(struct inode *inode, struct file *file)
{
    struct seq_file *seq = file->private_data;
    kfree(seq->private);
    return single_release(inode, file);
}

static const struct file_operations cgroup_seqfile_operations =
{
    .read = seq_read,
    .write = cgroup_file_write,
    .llseek = seq_lseek,
    .release = cgroup_seqfile_release,
};

static int cgroup_file_open(struct inode *inode, struct file *file)
{
    int err;
    struct cftype *cft;

    err = generic_file_open(inode, file);
    if (err)
        return err;
    cft = __d_cft(file->f_dentry);

    if (cft->read_map || cft->read_seq_string)
    {
        struct cgroup_seqfile_state *state;

        state = kzalloc(sizeof(*state), GFP_USER);
        if (!state)
            return -ENOMEM;

        state->cft = cft;
        state->cgroup = __d_cgrp(file->f_dentry->d_parent);
        file->f_op = &cgroup_seqfile_operations;
        err = single_open(file, cgroup_seqfile_show, state);
        if (err < 0)
            kfree(state);
    }
    else if (cft->open)
        err = cft->open(inode, file);
    else
        err = 0;

    return err;
}

static int cgroup_file_release(struct inode *inode, struct file *file)
{
    struct cftype *cft = __d_cft(file->f_dentry);
    if (cft->release)
        return cft->release(inode, file);
    return 0;
}

/*
 * cgroup_rename - Only allow simple rename of directories in place.
 */
static int cgroup_rename(struct inode *old_dir, struct dentry *old_dentry,
                         struct inode *new_dir, struct dentry *new_dentry)
{
    int ret;
    struct cgroup_name *name, *old_name;
    struct cgroup *cgrp;

    /*
     * It's convinient to use parent dir's i_mutex to protected
     * cgrp->name.
     */
    lockdep_assert_held(&old_dir->i_mutex);

    if (!S_ISDIR(old_dentry->d_inode->i_mode))
        return -ENOTDIR;
    if (new_dentry->d_inode)
        return -EEXIST;
    if (old_dir != new_dir)
        return -EIO;

    cgrp = __d_cgrp(old_dentry);

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

    name = cgroup_alloc_name(new_dentry);
    if (!name)
        return -ENOMEM;

    ret = simple_rename(old_dir, old_dentry, new_dir, new_dentry);
    if (ret)
    {
        kfree(name);
        return ret;
    }

    old_name = rcu_dereference_protected(cgrp->name, true);
    rcu_assign_pointer(cgrp->name, name);

    kfree_rcu(old_name, rcu_head);
    return 0;
}

static struct simple_xattrs *__d_xattrs(struct dentry *dentry)
{
    if (S_ISDIR(dentry->d_inode->i_mode))
        return &__d_cgrp(dentry)->xattrs;
    else
        return &__d_cfe(dentry)->xattrs;
}

static inline int xattr_enabled(struct dentry *dentry)
{
    struct cgroupfs_root *root = dentry->d_sb->s_fs_info;
    return root->flags & CGRP_ROOT_XATTR;
}

static bool is_valid_xattr(const char *name)
{
    if (!strncmp(name, XATTR_TRUSTED_PREFIX, XATTR_TRUSTED_PREFIX_LEN) ||
            !strncmp(name, XATTR_SECURITY_PREFIX, XATTR_SECURITY_PREFIX_LEN))
        return true;
    return false;
}

static int cgroup_setxattr(struct dentry *dentry, const char *name,
                           const void *val, size_t size, int flags)
{
    if (!xattr_enabled(dentry))
        return -EOPNOTSUPP;
    if (!is_valid_xattr(name))
        return -EINVAL;
    return simple_xattr_set(__d_xattrs(dentry), name, val, size, flags);
}

static int cgroup_removexattr(struct dentry *dentry, const char *name)
{
    if (!xattr_enabled(dentry))
        return -EOPNOTSUPP;
    if (!is_valid_xattr(name))
        return -EINVAL;
    return simple_xattr_remove(__d_xattrs(dentry), name);
}

static ssize_t cgroup_getxattr(struct dentry *dentry, const char *name,
                               void *buf, size_t size)
{
    if (!xattr_enabled(dentry))
        return -EOPNOTSUPP;
    if (!is_valid_xattr(name))
        return -EINVAL;
    return simple_xattr_get(__d_xattrs(dentry), name, buf, size);
}

static ssize_t cgroup_listxattr(struct dentry *dentry, char *buf, size_t size)
{
    if (!xattr_enabled(dentry))
        return -EOPNOTSUPP;
    return simple_xattr_list(__d_xattrs(dentry), buf, size);
}

static const struct file_operations cgroup_file_operations =
{
    .read = cgroup_file_read,
    .write = cgroup_file_write,
    .llseek = generic_file_llseek,
    .open = cgroup_file_open,
    .release = cgroup_file_release,
};

static const struct inode_operations cgroup_file_inode_operations =
{
    .setxattr = cgroup_setxattr,
    .getxattr = cgroup_getxattr,
    .listxattr = cgroup_listxattr,
    .removexattr = cgroup_removexattr,
};

static const struct inode_operations cgroup_dir_inode_operations =
{
    .lookup = simple_lookup,
    .mkdir = cgroup_mkdir,
    .rmdir = cgroup_rmdir,
    .rename = cgroup_rename,
    .setxattr = cgroup_setxattr,
    .getxattr = cgroup_getxattr,
    .listxattr = cgroup_listxattr,
    .removexattr = cgroup_removexattr,
};

/*
 * Check if a file is a control file
 */
static inline struct cftype *__file_cft(struct file *file)
{
    if (file_inode(file)->i_fop != &cgroup_file_operations)
        return ERR_PTR(-EINVAL);
    return __d_cft(file->f_dentry);
}

static int cgroup_create_file(struct dentry *dentry, umode_t mode,
                              struct super_block *sb)
{
    struct inode *inode;

    if (!dentry)
        return -ENOENT;
    if (dentry->d_inode)
        return -EEXIST;

    inode = cgroup_new_inode(mode, sb);
    if (!inode)
        return -ENOMEM;

    if (S_ISDIR(mode))
    {
        inode->i_op = &cgroup_dir_inode_operations;
        inode->i_fop = &simple_dir_operations;

        /* start off with i_nlink == 2 (for "." entry) */
        inc_nlink(inode);
        inc_nlink(dentry->d_parent->d_inode);

        /*
         * Control reaches here with cgroup_mutex held.
         * @inode->i_mutex should nest outside cgroup_mutex but we
         * want to populate it immediately without releasing
         * cgroup_mutex.  As @inode isn't visible to anyone else
         * yet, trylock will always succeed without affecting
         * lockdep checks.
         */
        WARN_ON_ONCE(!mutex_trylock(&inode->i_mutex));
    }
    else if (S_ISREG(mode))
    {
        inode->i_size = 0;
        inode->i_fop = &cgroup_file_operations;
        inode->i_op = &cgroup_file_inode_operations;
    }
    d_instantiate(dentry, inode);
    dget(dentry);	/* Extra count - pin the dentry in core */
    return 0;
}

/**
 * cgroup_file_mode - deduce file mode of a control file
 * @cft: the control file in question
 *
 * returns cft->mode if ->mode is not 0
 * returns S_IRUGO|S_IWUSR if it has both a read and a write handler
 * returns S_IRUGO if it has only a read handler
 * returns S_IWUSR if it has only a write hander
 */
static umode_t cgroup_file_mode(const struct cftype *cft)
{
    umode_t mode = 0;

    if (cft->mode)
        return cft->mode;

    if (cft->read || cft->read_u64 || cft->read_s64 ||
            cft->read_map || cft->read_seq_string)
        mode |= S_IRUGO;

    if (cft->write || cft->write_u64 || cft->write_s64 ||
            cft->write_string || cft->trigger)
        mode |= S_IWUSR;

    return mode;
}

static int cgroup_add_file(struct cgroup *cgrp, struct cgroup_subsys *subsys,
                           struct cftype *cft)
{
    struct dentry *dir = cgrp->dentry;
    struct cgroup *parent = __d_cgrp(dir);
    struct dentry *dentry;
    struct cfent *cfe;
    int error;
    umode_t mode;
    char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };

    if (subsys && !(cgrp->root->flags & CGRP_ROOT_NOPREFIX))
    {
        strcpy(name, subsys->name);
        strcat(name, ".");
    }
    strcat(name, cft->name);

    BUG_ON(!mutex_is_locked(&dir->d_inode->i_mutex));

    cfe = kzalloc(sizeof(*cfe), GFP_KERNEL);
    if (!cfe)
        return -ENOMEM;

    dentry = lookup_one_len(name, dir, strlen(name));
    if (IS_ERR(dentry))
    {
        error = PTR_ERR(dentry);
        goto out;
    }

    cfe->type = (void *)cft;
    cfe->dentry = dentry;
    dentry->d_fsdata = cfe;
    simple_xattrs_init(&cfe->xattrs);

    mode = cgroup_file_mode(cft);
    error = cgroup_create_file(dentry, mode | S_IFREG, cgrp->root->sb);
    if (!error)
    {
        list_add_tail(&cfe->node, &parent->files);
        cfe = NULL;
    }
    dput(dentry);
out:
    kfree(cfe);
    return error;
}

static int cgroup_addrm_files(struct cgroup *cgrp, struct cgroup_subsys *subsys,
                              struct cftype cfts[], bool is_add)
{
    struct cftype *cft;
    int err, ret = 0;

    for (cft = cfts; cft->name[0] != '\0'; cft++)
    {
        /* does cft->flags tell us to skip this file on @cgrp? */
        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)
        {
            err = cgroup_add_file(cgrp, subsys, cft);
            if (err)
                pr_warn("cgroup_addrm_files: failed to add %s, err=%d\n",
                        cft->name, err);
            ret = err;
        }
        else
        {
            cgroup_rm_file(cgrp, cft);
        }
    }
    return ret;
}

static void cgroup_cfts_prepare(void)
__acquires(&cgroup_mutex)
{
    /*
     * Thanks to the entanglement with vfs inode locking, we can't walk
     * the existing cgroups under cgroup_mutex and create files.
     * Instead, we use cgroup_for_each_descendant_pre() and drop RCU
     * read lock before calling cgroup_addrm_files().
     */
    mutex_lock(&cgroup_mutex);
}

static void cgroup_cfts_commit(struct cgroup_subsys *ss,
                               struct cftype *cfts, bool is_add)
__releases(&cgroup_mutex)
{
    LIST_HEAD(pending);
    struct cgroup *cgrp, *root = &ss->root->top_cgroup;
    struct super_block *sb = ss->root->sb;
    struct dentry *prev = NULL;
    struct inode *inode;
    u64 update_before;

    /* %NULL @cfts indicates abort and don't bother if @ss isn't attached */
    if (!cfts || ss->root == &cgroup_dummy_root ||
            !atomic_inc_not_zero(&sb->s_active))
    {
        mutex_unlock(&cgroup_mutex);
        return;
    }

    /*
     * All cgroups which are created after we drop cgroup_mutex will
     * have the updated set of files, so we only need to update the
     * cgroups created before the current @cgroup_serial_nr_next.
     */
    update_before = cgroup_serial_nr_next;

    mutex_unlock(&cgroup_mutex);

    /* @root always needs to be updated */
    inode = root->dentry->d_inode;
    mutex_lock(&inode->i_mutex);
    mutex_lock(&cgroup_mutex);
    cgroup_addrm_files(root, ss, cfts, is_add);
    mutex_unlock(&cgroup_mutex);
    mutex_unlock(&inode->i_mutex);

    /* add/rm files for all cgroups created before */
    rcu_read_lock();
    cgroup_for_each_descendant_pre(cgrp, root)
    {
        if (cgroup_is_dead(cgrp))
            continue;

        inode = cgrp->dentry->d_inode;
        dget(cgrp->dentry);
        rcu_read_unlock();

        dput(prev);
        prev = cgrp->dentry;

        mutex_lock(&inode->i_mutex);
        mutex_lock(&cgroup_mutex);
        if (cgrp->serial_nr < update_before && !cgroup_is_dead(cgrp))
            cgroup_addrm_files(cgrp, ss, cfts, is_add);
        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&inode->i_mutex);

        rcu_read_lock();
    }
    rcu_read_unlock();
    dput(prev);
    deactivate_super(sb);
}

/**
 * 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)
{
    struct cftype_set *set;

    set = kzalloc(sizeof(*set), GFP_KERNEL);
    if (!set)
        return -ENOMEM;

    cgroup_cfts_prepare();
    set->cfts = cfts;
    list_add_tail(&set->node, &ss->cftsets);
    cgroup_cfts_commit(ss, cfts, true);

    return 0;
}
EXPORT_SYMBOL_GPL(cgroup_add_cftypes);

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

    cgroup_cfts_prepare();

    list_for_each_entry(set, &ss->cftsets, node)
    {
        if (set->cfts == cfts)
        {
            list_del(&set->node);
            kfree(set);
            cgroup_cfts_commit(ss, cfts, false);
            return 0;
        }
    }

    cgroup_cfts_commit(ss, NULL, false);
    return -ENOENT;
}

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

    read_lock(&css_set_lock);
    list_for_each_entry(link, &cgrp->cset_links, cset_link)
    count += atomic_read(&link->cset->refcount);
    read_unlock(&css_set_lock);
    return count;
}

/*
 * Advance a list_head iterator.  The iterator should be positioned at
 * the start of a css_set
 */
static void cgroup_advance_iter(struct cgroup *cgrp, struct cgroup_iter *it)
{
    struct list_head *l = it->cset_link;
    struct cgrp_cset_link *link;
    struct css_set *cset;

    /* Advance to the next non-empty css_set */
    do
    {
        l = l->next;
        if (l == &cgrp->cset_links)
        {
            it->cset_link = NULL;
            return;
        }
        link = list_entry(l, struct cgrp_cset_link, cset_link);
        cset = link->cset;
    }
    while (list_empty(&cset->tasks));
    it->cset_link = l;
    it->task = cset->tasks.next;
}

/*
 * To reduce the fork() overhead for systems that are not actually
 * using their cgroups capability, we don't maintain the lists running
 * through each css_set to its tasks until we see the list actually
 * used - in other words after the first call to cgroup_iter_start().
 */
static void cgroup_enable_task_cg_lists(void)
{
    struct task_struct *p, *g;
    write_lock(&css_set_lock);
    use_task_css_set_links = 1;
    /*
     * We need tasklist_lock because RCU is not safe against
     * while_each_thread(). Besides, a forking task that has passed
     * cgroup_post_fork() without seeing use_task_css_set_links = 1
     * is not guaranteed to have its child immediately visible in the
     * tasklist if we walk through it with RCU.
     */
    read_lock(&tasklist_lock);
    do_each_thread(g, p)
    {
        task_lock(p);
        /*
         * We should check if the process is exiting, otherwise
         * it will race with cgroup_exit() in that the list
         * entry won't be deleted though the process has exited.
         */
        if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))
            list_add(&p->cg_list, &task_css_set(p)->tasks);
        task_unlock(p);
    }
    while_each_thread(g, p);
    read_unlock(&tasklist_lock);
    write_unlock(&css_set_lock);
}

/**
 * cgroup_next_sibling - find the next sibling of a given cgroup
 * @pos: the current cgroup
 *
 * This function returns the next sibling of @pos and should be called
 * under RCU read lock.  The only requirement is that @pos is accessible.
 * The next sibling is guaranteed to be returned regardless of @pos's
 * state.
 */
struct cgroup *cgroup_next_sibling(struct cgroup *pos)
{
    struct cgroup *next;

    WARN_ON_ONCE(!rcu_read_lock_held());

    /*
     * @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 (likely(!cgroup_is_dead(pos)))
    {
        next = list_entry_rcu(pos->sibling.next, struct cgroup, sibling);
        if (&next->sibling != &pos->parent->children)
            return next;
        return NULL;
    }

    /*
     * Can't dereference the next pointer.  Each cgroup is given a
     * monotonically increasing unique serial number and always
     * appended to the sibling list, so 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 slow, it's taken only when either the
     * current cgroup is removed or iteration and removal race.
     */
    list_for_each_entry_rcu(next, &pos->parent->children, sibling)
    if (next->serial_nr > pos->serial_nr)
        return next;
    return NULL;
}
EXPORT_SYMBOL_GPL(cgroup_next_sibling);

/**
 * cgroup_next_descendant_pre - find the next descendant for pre-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @cgroup: cgroup whose descendants to walk
 *
 * To be used by cgroup_for_each_descendant_pre().  Find the next
 * descendant to visit for pre-order traversal of @cgroup's descendants.
 *
 * While this function requires RCU read locking, it doesn't require the
 * whole traversal to be contained in a single RCU 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 *cgroup_next_descendant_pre(struct cgroup *pos,
        struct cgroup *cgroup)
{
    struct cgroup *next;

    WARN_ON_ONCE(!rcu_read_lock_held());

    /* if first iteration, pretend we just visited @cgroup */
    if (!pos)
        pos = cgroup;

    /* visit the first child if exists */
    next = list_first_or_null_rcu(&pos->children, struct cgroup, sibling);
    if (next)
        return next;

    /* no child, visit my or the closest ancestor's next sibling */
    while (pos != cgroup)
    {
        next = cgroup_next_sibling(pos);
        if (next)
            return next;
        pos = pos->parent;
    }

    return NULL;
}
EXPORT_SYMBOL_GPL(cgroup_next_descendant_pre);

/**
 * cgroup_rightmost_descendant - return the rightmost descendant of a cgroup
 * @pos: cgroup 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 RCU read locking, it doesn't require the
 * whole traversal to be contained in a single RCU critical section.  This
 * function will return the correct rightmost descendant as long as @pos is
 * accessible.
 */
struct cgroup *cgroup_rightmost_descendant(struct cgroup *pos)
{
    struct cgroup *last, *tmp;

    WARN_ON_ONCE(!rcu_read_lock_held());

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

    return last;
}
EXPORT_SYMBOL_GPL(cgroup_rightmost_descendant);

static struct cgroup *cgroup_leftmost_descendant(struct cgroup *pos)
{
    struct cgroup *last;

    do
    {
        last = pos;
        pos = list_first_or_null_rcu(&pos->children, struct cgroup,
                                     sibling);
    }
    while (pos);

    return last;
}

/**
 * cgroup_next_descendant_post - find the next descendant for post-order walk
 * @pos: the current position (%NULL to initiate traversal)
 * @cgroup: cgroup whose descendants to walk
 *
 * To be used by cgroup_for_each_descendant_post().  Find the next
 * descendant to visit for post-order traversal of @cgroup's descendants.
 *
 * While this function requires RCU read locking, it doesn't require the
 * whole traversal to be contained in a single RCU 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 *cgroup_next_descendant_post(struct cgroup *pos,
        struct cgroup *cgroup)
{
    struct cgroup *next;

    WARN_ON_ONCE(!rcu_read_lock_held());

    /* if first iteration, visit the leftmost descendant */
    if (!pos)
    {
        next = cgroup_leftmost_descendant(cgroup);
        return next != cgroup ? next : NULL;
    }

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

    /* no sibling left, visit parent */
    next = pos->parent;
    return next != cgroup ? next : NULL;
}
EXPORT_SYMBOL_GPL(cgroup_next_descendant_post);

void cgroup_iter_start(struct cgroup *cgrp, struct cgroup_iter *it)
__acquires(css_set_lock)
{
    /*
     * The first time anyone tries to iterate across a cgroup,
     * we need to enable the list linking each css_set to its
     * tasks, and fix up all existing tasks.
     */
    if (!use_task_css_set_links)
        cgroup_enable_task_cg_lists();

    read_lock(&css_set_lock);
    it->cset_link = &cgrp->cset_links;
    cgroup_advance_iter(cgrp, it);
}

struct task_struct *cgroup_iter_next(struct cgroup *cgrp,
                                     struct cgroup_iter *it)
{
    struct task_struct *res;
    struct list_head *l = it->task;
    struct cgrp_cset_link *link;

    /* If the iterator cg is NULL, we have no tasks */
    if (!it->cset_link)
        return NULL;
    res = list_entry(l, struct task_struct, cg_list);
    /* Advance iterator to find next entry */
    l = l->next;
    link = list_entry(it->cset_link, struct cgrp_cset_link, cset_link);
    if (l == &link->cset->tasks)
    {
        /* We reached the end of this task list - move on to
         * the next cg_cgroup_link */
        cgroup_advance_iter(cgrp, it);
    }
    else
    {
        it->task = l;
    }
    return res;
}

void cgroup_iter_end(struct cgroup *cgrp, struct cgroup_iter *it)
__releases(css_set_lock)
{
    read_unlock(&css_set_lock);
}

static inline int started_after_time(struct task_struct *t1,
                                     struct timespec *time,
                                     struct task_struct *t2)
{
    int start_diff = timespec_compare(&t1->start_time, time);
    if (start_diff > 0)
    {
        return 1;
    }
    else if (start_diff < 0)
    {
        return 0;
    }
    else
    {
        /*
         * Arbitrarily, if two processes started at the same
         * time, we'll say that the lower pointer value
         * started first. Note that t2 may have exited by now
         * so this may not be a valid pointer any longer, but
         * that's fine - it still serves to distinguish
         * between two tasks started (effectively) simultaneously.
         */
        return t1 > t2;
    }
}

/*
 * This function is a callback from heap_insert() and is used to order
 * the heap.
 * In this case we order the heap in descending task start time.
 */
static inline int started_after(void *p1, void *p2)
{
    struct task_struct *t1 = p1;
    struct task_struct *t2 = p2;
    return started_after_time(t1, &t2->start_time, t2);
}

/**
 * cgroup_scan_tasks - iterate though all the tasks in a cgroup
 * @scan: struct cgroup_scanner containing arguments for the scan
 *
 * Arguments include pointers to callback functions test_task() and
 * process_task().
 * Iterate through all the tasks in a cgroup, calling test_task() for each,
 * and if it returns true, call process_task() for it also.
 * The test_task pointer may be NULL, meaning always true (select all tasks).
 * Effectively duplicates cgroup_iter_{start,next,end}()
 * but does not lock css_set_lock for the call to process_task().
 * The struct cgroup_scanner may be embedded in any structure of the caller's
 * creation.
 * It is guaranteed that process_task() will act on every task that
 * is a member of the cgroup for the duration of this call. This
 * function may or may not call process_task() for tasks that exit
 * or move to a different cgroup during the call, or are forked or
 * move into the cgroup during the call.
 *
 * Note that test_task() may be called with locks held, and may in some
 * situations be called multiple times for the same task, so it should
 * be cheap.
 * If the heap pointer in the struct cgroup_scanner is non-NULL, a heap has been
 * pre-allocated and will be used for heap operations (and its "gt" member will
 * be overwritten), else a temporary heap will be used (allocation of which
 * may cause this function to fail).
 */
int cgroup_scan_tasks(struct cgroup_scanner *scan)
{
    int retval, i;
    struct cgroup_iter it;
    struct task_struct *p, *dropped;
    /* Never dereference latest_task, since it's not refcounted */
    struct task_struct *latest_task = NULL;
    struct ptr_heap tmp_heap;
    struct ptr_heap *heap;
    struct timespec latest_time = { 0, 0 };

    if (scan->heap)
    {
        /* The caller supplied our heap and pre-allocated its memory */
        heap = scan->heap;
        heap->gt = &started_after;
    }
    else
    {
        /* We need to allocate our own heap memory */
        heap = &tmp_heap;
        retval = heap_init(heap, PAGE_SIZE, GFP_KERNEL, &started_after);
        if (retval)
            /* cannot allocate the heap */
            return retval;
    }

again:
    /*
     * Scan tasks in the cgroup, using the scanner's "test_task" callback
     * to determine which are of interest, and using the scanner's
     * "process_task" callback to process any of them that need an update.
     * Since we don't want to hold any locks during the task updates,
     * gather tasks to be processed in a heap structure.
     * The heap is sorted by descending task start time.
     * If the statically-sized heap fills up, we overflow tasks that
     * started later, and in future iterations only consider tasks that
     * started after the latest task in the previous pass. This
     * guarantees forward progress and that we don't miss any tasks.
     */
    heap->size = 0;
    cgroup_iter_start(scan->cg, &it);
    while ((p = cgroup_iter_next(scan->cg, &it)))
    {
        /*
         * Only affect tasks that qualify per the caller's callback,
         * if he provided one
         */
        if (scan->test_task && !scan->test_task(p, scan))
            continue;
        /*
         * Only process tasks that started after the last task
         * we processed
         */
        if (!started_after_time(p, &latest_time, latest_task))
            continue;
        dropped = heap_insert(heap, p);
        if (dropped == NULL)
        {
            /*
             * The new task was inserted; the heap wasn't
             * previously full
             */
            get_task_struct(p);
        }
        else if (dropped != p)
        {
            /*
             * The new task was inserted, and pushed out a
             * different task
             */
            get_task_struct(p);
            put_task_struct(dropped);
        }
        /*
         * Else the new task was newer than anything already in
         * the heap and wasn't inserted
         */
    }
    cgroup_iter_end(scan->cg, &it);

    if (heap->size)
    {
        for (i = 0; i < heap->size; i++)
        {
            struct task_struct *q = heap->ptrs[i];
            if (i == 0)
            {
                latest_time = q->start_time;
                latest_task = q;
            }
            /* Process the task per the caller's callback */
            scan->process_task(q, scan);
            put_task_struct(q);
        }
        /*
         * If we had to process any tasks at all, scan again
         * in case some of them were in the middle of forking
         * children that didn't get processed.
         * Not the most efficient way to do it, but it avoids
         * having to take callback_mutex in the fork path
         */
        goto again;
    }
    if (heap == &tmp_heap)
        heap_free(&tmp_heap);
    return 0;
}

static void cgroup_transfer_one_task(struct task_struct *task,
                                     struct cgroup_scanner *scan)
{
    struct cgroup *new_cgroup = scan->data;

    mutex_lock(&cgroup_mutex);
    cgroup_attach_task(new_cgroup, task, false);
    mutex_unlock(&cgroup_mutex);
}

/**
 * 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
 */
int cgroup_transfer_tasks(struct cgroup *to, struct cgroup *from)
{
    struct cgroup_scanner scan;

    scan.cg = from;
    scan.test_task = NULL; /* select all tasks in cgroup */
    scan.process_task = cgroup_transfer_one_task;
    scan.heap = NULL;
    scan.data = to;

    return cgroup_scan_tasks(&scan);
}

/*
 * Stuff for reading the 'tasks'/'procs' files.
 *
 * Reading this file can return large amounts of data if a cgroup has
 * *lots* of attached tasks. So it may need several calls to read(),
 * but we cannot guarantee that the information we produce is correct
 * unless we produce it entirely atomically.
 *
 */

/* which pidlist file are we talking about? */
enum cgroup_filetype
{
    CGROUP_FILE_PROCS,
    CGROUP_FILE_TASKS,
};

/*
 * A pidlist is a list of pids that virtually represents the contents of one
 * of the cgroup files ("procs" or "tasks"). We keep a list of such pidlists,
 * a pair (one each for procs, tasks) for each pid namespace that's relevant
 * to the cgroup.
 */
struct cgroup_pidlist
{
    /*
     * used to find which pidlist is wanted. doesn't change as long as
     * this particular list stays in the list.
    */
    struct
    {
        enum cgroup_filetype type;
        struct pid_namespace *ns;
    } key;
    /* array of xids */
    pid_t *list;
    /* how many elements the above list has */
    int length;
    /* how many files are using the current array */
    int use_count;
    /* each of these stored in a list by its cgroup */
    struct list_head links;
    /* pointer to the cgroup we belong to, for list removal purposes */
    struct cgroup *owner;
    /* protects the other fields */
    struct rw_semaphore mutex;
};

/*
 * The following two functions "fix" the issue where there are more pids
 * than kmalloc will give memory for; in such cases, we use vmalloc/vfree.
 * TODO: replace with a kernel-wide solution to this problem
 */
#define PIDLIST_TOO_LARGE(c) ((c) * sizeof(pid_t) > (PAGE_SIZE * 2))
static void *pidlist_allocate(int count)
{
    if (PIDLIST_TOO_LARGE(count))
        return vmalloc(count * sizeof(pid_t));
    else
        return kmalloc(count * sizeof(pid_t), GFP_KERNEL);
}
static void pidlist_free(void *p)
{
    if (is_vmalloc_addr(p))
        vfree(p);
    else
        kfree(p);
}

/*
 * pidlist_uniq - given a kmalloc()ed list, strip out all duplicate entries
 * Returns the number of unique elements.
 */
static int pidlist_uniq(pid_t *list, int length)
{
    int src, dest = 1;

    /*
     * we presume the 0th element is unique, so i starts at 1. trivial
     * edge cases first; no work needs to be done for either
     */
    if (length == 0 || length == 1)
        return length;
    /* src and dest walk down the list; dest counts unique elements */
    for (src = 1; src < length; src++)
    {
        /* find next unique element */
        while (list[src] == list[src-1])
        {
            src++;
            if (src == length)
                goto after;
        }
        /* dest always points to where the next unique element goes */
        list[dest] = list[src];
        dest++;
    }
after:
    return dest;
}

static int cmppid(const void *a, const void *b)
{
    return *(pid_t *)a - *(pid_t *)b;
}

/*
 * find the appropriate pidlist for our purpose (given procs vs tasks)
 * returns with the lock on that pidlist already held, and takes care
 * of the use count, or returns NULL with no locks held if we're out of
 * memory.
 */
static struct cgroup_pidlist *cgroup_pidlist_find(struct cgroup *cgrp,
        enum cgroup_filetype type)
{
    struct cgroup_pidlist *l;
    /* don't need task_nsproxy() if we're looking at ourself */
    struct pid_namespace *ns = task_active_pid_ns(current);

    /*
     * We can't drop the pidlist_mutex before taking the l->mutex in case
     * the last ref-holder is trying to remove l from the list at the same
     * time. Holding the pidlist_mutex precludes somebody taking whichever
     * list we find out from under us - compare release_pid_array().
     */
    mutex_lock(&cgrp->pidlist_mutex);
    list_for_each_entry(l, &cgrp->pidlists, links)
    {
        if (l->key.type == type && l->key.ns == ns)
        {
            /* make sure l doesn't vanish out from under us */
            down_write(&l->mutex);
            mutex_unlock(&cgrp->pidlist_mutex);
            return l;
        }
    }
    /* entry not found; create a new one */
    l = kzalloc(sizeof(struct cgroup_pidlist), GFP_KERNEL);
    if (!l)
    {
        mutex_unlock(&cgrp->pidlist_mutex);
        return l;
    }
    init_rwsem(&l->mutex);
    down_write(&l->mutex);
    l->key.type = type;
    l->key.ns = get_pid_ns(ns);
    l->owner = cgrp;
    list_add(&l->links, &cgrp->pidlists);
    mutex_unlock(&cgrp->pidlist_mutex);
    return l;
}

/*
 * Load a cgroup's pidarray with either procs' tgids or tasks' pids
 */
static int pidlist_array_load(struct cgroup *cgrp, enum cgroup_filetype type,
                              struct cgroup_pidlist **lp)
{
    pid_t *array;
    int length;
    int pid, n = 0; /* used for populating the array */
    struct cgroup_iter it;
    struct task_struct *tsk;
    struct cgroup_pidlist *l;

    /*
     * If cgroup gets more users after we read count, we won't have
     * enough space - tough.  This race is indistinguishable to the
     * caller from the case that the additional cgroup users didn't
     * show up until sometime later on.
     */
    length = cgroup_task_count(cgrp);
    array = pidlist_allocate(length);
    if (!array)
        return -ENOMEM;
    /* now, populate the array */
    cgroup_iter_start(cgrp, &it);
    while ((tsk = cgroup_iter_next(cgrp, &it)))
    {
        if (unlikely(n == length))
            break;
        /* get tgid or pid for procs or tasks file respectively */
        if (type == CGROUP_FILE_PROCS)
            pid = task_tgid_vnr(tsk);
        else
            pid = task_pid_vnr(tsk);
        if (pid > 0) /* make sure to only use valid results */
            array[n++] = pid;
    }
    cgroup_iter_end(cgrp, &it);
    length = n;
    /* now sort & (if procs) strip out duplicates */
    sort(array, length, sizeof(pid_t), cmppid, NULL);
    if (type == CGROUP_FILE_PROCS)
        length = pidlist_uniq(array, length);
    l = cgroup_pidlist_find(cgrp, type);
    if (!l)
    {
        pidlist_free(array);
        return -ENOMEM;
    }
    /* store array, freeing old if necessary - lock already held */
    pidlist_free(l->list);
    l->list = array;
    l->length = length;
    l->use_count++;
    up_write(&l->mutex);
    *lp = l;
    return 0;
}

/**
 * cgroupstats_build - build and fill cgroupstats
 * @stats: cgroupstats to fill information into
 * @dentry: A dentry entry belonging to the cgroup for which stats have
 * been requested.
 *
 * Build and fill cgroupstats so that taskstats can export it to user
 * space.
 */
int cgroupstats_build(struct cgroupstats *stats, struct dentry *dentry)
{
    int ret = -EINVAL;
    struct cgroup *cgrp;
    struct cgroup_iter it;
    struct task_struct *tsk;

    /*
     * Validate dentry by checking the superblock operations,
     * and make sure it's a directory.
     */
    if (dentry->d_sb->s_op != &cgroup_ops ||
            !S_ISDIR(dentry->d_inode->i_mode))
        goto err;

    ret = 0;
    cgrp = dentry->d_fsdata;

    cgroup_iter_start(cgrp, &it);
    while ((tsk = cgroup_iter_next(cgrp, &it)))
    {
        switch (tsk->state)
        {
        case TASK_RUNNING:
            stats->nr_running++;
            break;
        case TASK_INTERRUPTIBLE:
            stats->nr_sleeping++;
            break;
        case TASK_UNINTERRUPTIBLE:
            stats->nr_uninterruptible++;
            break;
        case TASK_STOPPED:
            stats->nr_stopped++;
            break;
        default:
            if (delayacct_is_task_waiting_on_io(tsk))
                stats->nr_io_wait++;
            break;
        }
    }
    cgroup_iter_end(cgrp, &it);

err:
    return ret;
}


/*
 * seq_file methods for the tasks/procs files. The seq_file position is the
 * next pid to display; the seq_file iterator is a pointer to the pid
 * in the cgroup->l->list array.
 */

static void *cgroup_pidlist_start(struct seq_file *s, loff_t *pos)
{
    /*
     * Initially we receive a position value that corresponds to
     * one more than the last pid shown (or 0 on the first call or
     * after a seek to the start). Use a binary-search to find the
     * next pid to display, if any
     */
    struct cgroup_pidlist *l = s->private;
    int index = 0, pid = *pos;
    int *iter;

    down_read(&l->mutex);
    if (pid)
    {
        int end = l->length;

        while (index < end)
        {
            int mid = (index + end) / 2;
            if (l->list[mid] == pid)
            {
                index = mid;
                break;
            }
            else if (l->list[mid] <= pid)
                index = mid + 1;
            else
                end = mid;
        }
    }
    /* If we're off the end of the array, we're done */
    if (index >= l->length)
        return NULL;
    /* Update the abstract position to be the actual pid that we found */
    iter = l->list + index;
    *pos = *iter;
    return iter;
}

static void cgroup_pidlist_stop(struct seq_file *s, void *v)
{
    struct cgroup_pidlist *l = s->private;
    up_read(&l->mutex);
}

static void *cgroup_pidlist_next(struct seq_file *s, void *v, loff_t *pos)
{
    struct cgroup_pidlist *l = s->private;
    pid_t *p = v;
    pid_t *end = l->list + l->length;
    /*
     * Advance to the next pid in the array. If this goes off the
     * end, we're done
     */
    p++;
    if (p >= end)
    {
        return NULL;
    }
    else
    {
        *pos = *p;
        return p;
    }
}

static int cgroup_pidlist_show(struct seq_file *s, void *v)
{
    return seq_printf(s, "%d\n", *(int *)v);
}

/*
 * seq_operations functions for iterating on pidlists through seq_file -
 * independent of whether it's tasks or procs
 */
static const struct seq_operations cgroup_pidlist_seq_operations =
{
    .start = cgroup_pidlist_start,
    .stop = cgroup_pidlist_stop,
    .next = cgroup_pidlist_next,
    .show = cgroup_pidlist_show,
};

static void cgroup_release_pid_array(struct cgroup_pidlist *l)
{
    /*
     * the case where we're the last user of this particular pidlist will
     * have us remove it from the cgroup's list, which entails taking the
     * mutex. since in pidlist_find the pidlist->lock depends on cgroup->
     * pidlist_mutex, we have to take pidlist_mutex first.
     */
    mutex_lock(&l->owner->pidlist_mutex);
    down_write(&l->mutex);
    BUG_ON(!l->use_count);
    if (!--l->use_count)
    {
        /* we're the last user if refcount is 0; remove and free */
        list_del(&l->links);
        mutex_unlock(&l->owner->pidlist_mutex);
        pidlist_free(l->list);
        put_pid_ns(l->key.ns);
        up_write(&l->mutex);
        kfree(l);
        return;
    }
    mutex_unlock(&l->owner->pidlist_mutex);
    up_write(&l->mutex);
}

static int cgroup_pidlist_release(struct inode *inode, struct file *file)
{
    struct cgroup_pidlist *l;
    if (!(file->f_mode & FMODE_READ))
        return 0;
    /*
     * the seq_file will only be initialized if the file was opened for
     * reading; hence we check if it's not null only in that case.
     */
    l = ((struct seq_file *)file->private_data)->private;
    cgroup_release_pid_array(l);
    return seq_release(inode, file);
}

static const struct file_operations cgroup_pidlist_operations =
{
    .read = seq_read,
    .llseek = seq_lseek,
    .write = cgroup_file_write,
    .release = cgroup_pidlist_release,
};

/*
 * The following functions handle opens on a file that displays a pidlist
 * (tasks or procs). Prepare an array of the process/thread IDs of whoever's
 * in the cgroup.
 */
/* helper function for the two below it */
static int cgroup_pidlist_open(struct file *file, enum cgroup_filetype type)
{
    struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
    struct cgroup_pidlist *l;
    int retval;

    /* Nothing to do for write-only files */
    if (!(file->f_mode & FMODE_READ))
        return 0;

    /* have the array populated */
    retval = pidlist_array_load(cgrp, type, &l);
    if (retval)
        return retval;
    /* configure file information */
    file->f_op = &cgroup_pidlist_operations;

    retval = seq_open(file, &cgroup_pidlist_seq_operations);
    if (retval)
    {
        cgroup_release_pid_array(l);
        return retval;
    }
    ((struct seq_file *)file->private_data)->private = l;
    return 0;
}
static int cgroup_tasks_open(struct inode *unused, struct file *file)
{
    return cgroup_pidlist_open(file, CGROUP_FILE_TASKS);
}
static int cgroup_procs_open(struct inode *unused, struct file *file)
{
    return cgroup_pidlist_open(file, CGROUP_FILE_PROCS);
}

static u64 cgroup_read_notify_on_release(struct cgroup *cgrp,
        struct cftype *cft)
{
    return notify_on_release(cgrp);
}

static int cgroup_write_notify_on_release(struct cgroup *cgrp,
        struct cftype *cft,
        u64 val)
{
    clear_bit(CGRP_RELEASABLE, &cgrp->flags);
    if (val)
        set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
    else
        clear_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
    return 0;
}

/*
 * When dput() is called asynchronously, if umount has been done and
 * then deactivate_super() in cgroup_free_fn() kills the superblock,
 * there's a small window that vfs will see the root dentry with non-zero
 * refcnt and trigger BUG().
 *
 * That's why we hold a reference before dput() and drop it right after.
 */
static void cgroup_dput(struct cgroup *cgrp)
{
    struct super_block *sb = cgrp->root->sb;

    atomic_inc(&sb->s_active);
    dput(cgrp->dentry);
    deactivate_super(sb);
}

/*
 * Unregister event and free resources.
 *
 * Gets called from workqueue.
 */
static void cgroup_event_remove(struct work_struct *work)
{
    struct cgroup_event *event = container_of(work, struct cgroup_event,
            remove);
    struct cgroup *cgrp = event->cgrp;

    remove_wait_queue(event->wqh, &event->wait);

    event->cft->unregister_event(cgrp, event->cft, event->eventfd);

    /* Notify userspace the event is going away. */
    eventfd_signal(event->eventfd, 1);

    eventfd_ctx_put(event->eventfd);
    kfree(event);
    cgroup_dput(cgrp);
}

/*
 * Gets called on POLLHUP on eventfd when user closes it.
 *
 * Called with wqh->lock held and interrupts disabled.
 */
static int cgroup_event_wake(wait_queue_t *wait, unsigned mode,
                             int sync, void *key)
{
    struct cgroup_event *event = container_of(wait,
            struct cgroup_event, wait);
    struct cgroup *cgrp = event->cgrp;
    unsigned long flags = (unsigned long)key;

    if (flags & POLLHUP)
    {
        /*
         * If the event has been detached at cgroup removal, we
         * can simply return knowing the other side will cleanup
         * for us.
         *
         * We can't race against event freeing since the other
         * side will require wqh->lock via remove_wait_queue(),
         * which we hold.
         */
        spin_lock(&cgrp->event_list_lock);
        if (!list_empty(&event->list))
        {
            list_del_init(&event->list);
            /*
             * We are in atomic context, but cgroup_event_remove()
             * may sleep, so we have to call it in workqueue.
             */
            schedule_work(&event->remove);
        }
        spin_unlock(&cgrp->event_list_lock);
    }

    return 0;
}

static void cgroup_event_ptable_queue_proc(struct file *file,
        wait_queue_head_t *wqh, poll_table *pt)
{
    struct cgroup_event *event = container_of(pt,
            struct cgroup_event, pt);

    event->wqh = wqh;
    add_wait_queue(wqh, &event->wait);
}

/*
 * Parse input and register new cgroup event handler.
 *
 * Input must be in format '<event_fd> <control_fd> <args>'.
 * Interpretation of args is defined by control file implementation.
 */
static int cgroup_write_event_control(struct cgroup *cgrp, struct cftype *cft,
                                      const char *buffer)
{
    struct cgroup_event *event = NULL;
    struct cgroup *cgrp_cfile;
    unsigned int efd, cfd;
    struct file *efile = NULL;
    struct file *cfile = NULL;
    char *endp;
    int ret;

    efd = simple_strtoul(buffer, &endp, 10);
    if (*endp != ' ')
        return -EINVAL;
    buffer = endp + 1;

    cfd = simple_strtoul(buffer, &endp, 10);
    if ((*endp != ' ') && (*endp != '\0'))
        return -EINVAL;
    buffer = endp + 1;

    event = kzalloc(sizeof(*event), GFP_KERNEL);
    if (!event)
        return -ENOMEM;
    event->cgrp = cgrp;
    INIT_LIST_HEAD(&event->list);
    init_poll_funcptr(&event->pt, cgroup_event_ptable_queue_proc);
    init_waitqueue_func_entry(&event->wait, cgroup_event_wake);
    INIT_WORK(&event->remove, cgroup_event_remove);

    efile = eventfd_fget(efd);
    if (IS_ERR(efile))
    {
        ret = PTR_ERR(efile);
        goto fail;
    }

    event->eventfd = eventfd_ctx_fileget(efile);
    if (IS_ERR(event->eventfd))
    {
        ret = PTR_ERR(event->eventfd);
        goto fail;
    }

    cfile = fget(cfd);
    if (!cfile)
    {
        ret = -EBADF;
        goto fail;
    }

    /* the process need read permission on control file */
    /* AV: shouldn't we check that it's been opened for read instead? */
    ret = inode_permission(file_inode(cfile), MAY_READ);
    if (ret < 0)
        goto fail;

    event->cft = __file_cft(cfile);
    if (IS_ERR(event->cft))
    {
        ret = PTR_ERR(event->cft);
        goto fail;
    }

    /*
     * The file to be monitored must be in the same cgroup as
     * cgroup.event_control is.
     */
    cgrp_cfile = __d_cgrp(cfile->f_dentry->d_parent);
    if (cgrp_cfile != cgrp)
    {
        ret = -EINVAL;
        goto fail;
    }

    if (!event->cft->register_event || !event->cft->unregister_event)
    {
        ret = -EINVAL;
        goto fail;
    }

    ret = event->cft->register_event(cgrp, event->cft,
                                     event->eventfd, buffer);
    if (ret)
        goto fail;

    efile->f_op->poll(efile, &event->pt);

    /*
     * Events should be removed after rmdir of cgroup directory, but before
     * destroying subsystem state objects. Let's take reference to cgroup
     * directory dentry to do that.
     */
    dget(cgrp->dentry);

    spin_lock(&cgrp->event_list_lock);
    list_add(&event->list, &cgrp->event_list);
    spin_unlock(&cgrp->event_list_lock);

    fput(cfile);
    fput(efile);

    return 0;

fail:
    if (cfile)
        fput(cfile);

    if (event && event->eventfd && !IS_ERR(event->eventfd))
        eventfd_ctx_put(event->eventfd);

    if (!IS_ERR_OR_NULL(efile))
        fput(efile);

    kfree(event);

    return ret;
}

static u64 cgroup_clone_children_read(struct cgroup *cgrp,
                                      struct cftype *cft)
{
    return test_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);
}

static int cgroup_clone_children_write(struct cgroup *cgrp,
                                       struct cftype *cft,
                                       u64 val)
{
    if (val)
        set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);
    else
        clear_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);
    return 0;
}

static struct cftype cgroup_base_files[] =
{
    {
        .name = "cgroup.procs",
        .open = cgroup_procs_open,
        .write_u64 = cgroup_procs_write,
        .release = cgroup_pidlist_release,
        .mode = S_IRUGO | S_IWUSR,
    },
    {
        .name = "cgroup.event_control",
        .write_string = cgroup_write_event_control,
        .mode = S_IWUGO,
    },
    {
        .name = "cgroup.clone_children",
        .flags = CFTYPE_INSANE,
        .read_u64 = cgroup_clone_children_read,
        .write_u64 = cgroup_clone_children_write,
    },
    {
        .name = "cgroup.sane_behavior",
        .flags = CFTYPE_ONLY_ON_ROOT,
        .read_seq_string = cgroup_sane_behavior_show,
    },

    /*
     * Historical crazy stuff.  These don't have "cgroup."  prefix and
     * don't exist if sane_behavior.  If you're depending on these, be
     * prepared to be burned.
     */
    {
        .name = "tasks",
        .flags = CFTYPE_INSANE,		/* use "procs" instead */
        .open = cgroup_tasks_open,
        .write_u64 = cgroup_tasks_write,
        .release = cgroup_pidlist_release,
        .mode = S_IRUGO | S_IWUSR,
    },
    {
        .name = "notify_on_release",
        .flags = CFTYPE_INSANE,
        .read_u64 = cgroup_read_notify_on_release,
        .write_u64 = cgroup_write_notify_on_release,
    },
    {
        .name = "release_agent",
        .flags = CFTYPE_INSANE | CFTYPE_ONLY_ON_ROOT,
        .read_seq_string = cgroup_release_agent_show,
        .write_string = cgroup_release_agent_write,
        .max_write_len = PATH_MAX,
    },
    { }	/* terminate */
};

/**
 * cgroup_populate_dir - selectively creation of files in a directory
 * @cgrp: target cgroup
 * @base_files: true if the base files should be added
 * @subsys_mask: mask of the subsystem ids whose files should be added
 */
static int cgroup_populate_dir(struct cgroup *cgrp, bool base_files,
                               unsigned long subsys_mask)
{
    int err;
    struct cgroup_subsys *ss;

    if (base_files)
    {
        err = cgroup_addrm_files(cgrp, NULL, cgroup_base_files, true);
        if (err < 0)
            return err;
    }

    /* process cftsets of each subsystem */
    for_each_root_subsys(cgrp->root, ss)
    {
        struct cftype_set *set;
        if (!test_bit(ss->subsys_id, &subsys_mask))
            continue;

        list_for_each_entry(set, &ss->cftsets, node)
        cgroup_addrm_files(cgrp, ss, set->cfts, true);
    }

    /* This cgroup is ready now */
    for_each_root_subsys(cgrp->root, ss)
    {
        struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
        struct css_id *id = rcu_dereference_protected(css->id, true);

        /*
         * Update id->css pointer and make this css visible from
         * CSS ID functions. This pointer will be dereferened
         * from RCU-read-side without locks.
         */
        if (id)
            rcu_assign_pointer(id->css, css);
    }

    return 0;
}

static void css_dput_fn(struct work_struct *work)
{
    struct cgroup_subsys_state *css =
    container_of(work, struct cgroup_subsys_state, dput_work);

    cgroup_dput(css->cgroup);
}

static void css_release(struct percpu_ref *ref)
{
    struct cgroup_subsys_state *css =
    container_of(ref, struct cgroup_subsys_state, refcnt);

    schedule_work(&css->dput_work);
}

static void init_cgroup_css(struct cgroup_subsys_state *css,
                            struct cgroup_subsys *ss,
                            struct cgroup *cgrp)
{
    css->cgroup = cgrp;
    css->flags = 0;
    css->id = NULL;
    if (cgrp == cgroup_dummy_top)
        css->flags |= CSS_ROOT;
    BUG_ON(cgrp->subsys[ss->subsys_id]);
    cgrp->subsys[ss->subsys_id] = css;

    /*
     * css holds an extra ref to @cgrp->dentry which is put on the last
     * css_put().  dput() requires process context, which css_put() may
     * be called without.  @css->dput_work will be used to invoke
     * dput() asynchronously from css_put().
     */
    INIT_WORK(&css->dput_work, css_dput_fn);
}

/* invoke ->post_create() on a new CSS and mark it online if successful */
static int online_css(struct cgroup_subsys *ss, struct cgroup *cgrp)
{
    int ret = 0;

    lockdep_assert_held(&cgroup_mutex);

    if (ss->css_online)
        ret = ss->css_online(cgrp);
    if (!ret)
        cgrp->subsys[ss->subsys_id]->flags |= CSS_ONLINE;
    return ret;
}

/* if the CSS is online, invoke ->pre_destory() on it and mark it offline */
static void offline_css(struct cgroup_subsys *ss, struct cgroup *cgrp)
__releases(&cgroup_mutex) __acquires(&cgroup_mutex)
{
    struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];

    lockdep_assert_held(&cgroup_mutex);

    if (!(css->flags & CSS_ONLINE))
        return;

    if (ss->css_offline)
        ss->css_offline(cgrp);

    cgrp->subsys[ss->subsys_id]->flags &= ~CSS_ONLINE;
}

/*
 * cgroup_create - create a cgroup
 * @parent: cgroup that will be parent of the new cgroup
 * @dentry: dentry of the new cgroup
 * @mode: mode to set on new inode
 *
 * Must be called with the mutex on the parent inode held
 */
static long cgroup_create(struct cgroup *parent, struct dentry *dentry,
                          umode_t mode)
{
    struct cgroup *cgrp;
    struct cgroup_name *name;
    struct cgroupfs_root *root = parent->root;
    int err = 0;
    struct cgroup_subsys *ss;
    struct super_block *sb = root->sb;

    /* allocate the cgroup and its ID, 0 is reserved for the root */
    cgrp = kzalloc(sizeof(*cgrp), GFP_KERNEL);
    if (!cgrp)
        return -ENOMEM;

    name = cgroup_alloc_name(dentry);
    if (!name)
        goto err_free_cgrp;
    rcu_assign_pointer(cgrp->name, name);

    cgrp->id = ida_simple_get(&root->cgroup_ida, 1, 0, GFP_KERNEL);
    if (cgrp->id < 0)
        goto err_free_name;

    /*
     * Only live parents can have children.  Note that the liveliness
     * check isn't strictly necessary because cgroup_mkdir() and
     * cgroup_rmdir() are fully synchronized by i_mutex; however, do it
     * anyway so that locking is contained inside cgroup proper and we
     * don't get nasty surprises if we ever grow another caller.
     */
    if (!cgroup_lock_live_group(parent))
    {
        err = -ENODEV;
        goto err_free_id;
    }

    /* Grab a reference on the superblock so the hierarchy doesn't
     * get deleted on unmount if there are child cgroups.  This
     * can be done outside cgroup_mutex, since the sb can't
     * disappear while someone has an open control file on the
     * fs */
    atomic_inc(&sb->s_active);

    init_cgroup_housekeeping(cgrp);

    dentry->d_fsdata = cgrp;
    cgrp->dentry = dentry;

    cgrp->parent = parent;
    cgrp->root = parent->root;

    if (notify_on_release(parent))
        set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);

    if (test_bit(CGRP_CPUSET_CLONE_CHILDREN, &parent->flags))
        set_bit(CGRP_CPUSET_CLONE_CHILDREN, &cgrp->flags);

    for_each_root_subsys(root, ss)
    {
        struct cgroup_subsys_state *css;

        css = ss->css_alloc(cgrp);
        if (IS_ERR(css))
        {
            err = PTR_ERR(css);
            goto err_free_all;
        }

        err = percpu_ref_init(&css->refcnt, css_release);
        if (err)
        {
            ss->css_free(cgrp);
            goto err_free_all;
        }

        init_cgroup_css(css, ss, cgrp);

        if (ss->use_id)
        {
            err = alloc_css_id(ss, parent, cgrp);
            if (err)
                goto err_free_all;
        }
    }

    /*
     * Create directory.  cgroup_create_file() returns with the new
     * directory locked on success so that it can be populated without
     * dropping cgroup_mutex.
     */
    err = cgroup_create_file(dentry, S_IFDIR | mode, sb);
    if (err < 0)
        goto err_free_all;
    lockdep_assert_held(&dentry->d_inode->i_mutex);

    cgrp->serial_nr = cgroup_serial_nr_next++;

    /* allocation complete, commit to creation */
    list_add_tail_rcu(&cgrp->sibling, &cgrp->parent->children);
    root->number_of_cgroups++;

    /* each css holds a ref to the cgroup's dentry */
    for_each_root_subsys(root, ss)
    dget(dentry);

    /* hold a ref to the parent's dentry */
    dget(parent->dentry);

    /* creation succeeded, notify subsystems */
    for_each_root_subsys(root, ss)
    {
        err = online_css(ss, cgrp);
        if (err)
            goto err_destroy;

        if (ss->broken_hierarchy && !ss->warned_broken_hierarchy &&
                parent->parent)
        {
            pr_warning("cgroup: %s (%d) created nested cgroup for controller \"%s\" which has incomplete hierarchy support. Nested cgroups may change behavior in the future.\n",
                       current->comm, current->pid, ss->name);
            if (!strcmp(ss->name, "memory"))
                pr_warning("cgroup: \"memory\" requires setting use_hierarchy to 1 on the root.\n");
            ss->warned_broken_hierarchy = true;
        }
    }

    err = cgroup_populate_dir(cgrp, true, root->subsys_mask);
    if (err)
        goto err_destroy;

    mutex_unlock(&cgroup_mutex);
    mutex_unlock(&cgrp->dentry->d_inode->i_mutex);

    return 0;

err_free_all:
    for_each_root_subsys(root, ss)
    {
        struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];

        if (css)
        {
            percpu_ref_cancel_init(&css->refcnt);
            ss->css_free(cgrp);
        }
    }
    mutex_unlock(&cgroup_mutex);
    /* Release the reference count that we took on the superblock */
    deactivate_super(sb);
err_free_id:
    ida_simple_remove(&root->cgroup_ida, cgrp->id);
err_free_name:
    kfree(rcu_dereference_raw(cgrp->name));
err_free_cgrp:
    kfree(cgrp);
    return err;

err_destroy:
    cgroup_destroy_locked(cgrp);
    mutex_unlock(&cgroup_mutex);
    mutex_unlock(&dentry->d_inode->i_mutex);
    return err;
}

static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
{
    struct cgroup *c_parent = dentry->d_parent->d_fsdata;

    /* the vfs holds inode->i_mutex already */
    return cgroup_create(c_parent, dentry, mode | S_IFDIR);
}

static void cgroup_css_killed(struct cgroup *cgrp)
{
    if (!atomic_dec_and_test(&cgrp->css_kill_cnt))
        return;

    /* percpu ref's of all css's are killed, kick off the next step */
    INIT_WORK(&cgrp->destroy_work, cgroup_offline_fn);
    schedule_work(&cgrp->destroy_work);
}

static void css_ref_killed_fn(struct percpu_ref *ref)
{
    struct cgroup_subsys_state *css =
    container_of(ref, struct cgroup_subsys_state, refcnt);

    cgroup_css_killed(css->cgroup);
}

/**
 * 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 dentry *d = cgrp->dentry;
    struct cgroup_event *event, *tmp;
    struct cgroup_subsys *ss;
    struct cgroup *child;
    bool empty;

    lockdep_assert_held(&d->d_inode->i_mutex);
    lockdep_assert_held(&cgroup_mutex);

    /*
     * css_set_lock synchronizes access to ->cset_links and prevents
     * @cgrp from being removed while __put_css_set() is in progress.
     */
    read_lock(&css_set_lock);
    empty = list_empty(&cgrp->cset_links);
    read_unlock(&css_set_lock);
    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;

    /*
     * Block new css_tryget() by killing css refcnts.  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.  The
     * notification callback keeps track of the number of css's to be
     * killed and schedules cgroup_offline_fn() to perform the rest of
     * destruction once the percpu refs of all css's are confirmed to
     * be killed.
     */
    atomic_set(&cgrp->css_kill_cnt, 1);
    for_each_root_subsys(cgrp->root, ss)
    {
        struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];

        /*
         * Killing would put the base ref, but we need to keep it
         * alive until after ->css_offline.
         */
        percpu_ref_get(&css->refcnt);

        atomic_inc(&cgrp->css_kill_cnt);
        percpu_ref_kill_and_confirm(&css->refcnt, css_ref_killed_fn);
    }
    cgroup_css_killed(cgrp);

    /*
     * 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 cgroup_next_sibling() to
     * resume iteration after dropping RCU read lock.  See
     * cgroup_next_sibling() for details.
     */
    set_bit(CGRP_DEAD, &cgrp->flags);

    /* 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);

    /*
     * Remove @cgrp directory.  The removal puts the base ref but we
     * aren't quite done with @cgrp yet, so hold onto it.
     */
    dget(d);
    cgroup_d_remove_dir(d);

    /*
     * Unregister events and notify userspace.
     * Notify userspace about cgroup removing only after rmdir of cgroup
     * directory to avoid race between userspace and kernelspace.
     */
    spin_lock(&cgrp->event_list_lock);
    list_for_each_entry_safe(event, tmp, &cgrp->event_list, list)
    {
        list_del_init(&event->list);
        schedule_work(&event->remove);
    }
    spin_unlock(&cgrp->event_list_lock);

    return 0;
};

/**
 * cgroup_offline_fn - 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 the percpu refcnts of all css's are guaranteed to be
 * seen as killed on all CPUs, and performs the rest of destruction.  This
 * is the second step of destruction described in the comment above
 * cgroup_destroy_locked().
 */
static void cgroup_offline_fn(struct work_struct *work)
{
    struct cgroup *cgrp = container_of(work, struct cgroup, destroy_work);
    struct cgroup *parent = cgrp->parent;
    struct dentry *d = cgrp->dentry;
    struct cgroup_subsys *ss;

    mutex_lock(&cgroup_mutex);

    /*
     * css_tryget() is guaranteed to fail now.  Tell subsystems to
     * initate destruction.
     */
    for_each_root_subsys(cgrp->root, ss)
    offline_css(ss, cgrp);

    /*
     * Put the css refs from cgroup_destroy_locked().  Each css holds
     * an extra reference to the cgroup's dentry and cgroup removal
     * proceeds regardless of css refs.  On the last put of each css,
     * whenever that may be, the extra dentry ref is put so that dentry
     * destruction happens only after all css's are released.
     */
    for_each_root_subsys(cgrp->root, ss)
    css_put(cgrp->subsys[ss->subsys_id]);

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

    dput(d);

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

    mutex_unlock(&cgroup_mutex);
}

static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
{
    int ret;

    mutex_lock(&cgroup_mutex);
    ret = cgroup_destroy_locked(dentry->d_fsdata);
    mutex_unlock(&cgroup_mutex);

    return ret;
}

static void __init_or_module cgroup_init_cftsets(struct cgroup_subsys *ss)
{
    INIT_LIST_HEAD(&ss->cftsets);

    /*
     * base_cftset is embedded in subsys itself, no need to worry about
     * deregistration.
     */
    if (ss->base_cftypes)
    {
        ss->base_cftset.cfts = ss->base_cftypes;
        list_add_tail(&ss->base_cftset.node, &ss->cftsets);
    }
}

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_mutex);

    /* init base cftset */
    cgroup_init_cftsets(ss);

    /* Create the top cgroup state for this subsystem */
    list_add(&ss->sibling, &cgroup_dummy_root.subsys_list);
    ss->root = &cgroup_dummy_root;
    css = ss->css_alloc(cgroup_dummy_top);
    /* We don't handle early failures gracefully */
    BUG_ON(IS_ERR(css));
    init_cgroup_css(css, ss, cgroup_dummy_top);

    /* 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 top cgroup. */
    init_css_set.subsys[ss->subsys_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(ss, cgroup_dummy_top));

    mutex_unlock(&cgroup_mutex);

    /* this function shouldn't be used with modular subsystems, since they
     * need to register a subsys_id, among other things */
    BUG_ON(ss->module);
}

/**
 * cgroup_load_subsys: load and register a modular subsystem at runtime
 * @ss: the subsystem to load
 *
 * This function should be called in a modular subsystem's initcall. If the
 * subsystem is built as a module, it will be assigned a new subsys_id and set
 * up for use. If the subsystem is built-in anyway, work is delegated to the
 * simpler cgroup_init_subsys.
 */
int __init_or_module cgroup_load_subsys(struct cgroup_subsys *ss)
{
    struct cgroup_subsys_state *css;
    int i, ret;
    struct hlist_node *tmp;
    struct css_set *cset;
    unsigned long key;

    /* check name and function validity */
    if (ss->name == NULL || strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN ||
            ss->css_alloc == NULL || ss->css_free == NULL)
        return -EINVAL;

    /*
     * we don't support callbacks in modular subsystems. this check is
     * before the ss->module check for consistency; a subsystem that could
     * be a module should still have no callbacks even if the user isn't
     * compiling it as one.
     */
    if (ss->fork || ss->exit)
        return -EINVAL;

    /*
     * an optionally modular subsystem is built-in: we want to do nothing,
     * since cgroup_init_subsys will have already taken care of it.
     */
    if (ss->module == NULL)
    {
        /* a sanity check */
        BUG_ON(cgroup_subsys[ss->subsys_id] != ss);
        return 0;
    }

    /* init base cftset */
    cgroup_init_cftsets(ss);

    mutex_lock(&cgroup_mutex);
    cgroup_subsys[ss->subsys_id] = ss;

    /*
     * no ss->css_alloc seems to need anything important in the ss
     * struct, so this can happen first (i.e. before the dummy root
     * attachment).
     */
    css = ss->css_alloc(cgroup_dummy_top);
    if (IS_ERR(css))
    {
        /* failure case - need to deassign the cgroup_subsys[] slot. */
        cgroup_subsys[ss->subsys_id] = NULL;
        mutex_unlock(&cgroup_mutex);
        return PTR_ERR(css);
    }

    list_add(&ss->sibling, &cgroup_dummy_root.subsys_list);
    ss->root = &cgroup_dummy_root;

    /* our new subsystem will be attached to the dummy hierarchy. */
    init_cgroup_css(css, ss, cgroup_dummy_top);
    /* init_idr must be after init_cgroup_css because it sets css->id. */
    if (ss->use_id)
    {
        ret = cgroup_init_idr(ss, css);
        if (ret)
            goto err_unload;
    }

    /*
     * Now we need to entangle the css into the existing css_sets. unlike
     * in cgroup_init_subsys, there are now multiple css_sets, so each one
     * will need a new pointer to it; done by iterating the css_set_table.
     * furthermore, modifying the existing css_sets will corrupt the hash
     * table state, so each changed css_set will need its hash recomputed.
     * this is all done under the css_set_lock.
     */
    write_lock(&css_set_lock);
    hash_for_each_safe(css_set_table, i, tmp, cset, hlist)
    {
        /* skip entries that we already rehashed */
        if (cset->subsys[ss->subsys_id])
            continue;
        /* remove existing entry */
        hash_del(&cset->hlist);
        /* set new value */
        cset->subsys[ss->subsys_id] = css;
        /* recompute hash and restore entry */
        key = css_set_hash(cset->subsys);
        hash_add(css_set_table, &cset->hlist, key);
    }
    write_unlock(&css_set_lock);

    ret = online_css(ss, cgroup_dummy_top);
    if (ret)
        goto err_unload;

    /* success! */
    mutex_unlock(&cgroup_mutex);
    return 0;

err_unload:
    mutex_unlock(&cgroup_mutex);
    /* @ss can't be mounted here as try_module_get() would fail */
    cgroup_unload_subsys(ss);
    return ret;
}
EXPORT_SYMBOL_GPL(cgroup_load_subsys);

/**
 * cgroup_unload_subsys: unload a modular subsystem
 * @ss: the subsystem to unload
 *
 * This function should be called in a modular subsystem's exitcall. When this
 * function is invoked, the refcount on the subsystem's module will be 0, so
 * the subsystem will not be attached to any hierarchy.
 */
void cgroup_unload_subsys(struct cgroup_subsys *ss)
{
    struct cgrp_cset_link *link;

    BUG_ON(ss->module == NULL);

    /*
     * we shouldn't be called if the subsystem is in use, and the use of
     * try_module_get in parse_cgroupfs_options should ensure that it
     * doesn't start being used while we're killing it off.
     */
    BUG_ON(ss->root != &cgroup_dummy_root);

    mutex_lock(&cgroup_mutex);

    offline_css(ss, cgroup_dummy_top);

    if (ss->use_id)
        idr_destroy(&ss->idr);

    /* deassign the subsys_id */
    cgroup_subsys[ss->subsys_id] = NULL;

    /* remove subsystem from the dummy root's list of subsystems */
    list_del_init(&ss->sibling);

    /*
     * disentangle the css from all css_sets attached to the dummy
     * top. as in loading, we need to pay our respects to the hashtable
     * gods.
     */
    write_lock(&css_set_lock);
    list_for_each_entry(link, &cgroup_dummy_top->cset_links, cset_link)
    {
        struct css_set *cset = link->cset;
        unsigned long key;

        hash_del(&cset->hlist);
        cset->subsys[ss->subsys_id] = NULL;
        key = css_set_hash(cset->subsys);
        hash_add(css_set_table, &cset->hlist, key);
    }
    write_unlock(&css_set_lock);

    /*
     * remove subsystem's css from the cgroup_dummy_top and free it -
     * need to free before marking as null because ss->css_free needs
     * the cgrp->subsys pointer to find their state. note that this
     * also takes care of freeing the css_id.
     */
    ss->css_free(cgroup_dummy_top);
    cgroup_dummy_top->subsys[ss->subsys_id] = NULL;

    mutex_unlock(&cgroup_mutex);
}
EXPORT_SYMBOL_GPL(cgroup_unload_subsys);

/**
 * 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)
{
    struct cgroup_subsys *ss;
    int i;

    atomic_set(&init_css_set.refcount, 1);
    INIT_LIST_HEAD(&init_css_set.cgrp_links);
    INIT_LIST_HEAD(&init_css_set.tasks);
    INIT_HLIST_NODE(&init_css_set.hlist);
    css_set_count = 1;
    init_cgroup_root(&cgroup_dummy_root);
    cgroup_root_count = 1;
    RCU_INIT_POINTER(init_task.cgroups, &init_css_set);

    init_cgrp_cset_link.cset = &init_css_set;
    init_cgrp_cset_link.cgrp = cgroup_dummy_top;
    list_add(&init_cgrp_cset_link.cset_link, &cgroup_dummy_top->cset_links);
    list_add(&init_cgrp_cset_link.cgrp_link, &init_css_set.cgrp_links);

    /* at bootup time, we don't worry about modular subsystems */
    for_each_builtin_subsys(ss, i)
    {
        BUG_ON(!ss->name);
        BUG_ON(strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN);
        BUG_ON(!ss->css_alloc);
        BUG_ON(!ss->css_free);
        if (ss->subsys_id != i)
        {
            printk(KERN_ERR "cgroup: Subsys %s id == %d\n",
                   ss->name, ss->subsys_id);
            BUG();
        }

        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 i, err;

    err = bdi_init(&cgroup_backing_dev_info);
    if (err)
        return err;

    for_each_builtin_subsys(ss, i)
    {
        if (!ss->early_init)
            cgroup_init_subsys(ss);
        if (ss->use_id)
            cgroup_init_idr(ss, init_css_set.subsys[ss->subsys_id]);
    }

    /* allocate id for the dummy hierarchy */
    mutex_lock(&cgroup_mutex);
    mutex_lock(&cgroup_root_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_init_root_id(&cgroup_dummy_root, 0, 1));

    mutex_unlock(&cgroup_root_mutex);
    mutex_unlock(&cgroup_mutex);

    cgroup_kobj = kobject_create_and_add("cgroup", fs_kobj);
    if (!cgroup_kobj)
    {
        err = -ENOMEM;
        goto out;
    }

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

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

out:
    if (err)
        bdi_destroy(&cgroup_backing_dev_info);

    return err;
}

/*
 * proc_cgroup_show()
 *  - Print task's cgroup paths into seq_file, one line for each hierarchy
 *  - Used for /proc/<pid>/cgroup.
 *  - No need to task_lock(tsk) on this tsk->cgroup reference, as it
 *    doesn't really matter if tsk->cgroup changes after we read it,
 *    and we take cgroup_mutex, keeping cgroup_attach_task() from changing it
 *    anyway.  No need to check that tsk->cgroup != NULL, thanks to
 *    the_top_cgroup_hack in cgroup_exit(), which sets an exiting tasks
 *    cgroup to top_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;
    int retval;
    struct cgroupfs_root *root;

    retval = -ENOMEM;
    buf = kmalloc(PAGE_SIZE, 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);

    for_each_active_root(root)
    {
        struct cgroup_subsys *ss;
        struct cgroup *cgrp;
        int count = 0;

        seq_printf(m, "%d:", root->hierarchy_id);
        for_each_root_subsys(root, ss)
        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);
        retval = cgroup_path(cgrp, buf, PAGE_SIZE);
        if (retval < 0)
            goto out_unlock;
        seq_puts(m, buf);
        seq_putc(m, '\n');
    }

out_unlock:
    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,
               ss->root->number_of_cgroups, !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 - attach newly forked task to its parents cgroup.
 * @child: pointer to task_struct of forking parent process.
 *
 * Description: A task inherits its parent's cgroup at fork().
 *
 * A pointer to the shared css_set was automatically copied in
 * fork.c by dup_task_struct().  However, we ignore that copy, since
 * it was not made under the protection of RCU or cgroup_mutex, so
 * might no longer be a valid cgroup pointer.  cgroup_attach_task() might
 * have already changed current->cgroups, allowing the previously
 * referenced cgroup group to be removed and freed.
 *
 * At the point that cgroup_fork() is called, 'current' is the parent
 * task, and the passed argument 'child' points to the child task.
 */
void cgroup_fork(struct task_struct *child)
{
    task_lock(current);
    get_css_set(task_css_set(current));
    child->cgroups = current->cgroups;
    task_unlock(current);
    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_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;

    /*
     * use_task_css_set_links is set to 1 before we walk the tasklist
     * under the tasklist_lock and we read it here after we added the child
     * to the tasklist under the tasklist_lock as well. If the child wasn't
     * yet in the tasklist when we walked through it from
     * cgroup_enable_task_cg_lists(), then use_task_css_set_links value
     * should be visible now due to the paired locking and barriers implied
     * by LOCK/UNLOCK: it is written before the tasklist_lock unlock
     * in cgroup_enable_task_cg_lists() and read here after the tasklist_lock
     * lock on fork.
     */
    if (use_task_css_set_links)
    {
        write_lock(&css_set_lock);
        task_lock(child);
        if (list_empty(&child->cg_list))
            list_add(&child->cg_list, &task_css_set(child)->tasks);
        task_unlock(child);
        write_unlock(&css_set_lock);
    }

    /*
     * 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)
    {
        /*
         * fork/exit callbacks are supported only for builtin
         * subsystems, and the builtin section of the subsys
         * array is immutable, so we don't need to lock the
         * subsys array here. On the other hand, modular section
         * of the array can be freed at module unload, so we
         * can't touch that.
         */
        for_each_builtin_subsys(ss, i)
        if (ss->fork)
            ss->fork(child);
    }
}

/**
 * cgroup_exit - detach cgroup from exiting task
 * @tsk: pointer to task_struct of exiting process
 * @run_callback: run exit callbacks?
 *
 * 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.
 *
 * the_top_cgroup_hack:
 *
 *    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.
 *
 *    To do this properly, we would increment the reference count on
 *    top_cgroup, and near the very end of the kernel/exit.c do_exit()
 *    code we would add a second cgroup function call, to drop that
 *    reference.  This would just create an unnecessary hot spot on
 *    the top_cgroup reference count, to no avail.
 *
 *    Normally, holding a reference to a cgroup without bumping its
 *    count is unsafe.   The cgroup could go away, or someone could
 *    attach us to a different cgroup, decrementing the count on
 *    the first cgroup that we never incremented.  But in this case,
 *    top_cgroup isn't going away, and either task has PF_EXITING set,
 *    which wards off any cgroup_attach_task() attempts, or task is a failed
 *    fork, never visible to cgroup_attach_task.
 */
void cgroup_exit(struct task_struct *tsk, int run_callbacks)
{
    struct cgroup_subsys *ss;
    struct css_set *cset;
    int i;

    /*
     * Unlink from the css_set task list if necessary.
     * Optimistically check cg_list before taking
     * css_set_lock
     */
    if (!list_empty(&tsk->cg_list))
    {
        write_lock(&css_set_lock);
        if (!list_empty(&tsk->cg_list))
            list_del_init(&tsk->cg_list);
        write_unlock(&css_set_lock);
    }

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

    if (run_callbacks && need_forkexit_callback)
    {
        /*
         * fork/exit callbacks are supported only for builtin
         * subsystems, see cgroup_post_fork() for details.
         */
        for_each_builtin_subsys(ss, i)
        {
            if (ss->exit)
            {
                struct cgroup *old_cgrp = cset->subsys[i]->cgroup;
                struct cgroup *cgrp = task_cgroup(tsk, i);

                ss->exit(cgrp, old_cgrp, tsk);
            }
        }
    }
    task_unlock(tsk);

    put_css_set_taskexit(cset);
}

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;
        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(PAGE_SIZE, GFP_KERNEL);
        if (!pathbuf)
            goto continue_free;
        if (cgroup_path(cgrp, pathbuf, PAGE_SIZE) < 0)
            goto continue_free;
        agentbuf = kstrdup(cgrp->root->release_agent_path, GFP_KERNEL);
        if (!agentbuf)
            goto continue_free;

        i = 0;
        argv[i++] = agentbuf;
        argv[i++] = pathbuf;
        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;

        /*
         * cgroup_disable, being at boot time, can't know about
         * module subsystems, so we don't worry about them.
         */
        for_each_builtin_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);

/*
 * Functons for CSS ID.
 */

/* to get ID other than 0, this should be called when !cgroup_is_dead() */
unsigned short css_id(struct cgroup_subsys_state *css)
{
    struct css_id *cssid;

    /*
     * This css_id() can return correct value when somone has refcnt
     * on this or this is under rcu_read_lock(). Once css->id is allocated,
     * it's unchanged until freed.
     */
    cssid = rcu_dereference_raw(css->id);

    if (cssid)
        return cssid->id;
    return 0;
}
EXPORT_SYMBOL_GPL(css_id);

/**
 *  css_is_ancestor - test "root" css is an ancestor of "child"
 * @child: the css to be tested.
 * @root: the css supporsed to be an ancestor of the child.
 *
 * Returns true if "root" is an ancestor of "child" in its hierarchy. Because
 * this function reads css->id, the caller must hold rcu_read_lock().
 * But, considering usual usage, the csses should be valid objects after test.
 * Assuming that the caller will do some action to the child if this returns
 * returns true, the caller must take "child";s reference count.
 * If "child" is valid object and this returns true, "root" is valid, too.
 */

bool css_is_ancestor(struct cgroup_subsys_state *child,
                     const struct cgroup_subsys_state *root)
{
    struct css_id *child_id;
    struct css_id *root_id;

    child_id  = rcu_dereference(child->id);
    if (!child_id)
        return false;
    root_id = rcu_dereference(root->id);
    if (!root_id)
        return false;
    if (child_id->depth < root_id->depth)
        return false;
    if (child_id->stack[root_id->depth] != root_id->id)
        return false;
    return true;
}

void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css)
{
    struct css_id *id = rcu_dereference_protected(css->id, true);

    /* When this is called before css_id initialization, id can be NULL */
    if (!id)
        return;

    BUG_ON(!ss->use_id);

    rcu_assign_pointer(id->css, NULL);
    rcu_assign_pointer(css->id, NULL);
    spin_lock(&ss->id_lock);
    idr_remove(&ss->idr, id->id);
    spin_unlock(&ss->id_lock);
    kfree_rcu(id, rcu_head);
}
EXPORT_SYMBOL_GPL(free_css_id);

/*
 * This is called by init or create(). Then, calls to this function are
 * always serialized (By cgroup_mutex() at create()).
 */

static struct css_id *get_new_cssid(struct cgroup_subsys *ss, int depth)
{
    struct css_id *newid;
    int ret, size;

    BUG_ON(!ss->use_id);

    size = sizeof(*newid) + sizeof(unsigned short) * (depth + 1);
    newid = kzalloc(size, GFP_KERNEL);
    if (!newid)
        return ERR_PTR(-ENOMEM);

    idr_preload(GFP_KERNEL);
    spin_lock(&ss->id_lock);
    /* Don't use 0. allocates an ID of 1-65535 */
    ret = idr_alloc(&ss->idr, newid, 1, CSS_ID_MAX + 1, GFP_NOWAIT);
    spin_unlock(&ss->id_lock);
    idr_preload_end();

    /* Returns error when there are no free spaces for new ID.*/
    if (ret < 0)
        goto err_out;

    newid->id = ret;
    newid->depth = depth;
    return newid;
err_out:
    kfree(newid);
    return ERR_PTR(ret);

}

static int __init_or_module cgroup_init_idr(struct cgroup_subsys *ss,
        struct cgroup_subsys_state *rootcss)
{
    struct css_id *newid;

    spin_lock_init(&ss->id_lock);
    idr_init(&ss->idr);

    newid = get_new_cssid(ss, 0);
    if (IS_ERR(newid))
        return PTR_ERR(newid);

    newid->stack[0] = newid->id;
    RCU_INIT_POINTER(newid->css, rootcss);
    RCU_INIT_POINTER(rootcss->id, newid);
    return 0;
}

static int alloc_css_id(struct cgroup_subsys *ss, struct cgroup *parent,
                        struct cgroup *child)
{
    int subsys_id, i, depth = 0;
    struct cgroup_subsys_state *parent_css, *child_css;
    struct css_id *child_id, *parent_id;

    subsys_id = ss->subsys_id;
    parent_css = parent->subsys[subsys_id];
    child_css = child->subsys[subsys_id];
    parent_id = rcu_dereference_protected(parent_css->id, true);
    depth = parent_id->depth + 1;

    child_id = get_new_cssid(ss, depth);
    if (IS_ERR(child_id))
        return PTR_ERR(child_id);

    for (i = 0; i < depth; i++)
        child_id->stack[i] = parent_id->stack[i];
    child_id->stack[depth] = child_id->id;
    /*
     * child_id->css pointer will be set after this cgroup is available
     * see cgroup_populate_dir()
     */
    rcu_assign_pointer(child_css->id, child_id);

    return 0;
}

/**
 * css_lookup - lookup css by id
 * @ss: cgroup subsys to be looked into.
 * @id: the id
 *
 * Returns pointer to cgroup_subsys_state if there is valid one with id.
 * NULL if not. Should be called under rcu_read_lock()
 */
struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id)
{
    struct css_id *cssid = NULL;

    BUG_ON(!ss->use_id);
    cssid = idr_find(&ss->idr, id);

    if (unlikely(!cssid))
        return NULL;

    return rcu_dereference(cssid->css);
}
EXPORT_SYMBOL_GPL(css_lookup);

/*
 * get corresponding css from file open on cgroupfs directory
 */
struct cgroup_subsys_state *cgroup_css_from_dir(struct file *f, int id)
{
    struct cgroup *cgrp;
    struct inode *inode;
    struct cgroup_subsys_state *css;

    inode = file_inode(f);
    /* check in cgroup filesystem dir */
    if (inode->i_op != &cgroup_dir_inode_operations)
        return ERR_PTR(-EBADF);

    if (id < 0 || id >= CGROUP_SUBSYS_COUNT)
        return ERR_PTR(-EINVAL);

    /* get cgroup */
    cgrp = __d_cgrp(f->f_dentry);
    css = cgrp->subsys[id];
    return css ? css : ERR_PTR(-ENOENT);
}

#ifdef CONFIG_CGROUP_DEBUG
static struct cgroup_subsys_state *debug_css_alloc(struct cgroup *cgrp)
{
    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 *cgrp)
{
    kfree(cgrp->subsys[debug_subsys_id]);
}

static u64 debug_taskcount_read(struct cgroup *cgrp, struct cftype *cft)
{
    return cgroup_task_count(cgrp);
}

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

static u64 current_css_set_refcount_read(struct cgroup *cgrp,
        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 cgroup *cgrp,
        struct cftype *cft,
        struct seq_file *seq)
{
    struct cgrp_cset_link *link;
    struct css_set *cset;

    read_lock(&css_set_lock);
    rcu_read_lock();
    cset = rcu_dereference(current->cgroups);
    list_for_each_entry(link, &cset->cgrp_links, cgrp_link)
    {
        struct cgroup *c = link->cgrp;
        const char *name;

        if (c->dentry)
            name = c->dentry->d_name.name;
        else
            name = "?";
        seq_printf(seq, "Root %d group %s\n",
                   c->root->hierarchy_id, name);
    }
    rcu_read_unlock();
    read_unlock(&css_set_lock);
    return 0;
}

#define MAX_TASKS_SHOWN_PER_CSS 25
static int cgroup_css_links_read(struct cgroup *cgrp,
                                 struct cftype *cft,
                                 struct seq_file *seq)
{
    struct cgrp_cset_link *link;

    read_lock(&css_set_lock);
    list_for_each_entry(link, &cgrp->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)
            {
                seq_puts(seq, "  ...\n");
                break;
            }
            else
            {
                seq_printf(seq, "  task %d\n",
                           task_pid_vnr(task));
            }
        }
    }
    read_unlock(&css_set_lock);
    return 0;
}

static u64 releasable_read(struct cgroup *cgrp, struct cftype *cft)
{
    return test_bit(CGRP_RELEASABLE, &cgrp->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",
        .read_seq_string = current_css_set_cg_links_read,
    },

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

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

    { }	/* terminate */
};

struct cgroup_subsys debug_subsys =
{
    .name = "debug",
    .css_alloc = debug_css_alloc,
    .css_free = debug_css_free,
    .subsys_id = debug_subsys_id,
    .base_cftypes = debug_files,
};
#endif /* CONFIG_CGROUP_DEBUG */