cgroup: introduce cancel_attach()

[safe/jmp/linux-2.6] / kernel / cgroup.c

/*
 *  Generic process-grouping system.
 *
 *  Based originally on the cpuset system, extracted by Paul Menage
 *  Copyright (C) 2006 Google, Inc
 *
 *  Copyright notices from the original cpuset code:
 *  --------------------------------------------------
 *  Copyright (C) 2003 BULL SA.
 *  Copyright (C) 2004-2006 Silicon Graphics, Inc.
 *
 *  Portions derived from Patrick Mochel's sysfs code.
 *  sysfs is Copyright (c) 2001-3 Patrick Mochel
 *
 *  2003-10-10 Written by Simon Derr.
 *  2003-10-22 Updates by Stephen Hemminger.
 *  2004 May-July Rework by Paul Jackson.
 *  ---------------------------------------------------
 *
 *  This file is subject to the terms and conditions of the GNU General Public
 *  License.  See the file COPYING in the main directory of the Linux
 *  distribution for more details.
 */

#include <linux/cgroup.h>
#include <linux/module.h>
#include <linux/ctype.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/list.h>
#include <linux/mm.h>
#include <linux/mutex.h>
#include <linux/mount.h>
#include <linux/pagemap.h>
#include <linux/proc_fs.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/backing-dev.h>
#include <linux/seq_file.h>
#include <linux/slab.h>
#include <linux/magic.h>
#include <linux/spinlock.h>
#include <linux/string.h>
#include <linux/sort.h>
#include <linux/kmod.h>
#include <linux/delayacct.h>
#include <linux/cgroupstats.h>
#include <linux/hash.h>
#include <linux/namei.h>
#include <linux/smp_lock.h>
#include <linux/pid_namespace.h>
#include <linux/idr.h>
#include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */

#include <asm/atomic.h>

static DEFINE_MUTEX(cgroup_mutex);

/* Generate an array of cgroup subsystem pointers */
#define SUBSYS(_x) &_x ## _subsys,

static struct cgroup_subsys *subsys[] = {
#include <linux/cgroup_subsys.h>
};

#define MAX_CGROUP_ROOT_NAMELEN 64

/*
 * A cgroupfs_root represents the root of a cgroup hierarchy,
 * and may be associated with a superblock to form an active
 * hierarchy
 */
struct cgroupfs_root {
        struct super_block *sb;

        /*
         * The bitmask of subsystems intended to be attached to this
         * hierarchy
         */
        unsigned long subsys_bits;

        /* Unique id for this hierarchy. */
        int hierarchy_id;

        /* The bitmask of subsystems currently attached to this hierarchy */
        unsigned long actual_subsys_bits;

        /* A list running through the attached subsystems */
        struct list_head subsys_list;

        /* The root cgroup for this hierarchy */
        struct cgroup top_cgroup;

        /* Tracks how many cgroups are currently defined in hierarchy.*/
        int number_of_cgroups;

        /* A list running through the active hierarchies */
        struct list_head root_list;

        /* Hierarchy-specific flags */
        unsigned long flags;

        /* The path to use for release notifications. */
        char release_agent_path[PATH_MAX];

        /* The name for this hierarchy - may be empty */
        char name[MAX_CGROUP_ROOT_NAMELEN];
};

/*
 * The "rootnode" hierarchy is 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 rootnode;

/*
 * 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_is_removed()
         * css_tryget() should be used for avoiding race.
         */
        struct cgroup_subsys_state *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) */
};


/* The list of hierarchy roots */

static LIST_HEAD(roots);
static int root_count;

static DEFINE_IDA(hierarchy_ida);
static int next_hierarchy_id;
static DEFINE_SPINLOCK(hierarchy_id_lock);

/* dummytop is a shorthand for the dummy hierarchy's top cgroup */
#define dummytop (&rootnode.top_cgroup)

/* 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;

#ifdef CONFIG_PROVE_LOCKING
int cgroup_lock_is_held(void)
{
        return lockdep_is_held(&cgroup_mutex);
}
#else /* #ifdef CONFIG_PROVE_LOCKING */
int cgroup_lock_is_held(void)
{
        return mutex_is_locked(&cgroup_mutex);
}
#endif /* #else #ifdef CONFIG_PROVE_LOCKING */

EXPORT_SYMBOL_GPL(cgroup_lock_is_held);

/* convenient tests for these bits */
inline int cgroup_is_removed(const struct cgroup *cgrp)
{
        return test_bit(CGRP_REMOVED, &cgrp->flags);
}

/* bits in struct cgroupfs_root flags field */
enum {
        ROOT_NOPREFIX, /* mounted subsystems have no named prefix */
};

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() allows you to iterate on each subsystem attached to
 * an active hierarchy
 */
#define for_each_subsys(_root, _ss) \
list_for_each_entry(_ss, &_root->subsys_list, sibling)

/* for_each_active_root() allows you to iterate across the active hierarchies */
#define for_each_active_root(_root) \
list_for_each_entry(_root, &roots, root_list)

/* the list of cgroups eligible for automatic release. Protected by
 * release_list_lock */
static LIST_HEAD(release_list);
static DEFINE_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);

/* Link structure for associating css_set objects with cgroups */
struct cg_cgroup_link {
        /*
         * List running through cg_cgroup_links associated with a
         * cgroup, anchored on cgroup->css_sets
         */
        struct list_head cgrp_link_list;
        struct cgroup *cgrp;
        /*
         * List running through cg_cgroup_links pointing at a
         * single css_set object, anchored on css_set->cg_links
         */
        struct list_head cg_link_list;
        struct css_set *cg;
};

/* 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 cg_cgroup_link init_css_set_link;

static int cgroup_subsys_init_idr(struct cgroup_subsys *ss);

/* 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
#define CSS_SET_TABLE_SIZE      (1 << CSS_SET_HASH_BITS)
static struct hlist_head css_set_table[CSS_SET_TABLE_SIZE];

static struct hlist_head *css_set_hash(struct cgroup_subsys_state *css[])
{
        int i;
        int index;
        unsigned long tmp = 0UL;

        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++)
                tmp += (unsigned long)css[i];
        tmp = (tmp >> 16) ^ tmp;

        index = hash_long(tmp, CSS_SET_HASH_BITS);

        return &css_set_table[index];
}

static void free_css_set_rcu(struct rcu_head *obj)
{
        struct css_set *cg = container_of(obj, struct css_set, rcu_head);
        kfree(cg);
}

/* 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 *cg, int taskexit)
{
        struct cg_cgroup_link *link;
        struct cg_cgroup_link *saved_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(&cg->refcount, -1, 1))
                return;
        write_lock(&css_set_lock);
        if (!atomic_dec_and_test(&cg->refcount)) {
                write_unlock(&css_set_lock);
                return;
        }

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

        list_for_each_entry_safe(link, saved_link, &cg->cg_links,
                                 cg_link_list) {
                struct cgroup *cgrp = link->cgrp;
                list_del(&link->cg_link_list);
                list_del(&link->cgrp_link_list);
                if (atomic_dec_and_test(&cgrp->count) &&
                    notify_on_release(cgrp)) {
                        if (taskexit)
                                set_bit(CGRP_RELEASABLE, &cgrp->flags);
                        check_for_release(cgrp);
                }

                kfree(link);
        }

        write_unlock(&css_set_lock);
        call_rcu(&cg->rcu_head, free_css_set_rcu);
}

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

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

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

/*
 * compare_css_sets - helper function for find_existing_css_set().
 * @cg: candidate css_set being tested
 * @old_cg: 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 *cg,
                             struct css_set *old_cg,
                             struct cgroup *new_cgrp,
                             struct cgroup_subsys_state *template[])
{
        struct list_head *l1, *l2;

        if (memcmp(template, cg->subsys, sizeof(cg->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 = &cg->cg_links;
        l2 = &old_cg->cg_links;
        while (1) {
                struct cg_cgroup_link *cgl1, *cgl2;
                struct cgroup *cg1, *cg2;

                l1 = l1->next;
                l2 = l2->next;
                /* See if we reached the end - both lists are equal length. */
                if (l1 == &cg->cg_links) {
                        BUG_ON(l2 != &old_cg->cg_links);
                        break;
                } else {
                        BUG_ON(l2 == &old_cg->cg_links);
                }
                /* Locate the cgroups associated with these links. */
                cgl1 = list_entry(l1, struct cg_cgroup_link, cg_link_list);
                cgl2 = list_entry(l2, struct cg_cgroup_link, cg_link_list);
                cg1 = cgl1->cgrp;
                cg2 = cgl2->cgrp;
                /* Hierarchies should be linked in the same order. */
                BUG_ON(cg1->root != cg2->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 (cg1->root == new_cgrp->root) {
                        if (cg1 != new_cgrp)
                                return false;
                } else {
                        if (cg1 != cg2)
                                return false;
                }
        }
        return true;
}

/*
 * find_existing_css_set() is a helper for
 * find_css_set(), and checks to see whether an existing
 * css_set is suitable.
 *
 * oldcg: the cgroup group that we're using before the cgroup
 * transition
 *
 * cgrp: the cgroup that we're moving into
 *
 * template: location in which to build the desired set of subsystem
 * state objects for the new cgroup group
 */
static struct css_set *find_existing_css_set(
        struct css_set *oldcg,
        struct cgroup *cgrp,
        struct cgroup_subsys_state *template[])
{
        int i;
        struct cgroupfs_root *root = cgrp->root;
        struct hlist_head *hhead;
        struct hlist_node *node;
        struct css_set *cg;

        /* Built the set of subsystem state objects that we want to
         * see in the new css_set */
        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                if (root->subsys_bits & (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] = oldcg->subsys[i];
                }
        }

        hhead = css_set_hash(template);
        hlist_for_each_entry(cg, node, hhead, hlist) {
                if (!compare_css_sets(cg, oldcg, cgrp, template))
                        continue;

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

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

static void free_cg_links(struct list_head *tmp)
{
        struct cg_cgroup_link *link;
        struct cg_cgroup_link *saved_link;

        list_for_each_entry_safe(link, saved_link, tmp, cgrp_link_list) {
                list_del(&link->cgrp_link_list);
                kfree(link);
        }
}

/*
 * allocate_cg_links() allocates "count" cg_cgroup_link structures
 * and chains them on tmp through their cgrp_link_list fields. Returns 0 on
 * success or a negative error
 */
static int allocate_cg_links(int count, struct list_head *tmp)
{
        struct cg_cgroup_link *link;
        int i;
        INIT_LIST_HEAD(tmp);
        for (i = 0; i < count; i++) {
                link = kmalloc(sizeof(*link), GFP_KERNEL);
                if (!link) {
                        free_cg_links(tmp);
                        return -ENOMEM;
                }
                list_add(&link->cgrp_link_list, tmp);
        }
        return 0;
}

/**
 * link_css_set - a helper function to link a css_set to a cgroup
 * @tmp_cg_links: cg_cgroup_link objects allocated by allocate_cg_links()
 * @cg: the css_set to be linked
 * @cgrp: the destination cgroup
 */
static void link_css_set(struct list_head *tmp_cg_links,
                         struct css_set *cg, struct cgroup *cgrp)
{
        struct cg_cgroup_link *link;

        BUG_ON(list_empty(tmp_cg_links));
        link = list_first_entry(tmp_cg_links, struct cg_cgroup_link,
                                cgrp_link_list);
        link->cg = cg;
        link->cgrp = cgrp;
        atomic_inc(&cgrp->count);
        list_move(&link->cgrp_link_list, &cgrp->css_sets);
        /*
         * Always add links to the tail of the list so that the list
         * is sorted by order of hierarchy creation
         */
        list_add_tail(&link->cg_link_list, &cg->cg_links);
}

/*
 * find_css_set() takes an existing cgroup group and a
 * cgroup object, and returns a css_set object that's
 * equivalent to the old group, but with the given cgroup
 * substituted into the appropriate hierarchy. Must be called with
 * cgroup_mutex held
 */
static struct css_set *find_css_set(
        struct css_set *oldcg, struct cgroup *cgrp)
{
        struct css_set *res;
        struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];

        struct list_head tmp_cg_links;

        struct hlist_head *hhead;
        struct cg_cgroup_link *link;

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

        if (res)
                return res;

        res = kmalloc(sizeof(*res), GFP_KERNEL);
        if (!res)
                return NULL;

        /* Allocate all the cg_cgroup_link objects that we'll need */
        if (allocate_cg_links(root_count, &tmp_cg_links) < 0) {
                kfree(res);
                return NULL;
        }

        atomic_set(&res->refcount, 1);
        INIT_LIST_HEAD(&res->cg_links);
        INIT_LIST_HEAD(&res->tasks);
        INIT_HLIST_NODE(&res->hlist);

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

        write_lock(&css_set_lock);
        /* Add reference counts and links from the new css_set. */
        list_for_each_entry(link, &oldcg->cg_links, cg_link_list) {
                struct cgroup *c = link->cgrp;
                if (c->root == cgrp->root)
                        c = cgrp;
                link_css_set(&tmp_cg_links, res, c);
        }

        BUG_ON(!list_empty(&tmp_cg_links));

        css_set_count++;

        /* Add this cgroup group to the hash table */
        hhead = css_set_hash(res->subsys);
        hlist_add_head(&res->hlist, hhead);

        write_unlock(&css_set_lock);

        return res;
}

/*
 * 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 *css;
        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.
         */
        css = task->cgroups;
        if (css == &init_css_set) {
                res = &root->top_cgroup;
        } else {
                struct cg_cgroup_link *link;
                list_for_each_entry(link, &css->cg_links, cg_link_list) {
                        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 tasks 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'ss 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()
 */

/**
 * cgroup_lock - lock out any changes to cgroup structures
 *
 */
void cgroup_lock(void)
{
        mutex_lock(&cgroup_mutex);
}

/**
 * cgroup_unlock - release lock on cgroup changes
 *
 * Undo the lock taken in a previous cgroup_lock() call.
 */
void cgroup_unlock(void)
{
        mutex_unlock(&cgroup_mutex);
}

/*
 * 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, int mode);
static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry);
static int cgroup_populate_dir(struct cgroup *cgrp);
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(mode_t mode, struct super_block *sb)
{
        struct inode *inode = new_inode(sb);

        if (inode) {
                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;
}

/*
 * Call subsys's pre_destroy handler.
 * This is called before css refcnt check.
 */
static int cgroup_call_pre_destroy(struct cgroup *cgrp)
{
        struct cgroup_subsys *ss;
        int ret = 0;

        for_each_subsys(cgrp->root, ss)
                if (ss->pre_destroy) {
                        ret = ss->pre_destroy(ss, cgrp);
                        if (ret)
                                break;
                }
        return ret;
}

static void free_cgroup_rcu(struct rcu_head *obj)
{
        struct cgroup *cgrp = container_of(obj, struct cgroup, rcu_head);

        kfree(cgrp);
}

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;
                struct cgroup_subsys *ss;
                BUG_ON(!(cgroup_is_removed(cgrp)));
                /* It's possible for external users to be holding css
                 * reference counts on a cgroup; css_put() needs to
                 * be able to access the cgroup after decrementing
                 * the reference count in order to know if it needs to
                 * queue the cgroup to be handled by the release
                 * agent */
                synchronize_rcu();

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

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

                /*
                 * Drop the active superblock reference that we took when we
                 * created the cgroup
                 */
                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));

                call_rcu(&cgrp->rcu_head, free_cgroup_rcu);
        }
        iput(inode);
}

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_clear_directory(struct dentry *dentry)
{
        struct list_head *node;

        BUG_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
        spin_lock(&dcache_lock);
        node = dentry->d_subdirs.next;
        while (node != &dentry->d_subdirs) {
                struct dentry *d = list_entry(node, struct dentry, d_u.d_child);
                list_del_init(node);
                if (d->d_inode) {
                        /* This should never be called on a cgroup
                         * directory with child cgroups */
                        BUG_ON(d->d_inode->i_mode & S_IFDIR);
                        d = dget_locked(d);
                        spin_unlock(&dcache_lock);
                        d_delete(d);
                        simple_unlink(dentry->d_inode, d);
                        dput(d);
                        spin_lock(&dcache_lock);
                }
                node = dentry->d_subdirs.next;
        }
        spin_unlock(&dcache_lock);
}

/*
 * NOTE : the dentry must have been dget()'ed
 */
static void cgroup_d_remove_dir(struct dentry *dentry)
{
        cgroup_clear_directory(dentry);

        spin_lock(&dcache_lock);
        list_del_init(&dentry->d_u.d_child);
        spin_unlock(&dcache_lock);
        remove_dir(dentry);
}

/*
 * A queue for waiters to do rmdir() cgroup. A tasks will sleep when
 * cgroup->count == 0 && list_empty(&cgroup->children) && subsys has some
 * reference to css->refcnt. In general, this refcnt is expected to goes down
 * to zero, soon.
 *
 * CGRP_WAIT_ON_RMDIR flag is set under cgroup's inode->i_mutex;
 */
DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);

static void cgroup_wakeup_rmdir_waiter(struct cgroup *cgrp)
{
        if (unlikely(test_and_clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)))
                wake_up_all(&cgroup_rmdir_waitq);
}

void cgroup_exclude_rmdir(struct cgroup_subsys_state *css)
{
        css_get(css);
}

void cgroup_release_and_wakeup_rmdir(struct cgroup_subsys_state *css)
{
        cgroup_wakeup_rmdir_waiter(css->cgroup);
        css_put(css);
}


static int rebind_subsystems(struct cgroupfs_root *root,
                              unsigned long final_bits)
{
        unsigned long added_bits, removed_bits;
        struct cgroup *cgrp = &root->top_cgroup;
        int i;

        removed_bits = root->actual_subsys_bits & ~final_bits;
        added_bits = final_bits & ~root->actual_subsys_bits;
        /* Check that any added subsystems are currently free */
        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                unsigned long bit = 1UL << i;
                struct cgroup_subsys *ss = subsys[i];
                if (!(bit & added_bits))
                        continue;
                if (ss->root != &rootnode) {
                        /* 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 (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup_subsys *ss = subsys[i];
                unsigned long bit = 1UL << i;
                if (bit & added_bits) {
                        /* We're binding this subsystem to this hierarchy */
                        BUG_ON(cgrp->subsys[i]);
                        BUG_ON(!dummytop->subsys[i]);
                        BUG_ON(dummytop->subsys[i]->cgroup != dummytop);
                        mutex_lock(&ss->hierarchy_mutex);
                        cgrp->subsys[i] = dummytop->subsys[i];
                        cgrp->subsys[i]->cgroup = cgrp;
                        list_move(&ss->sibling, &root->subsys_list);
                        ss->root = root;
                        if (ss->bind)
                                ss->bind(ss, cgrp);
                        mutex_unlock(&ss->hierarchy_mutex);
                } else if (bit & removed_bits) {
                        /* We're removing this subsystem */
                        BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
                        BUG_ON(cgrp->subsys[i]->cgroup != cgrp);
                        mutex_lock(&ss->hierarchy_mutex);
                        if (ss->bind)
                                ss->bind(ss, dummytop);
                        dummytop->subsys[i]->cgroup = dummytop;
                        cgrp->subsys[i] = NULL;
                        subsys[i]->root = &rootnode;
                        list_move(&ss->sibling, &rootnode.subsys_list);
                        mutex_unlock(&ss->hierarchy_mutex);
                } else if (bit & final_bits) {
                        /* Subsystem state should already exist */
                        BUG_ON(!cgrp->subsys[i]);
                } else {
                        /* Subsystem state shouldn't exist */
                        BUG_ON(cgrp->subsys[i]);
                }
        }
        root->subsys_bits = root->actual_subsys_bits = final_bits;
        synchronize_rcu();

        return 0;
}

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

        mutex_lock(&cgroup_mutex);
        for_each_subsys(root, ss)
                seq_printf(seq, ",%s", ss->name);
        if (test_bit(ROOT_NOPREFIX, &root->flags))
                seq_puts(seq, ",noprefix");
        if (strlen(root->release_agent_path))
                seq_printf(seq, ",release_agent=%s", root->release_agent_path);
        if (strlen(root->name))
                seq_printf(seq, ",name=%s", root->name);
        mutex_unlock(&cgroup_mutex);
        return 0;
}

struct cgroup_sb_opts {
        unsigned long subsys_bits;
        unsigned long flags;
        char *release_agent;
        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. */
static int parse_cgroupfs_options(char *data,
                                     struct cgroup_sb_opts *opts)
{
        char *token, *o = data ?: "all";
        unsigned long mask = (unsigned long)-1;

#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, "all")) {
                        /* Add all non-disabled subsystems */
                        int i;
                        opts->subsys_bits = 0;
                        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                                struct cgroup_subsys *ss = subsys[i];
                                if (!ss->disabled)
                                        opts->subsys_bits |= 1ul << i;
                        }
                } else if (!strcmp(token, "none")) {
                        /* Explicitly have no subsystems */
                        opts->none = true;
                } else if (!strcmp(token, "noprefix")) {
                        set_bit(ROOT_NOPREFIX, &opts->flags);
                } else 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, GFP_KERNEL);
                        if (!opts->release_agent)
                                return -ENOMEM;
                } else if (!strncmp(token, "name=", 5)) {
                        int i;
                        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,
                                              GFP_KERNEL);
                        if (!opts->name)
                                return -ENOMEM;
                } else {
                        struct cgroup_subsys *ss;
                        int i;
                        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                                ss = subsys[i];
                                if (!strcmp(token, ss->name)) {
                                        if (!ss->disabled)
                                                set_bit(i, &opts->subsys_bits);
                                        break;
                                }
                        }
                        if (i == CGROUP_SUBSYS_COUNT)
                                return -ENOENT;
                }
        }

        /* Consistency checks */

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


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

        return 0;
}

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;

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

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

        /* Don't allow flags to change at remount */
        if (opts.flags != root->flags) {
                ret = -EINVAL;
                goto out_unlock;
        }

        /* Don't allow name to change at remount */
        if (opts.name && strcmp(opts.name, root->name)) {
                ret = -EINVAL;
                goto out_unlock;
        }

        ret = rebind_subsystems(root, opts.subsys_bits);
        if (ret)
                goto out_unlock;

        /* (re)populate subsystem files */
        cgroup_populate_dir(cgrp);

        if (opts.release_agent)
                strcpy(root->release_agent_path, opts.release_agent);
 out_unlock:
        kfree(opts.release_agent);
        kfree(opts.name);
        mutex_unlock(&cgroup_mutex);
        mutex_unlock(&cgrp->dentry->d_inode->i_mutex);
        unlock_kernel();
        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->css_sets);
        INIT_LIST_HEAD(&cgrp->release_list);
        INIT_LIST_HEAD(&cgrp->pidlists);
        mutex_init(&cgrp->pidlist_mutex);
}

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;
        cgrp->top_cgroup = cgrp;
        init_cgroup_housekeeping(cgrp);
}

static bool init_root_id(struct cgroupfs_root *root)
{
        int ret = 0;

        do {
                if (!ida_pre_get(&hierarchy_ida, GFP_KERNEL))
                        return false;
                spin_lock(&hierarchy_id_lock);
                /* Try to allocate the next unused ID */
                ret = ida_get_new_above(&hierarchy_ida, next_hierarchy_id,
                                        &root->hierarchy_id);
                if (ret == -ENOSPC)
                        /* Try again starting from 0 */
                        ret = ida_get_new(&hierarchy_ida, &root->hierarchy_id);
                if (!ret) {
                        next_hierarchy_id = root->hierarchy_id + 1;
                } else if (ret != -EAGAIN) {
                        /* Can only get here if the 31-bit IDR is full ... */
                        BUG_ON(ret);
                }
                spin_unlock(&hierarchy_id_lock);
        } while (ret);
        return true;
}

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_bits || opts->none)
            && (opts->subsys_bits != root->subsys_bits))
                return 0;

        return 1;
}

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

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

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

        if (!init_root_id(root)) {
                kfree(root);
                return ERR_PTR(-ENOMEM);
        }
        init_cgroup_root(root);

        root->subsys_bits = opts->subsys_bits;
        root->flags = opts->flags;
        if (opts->release_agent)
                strcpy(root->release_agent_path, opts->release_agent);
        if (opts->name)
                strcpy(root->name, opts->name);
        return root;
}

static void cgroup_drop_root(struct cgroupfs_root *root)
{
        if (!root)
                return;

        BUG_ON(!root->hierarchy_id);
        spin_lock(&hierarchy_id_lock);
        ida_remove(&hierarchy_ida, root->hierarchy_id);
        spin_unlock(&hierarchy_id_lock);
        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_bits && !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)
{
        struct inode *inode =
                cgroup_new_inode(S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR, sb);
        struct dentry *dentry;

        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);
        dentry = d_alloc_root(inode);
        if (!dentry) {
                iput(inode);
                return -ENOMEM;
        }
        sb->s_root = dentry;
        return 0;
}

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

        /* First find the desired set of subsystems */
        ret = parse_cgroupfs_options(data, &opts);
        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 out_err;
        }
        opts.new_root = new_root;

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

        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_cg_links;
                struct cgroup *root_cgrp = &root->top_cgroup;
                struct inode *inode;
                struct cgroupfs_root *existing_root;
                int i;

                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);

                if (strlen(root->name)) {
                        /* Check for name clashes with existing mounts */
                        for_each_active_root(existing_root) {
                                if (!strcmp(existing_root->name, root->name)) {
                                        ret = -EBUSY;
                                        mutex_unlock(&cgroup_mutex);
                                        mutex_unlock(&inode->i_mutex);
                                        goto drop_new_super;
                                }
                        }
                }

                /*
                 * 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_cg_links(css_set_count, &tmp_cg_links);
                if (ret) {
                        mutex_unlock(&cgroup_mutex);
                        mutex_unlock(&inode->i_mutex);
                        goto drop_new_super;
                }

                ret = rebind_subsystems(root, root->subsys_bits);
                if (ret == -EBUSY) {
                        mutex_unlock(&cgroup_mutex);
                        mutex_unlock(&inode->i_mutex);
                        free_cg_links(&tmp_cg_links);
                        goto drop_new_super;
                }

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

                list_add(&root->root_list, &roots);
                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);
                for (i = 0; i < CSS_SET_TABLE_SIZE; i++) {
                        struct hlist_head *hhead = &css_set_table[i];
                        struct hlist_node *node;
                        struct css_set *cg;

                        hlist_for_each_entry(cg, node, hhead, hlist)
                                link_css_set(&tmp_cg_links, cg, root_cgrp);
                }
                write_unlock(&css_set_lock);

                free_cg_links(&tmp_cg_links);

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

                cgroup_populate_dir(root_cgrp);
                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_drop_root(opts.new_root);
        }

        simple_set_mnt(mnt, sb);
        kfree(opts.release_agent);
        kfree(opts.name);
        return 0;

 drop_new_super:
        deactivate_locked_super(sb);
 out_err:
        kfree(opts.release_agent);
        kfree(opts.name);

        return ret;
}

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

        BUG_ON(!root);

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

        mutex_lock(&cgroup_mutex);

        /* Rebind all subsystems back to the default hierarchy */
        ret = rebind_subsystems(root, 0);
        /* Shouldn't be able to fail ... */
        BUG_ON(ret);

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

        list_for_each_entry_safe(link, saved_link, &cgrp->css_sets,
                                 cgrp_link_list) {
                list_del(&link->cg_link_list);
                list_del(&link->cgrp_link_list);
                kfree(link);
        }
        write_unlock(&css_set_lock);

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

        mutex_unlock(&cgroup_mutex);

        kill_litter_super(sb);
        cgroup_drop_root(root);
}

static struct file_system_type cgroup_fs_type = {
        .name = "cgroup",
        .get_sb = cgroup_get_sb,
        .kill_sb = cgroup_kill_sb,
};

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

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

/**
 * 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
 *
 * Called with cgroup_mutex held or else with an RCU-protected cgroup
 * reference.  Writes path of cgroup into buf.  Returns 0 on success,
 * -errno on error.
 */
int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)
{
        char *start;
        struct dentry *dentry = rcu_dereference(cgrp->dentry);

        if (!dentry || cgrp == dummytop) {
                /*
                 * Inactive subsystems have no dentry for their root
                 * cgroup
                 */
                strcpy(buf, "/");
                return 0;
        }

        start = buf + buflen;

        *--start = '\0';
        for (;;) {
                int len = dentry->d_name.len;
                if ((start -= len) < buf)
                        return -ENAMETOOLONG;
                memcpy(start, cgrp->dentry->d_name.name, len);
                cgrp = cgrp->parent;
                if (!cgrp)
                        break;
                dentry = rcu_dereference(cgrp->dentry);
                if (!cgrp->parent)
                        continue;
                if (--start < buf)
                        return -ENAMETOOLONG;
                *start = '/';
        }
        memmove(buf, start, buf + buflen - start);
        return 0;
}

/**
 * cgroup_attach_task - attach task 'tsk' to cgroup 'cgrp'
 * @cgrp: the cgroup the task is attaching to
 * @tsk: the task to be attached
 *
 * Call holding cgroup_mutex. May take task_lock of
 * the task 'tsk' during call.
 */
int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
{
        int retval = 0;
        struct cgroup_subsys *ss, *failed_ss = NULL;
        struct cgroup *oldcgrp;
        struct css_set *cg;
        struct css_set *newcg;
        struct cgroupfs_root *root = cgrp->root;

        /* Nothing to do if the task is already in that cgroup */
        oldcgrp = task_cgroup_from_root(tsk, root);
        if (cgrp == oldcgrp)
                return 0;

        for_each_subsys(root, ss) {
                if (ss->can_attach) {
                        retval = ss->can_attach(ss, cgrp, tsk, false);
                        if (retval) {
                                /*
                                 * Remember on which subsystem the can_attach()
                                 * failed, so that we only call cancel_attach()
                                 * against the subsystems whose can_attach()
                                 * succeeded. (See below)
                                 */
                                failed_ss = ss;
                                goto out;
                        }
                }
        }

        task_lock(tsk);
        cg = tsk->cgroups;
        get_css_set(cg);
        task_unlock(tsk);
        /*
         * Locate or allocate a new css_set for this task,
         * based on its final set of cgroups
         */
        newcg = find_css_set(cg, cgrp);
        put_css_set(cg);
        if (!newcg) {
                retval = -ENOMEM;
                goto out;
        }

        task_lock(tsk);
        if (tsk->flags & PF_EXITING) {
                task_unlock(tsk);
                put_css_set(newcg);
                retval = -ESRCH;
                goto out;
        }
        rcu_assign_pointer(tsk->cgroups, newcg);
        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_del(&tsk->cg_list);
                list_add(&tsk->cg_list, &newcg->tasks);
        }
        write_unlock(&css_set_lock);

        for_each_subsys(root, ss) {
                if (ss->attach)
                        ss->attach(ss, cgrp, oldcgrp, tsk, false);
        }
        set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
        synchronize_rcu();
        put_css_set(cg);

        /*
         * wake up rmdir() waiter. the rmdir should fail since the cgroup
         * is no longer empty.
         */
        cgroup_wakeup_rmdir_waiter(cgrp);
out:
        if (retval) {
                for_each_subsys(root, ss) {
                        if (ss == failed_ss)
                                /*
                                 * This subsystem was the one that failed the
                                 * can_attach() check earlier, so we don't need
                                 * to call cancel_attach() against it or any
                                 * remaining subsystems.
                                 */
                                break;
                        if (ss->cancel_attach)
                                ss->cancel_attach(ss, cgrp, tsk, false);
                }
        }
        return retval;
}

/*
 * Attach task with pid 'pid' to cgroup 'cgrp'. Call with cgroup_mutex
 * held. May take task_lock of task
 */
static int attach_task_by_pid(struct cgroup *cgrp, u64 pid)
{
        struct task_struct *tsk;
        const struct cred *cred = current_cred(), *tcred;
        int ret;

        if (pid) {
                rcu_read_lock();
                tsk = find_task_by_vpid(pid);
                if (!tsk || tsk->flags & PF_EXITING) {
                        rcu_read_unlock();
                        return -ESRCH;
                }

                tcred = __task_cred(tsk);
                if (cred->euid &&
                    cred->euid != tcred->uid &&
                    cred->euid != tcred->suid) {
                        rcu_read_unlock();
                        return -EACCES;
                }
                get_task_struct(tsk);
                rcu_read_unlock();
        } else {
                tsk = current;
                get_task_struct(tsk);
        }

        ret = cgroup_attach_task(cgrp, tsk);
        put_task_struct(tsk);
        return ret;
}

static int cgroup_tasks_write(struct cgroup *cgrp, struct cftype *cft, u64 pid)
{
        int ret;
        if (!cgroup_lock_live_group(cgrp))
                return -ENODEV;
        ret = attach_task_by_pid(cgrp, pid);
        cgroup_unlock();
        return ret;
}

/**
 * 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 lock should be later released with
 * cgroup_unlock(). On failure returns false with no lock held.
 */
bool cgroup_lock_live_group(struct cgroup *cgrp)
{
        mutex_lock(&cgroup_mutex);
        if (cgroup_is_removed(cgrp)) {
                mutex_unlock(&cgroup_mutex);
                return false;
        }
        return 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 (!cgroup_lock_live_group(cgrp))
                return -ENODEV;
        strcpy(cgrp->root->release_agent_path, buffer);
        cgroup_unlock();
        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');
        cgroup_unlock();
        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_removed(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_removed(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 =
                        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)
{
        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;
        return simple_rename(old_dir, old_dentry, new_dir, new_dentry);
}

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_dir_inode_operations = {
        .lookup = simple_lookup,
        .mkdir = cgroup_mkdir,
        .rmdir = cgroup_rmdir,
        .rename = cgroup_rename,
};

static int cgroup_create_file(struct dentry *dentry, mode_t mode,
                                struct super_block *sb)
{
        static const struct dentry_operations cgroup_dops = {
                .d_iput = cgroup_diput,
        };

        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);

                /* start with the directory inode held, so that we can
                 * populate it without racing with another mkdir */
                mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);
        } else if (S_ISREG(mode)) {
                inode->i_size = 0;
                inode->i_fop = &cgroup_file_operations;
        }
        dentry->d_op = &cgroup_dops;
        d_instantiate(dentry, inode);
        dget(dentry);   /* Extra count - pin the dentry in core */
        return 0;
}

/*
 * cgroup_create_dir - create a directory for an object.
 * @cgrp: the cgroup we create the directory for. It must have a valid
 *        ->parent field. And we are going to fill its ->dentry field.
 * @dentry: dentry of the new cgroup
 * @mode: mode to set on new directory.
 */
static int cgroup_create_dir(struct cgroup *cgrp, struct dentry *dentry,
                                mode_t mode)
{
        struct dentry *parent;
        int error = 0;

        parent = cgrp->parent->dentry;
        error = cgroup_create_file(dentry, S_IFDIR | mode, cgrp->root->sb);
        if (!error) {
                dentry->d_fsdata = cgrp;
                inc_nlink(parent->d_inode);
                rcu_assign_pointer(cgrp->dentry, dentry);
                dget(dentry);
        }
        dput(dentry);

        return error;
}

/**
 * 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 mode_t cgroup_file_mode(const struct cftype *cft)
{
        mode_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;
}

int cgroup_add_file(struct cgroup *cgrp,
                       struct cgroup_subsys *subsys,
                       const struct cftype *cft)
{
        struct dentry *dir = cgrp->dentry;
        struct dentry *dentry;
        int error;
        mode_t mode;

        char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };
        if (subsys && !test_bit(ROOT_NOPREFIX, &cgrp->root->flags)) {
                strcpy(name, subsys->name);
                strcat(name, ".");
        }
        strcat(name, cft->name);
        BUG_ON(!mutex_is_locked(&dir->d_inode->i_mutex));
        dentry = lookup_one_len(name, dir, strlen(name));
        if (!IS_ERR(dentry)) {
                mode = cgroup_file_mode(cft);
                error = cgroup_create_file(dentry, mode | S_IFREG,
                                                cgrp->root->sb);
                if (!error)
                        dentry->d_fsdata = (void *)cft;
                dput(dentry);
        } else
                error = PTR_ERR(dentry);
        return error;
}

int cgroup_add_files(struct cgroup *cgrp,
                        struct cgroup_subsys *subsys,
                        const struct cftype cft[],
                        int count)
{
        int i, err;
        for (i = 0; i < count; i++) {
                err = cgroup_add_file(cgrp, subsys, &cft[i]);
                if (err)
                        return err;
        }
        return 0;
}

/**
 * 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 cg_cgroup_link *link;

        read_lock(&css_set_lock);
        list_for_each_entry(link, &cgrp->css_sets, cgrp_link_list) {
                count += atomic_read(&link->cg->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->cg_link;
        struct cg_cgroup_link *link;
        struct css_set *cg;

        /* Advance to the next non-empty css_set */
        do {
                l = l->next;
                if (l == &cgrp->css_sets) {
                        it->cg_link = NULL;
                        return;
                }
                link = list_entry(l, struct cg_cgroup_link, cgrp_link_list);
                cg = link->cg;
        } while (list_empty(&cg->tasks));
        it->cg_link = l;
        it->task = cg->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().
 *
 * The tasklist_lock is not held here, as do_each_thread() and
 * while_each_thread() are protected by RCU.
 */
static void cgroup_enable_task_cg_lists(void)
{
        struct task_struct *p, *g;
        write_lock(&css_set_lock);
        use_task_css_set_links = 1;
        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, &p->cgroups->tasks);
                task_unlock(p);
        } while_each_thread(g, p);
        write_unlock(&css_set_lock);
}

void cgroup_iter_start(struct cgroup *cgrp, struct cgroup_iter *it)
{
        /*
         * 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->cg_link = &cgrp->css_sets;
        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 cg_cgroup_link *link;

        /* If the iterator cg is NULL, we have no tasks */
        if (!it->cg_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->cg_link, struct cg_cgroup_link, cgrp_link_list);
        if (l == &link->cg->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)
{
        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;
}

/*
 * 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.
 *
 */

/*
 * 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);
}
static void *pidlist_resize(void *p, int newcount)
{
        void *newlist;
        /* note: if new alloc fails, old p will still be valid either way */
        if (is_vmalloc_addr(p)) {
                newlist = vmalloc(newcount * sizeof(pid_t));
                if (!newlist)
                        return NULL;
                memcpy(newlist, p, newcount * sizeof(pid_t));
                vfree(p);
        } else {
                newlist = krealloc(p, newcount * sizeof(pid_t), GFP_KERNEL);
        }
        return newlist;
}

/*
 * pidlist_uniq - given a kmalloc()ed list, strip out all duplicate entries
 * If the new stripped list is sufficiently smaller and there's enough memory
 * to allocate a new buffer, will let go of the unneeded memory. Returns the
 * number of unique elements.
 */
/* is the size difference enough that we should re-allocate the array? */
#define PIDLIST_REALLOC_DIFFERENCE(old, new) ((old) - PAGE_SIZE >= (new))
static int pidlist_uniq(pid_t **p, int length)
{
        int src, dest = 1;
        pid_t *list = *p;
        pid_t *newlist;

        /*
         * 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:
        /*
         * if the length difference is large enough, we want to allocate a
         * smaller buffer to save memory. if this fails due to out of memory,
         * we'll just stay with what we've got.
         */
        if (PIDLIST_REALLOC_DIFFERENCE(length, dest)) {
                newlist = pidlist_resize(list, dest);
                if (newlist)
                        *p = newlist;
        }
        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 = get_pid_ns(current->nsproxy->pid_ns);
        /*
         * 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) {
                        /* found a matching list - drop the extra refcount */
                        put_pid_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 = kmalloc(sizeof(struct cgroup_pidlist), GFP_KERNEL);
        if (!l) {
                mutex_unlock(&cgrp->pidlist_mutex);
                put_pid_ns(ns);
                return l;
        }
        init_rwsem(&l->mutex);
        down_write(&l->mutex);
        l->key.type = type;
        l->key.ns = ns;
        l->use_count = 0; /* don't increment here */
        l->list = NULL;
        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;
}

/*
 * for the common functions, 'private' gives the type of file
 */
/* for hysterical raisins, we can't put this on the older files */
#define CGROUP_FILE_GENERIC_PREFIX "cgroup."
static struct cftype files[] = {
        {
                .name = "tasks",
                .open = cgroup_tasks_open,
                .write_u64 = cgroup_tasks_write,
                .release = cgroup_pidlist_release,
                .mode = S_IRUGO | S_IWUSR,
        },
        {
                .name = CGROUP_FILE_GENERIC_PREFIX "procs",
                .open = cgroup_procs_open,
                /* .write_u64 = cgroup_procs_write, TODO */
                .release = cgroup_pidlist_release,
                .mode = S_IRUGO,
        },
        {
                .name = "notify_on_release",
                .read_u64 = cgroup_read_notify_on_release,
                .write_u64 = cgroup_write_notify_on_release,
        },
};

static struct cftype cft_release_agent = {
        .name = "release_agent",
        .read_seq_string = cgroup_release_agent_show,
        .write_string = cgroup_release_agent_write,
        .max_write_len = PATH_MAX,
};

static int cgroup_populate_dir(struct cgroup *cgrp)
{
        int err;
        struct cgroup_subsys *ss;

        /* First clear out any existing files */
        cgroup_clear_directory(cgrp->dentry);

        err = cgroup_add_files(cgrp, NULL, files, ARRAY_SIZE(files));
        if (err < 0)
                return err;

        if (cgrp == cgrp->top_cgroup) {
                if ((err = cgroup_add_file(cgrp, NULL, &cft_release_agent)) < 0)
                        return err;
        }

        for_each_subsys(cgrp->root, ss) {
                if (ss->populate && (err = ss->populate(ss, cgrp)) < 0)
                        return err;
        }
        /* This cgroup is ready now */
        for_each_subsys(cgrp->root, ss) {
                struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
                /*
                 * 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 (css->id)
                        rcu_assign_pointer(css->id->css, css);
        }

        return 0;
}

static void init_cgroup_css(struct cgroup_subsys_state *css,
                               struct cgroup_subsys *ss,
                               struct cgroup *cgrp)
{
        css->cgroup = cgrp;
        atomic_set(&css->refcnt, 1);
        css->flags = 0;
        css->id = NULL;
        if (cgrp == dummytop)
                set_bit(CSS_ROOT, &css->flags);
        BUG_ON(cgrp->subsys[ss->subsys_id]);
        cgrp->subsys[ss->subsys_id] = css;
}

static void cgroup_lock_hierarchy(struct cgroupfs_root *root)
{
        /* We need to take each hierarchy_mutex in a consistent order */
        int i;

        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup_subsys *ss = subsys[i];
                if (ss->root == root)
                        mutex_lock(&ss->hierarchy_mutex);
        }
}

static void cgroup_unlock_hierarchy(struct cgroupfs_root *root)
{
        int i;

        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup_subsys *ss = subsys[i];
                if (ss->root == root)
                        mutex_unlock(&ss->hierarchy_mutex);
        }
}

/*
 * 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,
                             mode_t mode)
{
        struct cgroup *cgrp;
        struct cgroupfs_root *root = parent->root;
        int err = 0;
        struct cgroup_subsys *ss;
        struct super_block *sb = root->sb;

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

        /* 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);

        mutex_lock(&cgroup_mutex);

        init_cgroup_housekeeping(cgrp);

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

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

        for_each_subsys(root, ss) {
                struct cgroup_subsys_state *css = ss->create(ss, cgrp);

                if (IS_ERR(css)) {
                        err = PTR_ERR(css);
                        goto err_destroy;
                }
                init_cgroup_css(css, ss, cgrp);
                if (ss->use_id) {
                        err = alloc_css_id(ss, parent, cgrp);
                        if (err)
                                goto err_destroy;
                }
                /* At error, ->destroy() callback has to free assigned ID. */
        }

        cgroup_lock_hierarchy(root);
        list_add(&cgrp->sibling, &cgrp->parent->children);
        cgroup_unlock_hierarchy(root);
        root->number_of_cgroups++;

        err = cgroup_create_dir(cgrp, dentry, mode);
        if (err < 0)
                goto err_remove;

        /* The cgroup directory was pre-locked for us */
        BUG_ON(!mutex_is_locked(&cgrp->dentry->d_inode->i_mutex));

        err = cgroup_populate_dir(cgrp);
        /* If err < 0, we have a half-filled directory - oh well ;) */

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

        return 0;

 err_remove:

        cgroup_lock_hierarchy(root);
        list_del(&cgrp->sibling);
        cgroup_unlock_hierarchy(root);
        root->number_of_cgroups--;

 err_destroy:

        for_each_subsys(root, ss) {
                if (cgrp->subsys[ss->subsys_id])
                        ss->destroy(ss, cgrp);
        }

        mutex_unlock(&cgroup_mutex);

        /* Release the reference count that we took on the superblock */
        deactivate_super(sb);

        kfree(cgrp);
        return err;
}

static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, int 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 int cgroup_has_css_refs(struct cgroup *cgrp)
{
        /* Check the reference count on each subsystem. Since we
         * already established that there are no tasks in the
         * cgroup, if the css refcount is also 1, then there should
         * be no outstanding references, so the subsystem is safe to
         * destroy. We scan across all subsystems rather than using
         * the per-hierarchy linked list of mounted subsystems since
         * we can be called via check_for_release() with no
         * synchronization other than RCU, and the subsystem linked
         * list isn't RCU-safe */
        int i;
        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup_subsys *ss = subsys[i];
                struct cgroup_subsys_state *css;
                /* Skip subsystems not in this hierarchy */
                if (ss->root != cgrp->root)
                        continue;
                css = cgrp->subsys[ss->subsys_id];
                /* When called from check_for_release() it's possible
                 * that by this point the cgroup has been removed
                 * and the css deleted. But a false-positive doesn't
                 * matter, since it can only happen if the cgroup
                 * has been deleted and hence no longer needs the
                 * release agent to be called anyway. */
                if (css && (atomic_read(&css->refcnt) > 1))
                        return 1;
        }
        return 0;
}

/*
 * Atomically mark all (or else none) of the cgroup's CSS objects as
 * CSS_REMOVED. Return true on success, or false if the cgroup has
 * busy subsystems. Call with cgroup_mutex held
 */

static int cgroup_clear_css_refs(struct cgroup *cgrp)
{
        struct cgroup_subsys *ss;
        unsigned long flags;
        bool failed = false;
        local_irq_save(flags);
        for_each_subsys(cgrp->root, ss) {
                struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
                int refcnt;
                while (1) {
                        /* We can only remove a CSS with a refcnt==1 */
                        refcnt = atomic_read(&css->refcnt);
                        if (refcnt > 1) {
                                failed = true;
                                goto done;
                        }
                        BUG_ON(!refcnt);
                        /*
                         * Drop the refcnt to 0 while we check other
                         * subsystems. This will cause any racing
                         * css_tryget() to spin until we set the
                         * CSS_REMOVED bits or abort
                         */
                        if (atomic_cmpxchg(&css->refcnt, refcnt, 0) == refcnt)
                                break;
                        cpu_relax();
                }
        }
 done:
        for_each_subsys(cgrp->root, ss) {
                struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
                if (failed) {
                        /*
                         * Restore old refcnt if we previously managed
                         * to clear it from 1 to 0
                         */
                        if (!atomic_read(&css->refcnt))
                                atomic_set(&css->refcnt, 1);
                } else {
                        /* Commit the fact that the CSS is removed */
                        set_bit(CSS_REMOVED, &css->flags);
                }
        }
        local_irq_restore(flags);
        return !failed;
}

static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
{
        struct cgroup *cgrp = dentry->d_fsdata;
        struct dentry *d;
        struct cgroup *parent;
        DEFINE_WAIT(wait);
        int ret;

        /* the vfs holds both inode->i_mutex already */
again:
        mutex_lock(&cgroup_mutex);
        if (atomic_read(&cgrp->count) != 0) {
                mutex_unlock(&cgroup_mutex);
                return -EBUSY;
        }
        if (!list_empty(&cgrp->children)) {
                mutex_unlock(&cgroup_mutex);
                return -EBUSY;
        }
        mutex_unlock(&cgroup_mutex);

        /*
         * In general, subsystem has no css->refcnt after pre_destroy(). But
         * in racy cases, subsystem may have to get css->refcnt after
         * pre_destroy() and it makes rmdir return with -EBUSY. This sometimes
         * make rmdir return -EBUSY too often. To avoid that, we use waitqueue
         * for cgroup's rmdir. CGRP_WAIT_ON_RMDIR is for synchronizing rmdir
         * and subsystem's reference count handling. Please see css_get/put
         * and css_tryget() and cgroup_wakeup_rmdir_waiter() implementation.
         */
        set_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);

        /*
         * Call pre_destroy handlers of subsys. Notify subsystems
         * that rmdir() request comes.
         */
        ret = cgroup_call_pre_destroy(cgrp);
        if (ret) {
                clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
                return ret;
        }

        mutex_lock(&cgroup_mutex);
        parent = cgrp->parent;
        if (atomic_read(&cgrp->count) || !list_empty(&cgrp->children)) {
                clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
                mutex_unlock(&cgroup_mutex);
                return -EBUSY;
        }
        prepare_to_wait(&cgroup_rmdir_waitq, &wait, TASK_INTERRUPTIBLE);
        if (!cgroup_clear_css_refs(cgrp)) {
                mutex_unlock(&cgroup_mutex);
                /*
                 * Because someone may call cgroup_wakeup_rmdir_waiter() before
                 * prepare_to_wait(), we need to check this flag.
                 */
                if (test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags))
                        schedule();
                finish_wait(&cgroup_rmdir_waitq, &wait);
                clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);
                if (signal_pending(current))
                        return -EINTR;
                goto again;
        }
        /* NO css_tryget() can success after here. */
        finish_wait(&cgroup_rmdir_waitq, &wait);
        clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags);

        spin_lock(&release_list_lock);
        set_bit(CGRP_REMOVED, &cgrp->flags);
        if (!list_empty(&cgrp->release_list))
                list_del(&cgrp->release_list);
        spin_unlock(&release_list_lock);

        cgroup_lock_hierarchy(cgrp->root);
        /* delete this cgroup from parent->children */
        list_del(&cgrp->sibling);
        cgroup_unlock_hierarchy(cgrp->root);

        spin_lock(&cgrp->dentry->d_lock);
        d = dget(cgrp->dentry);
        spin_unlock(&d->d_lock);

        cgroup_d_remove_dir(d);
        dput(d);

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

        mutex_unlock(&cgroup_mutex);
        return 0;
}

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);

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

        /* 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] = dummytop->subsys[ss->subsys_id];

        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));

        mutex_init(&ss->hierarchy_mutex);
        lockdep_set_class(&ss->hierarchy_mutex, &ss->subsys_key);
        ss->active = 1;
}

/**
 * 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)
{
        int i;
        atomic_set(&init_css_set.refcount, 1);
        INIT_LIST_HEAD(&init_css_set.cg_links);
        INIT_LIST_HEAD(&init_css_set.tasks);
        INIT_HLIST_NODE(&init_css_set.hlist);
        css_set_count = 1;
        init_cgroup_root(&rootnode);
        root_count = 1;
        init_task.cgroups = &init_css_set;

        init_css_set_link.cg = &init_css_set;
        init_css_set_link.cgrp = dummytop;
        list_add(&init_css_set_link.cgrp_link_list,
                 &rootnode.top_cgroup.css_sets);
        list_add(&init_css_set_link.cg_link_list,
                 &init_css_set.cg_links);

        for (i = 0; i < CSS_SET_TABLE_SIZE; i++)
                INIT_HLIST_HEAD(&css_set_table[i]);

        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup_subsys *ss = subsys[i];

                BUG_ON(!ss->name);
                BUG_ON(strlen(ss->name) > MAX_CGROUP_TYPE_NAMELEN);
                BUG_ON(!ss->create);
                BUG_ON(!ss->destroy);
                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)
{
        int err;
        int i;
        struct hlist_head *hhead;

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

        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup_subsys *ss = subsys[i];
                if (!ss->early_init)
                        cgroup_init_subsys(ss);
                if (ss->use_id)
                        cgroup_subsys_init_idr(ss);
        }

        /* Add init_css_set to the hash table */
        hhead = css_set_hash(init_css_set.subsys);
        hlist_add_head(&init_css_set.hlist, hhead);
        BUG_ON(!init_root_id(&rootnode));
        err = register_filesystem(&cgroup_fs_type);
        if (err < 0)
                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 */
static 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_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;
}

static int cgroup_open(struct inode *inode, struct file *file)
{
        struct pid *pid = PROC_I(inode)->pid;
        return single_open(file, proc_cgroup_show, pid);
}

const struct file_operations proc_cgroup_operations = {
        .open           = cgroup_open,
        .read           = seq_read,
        .llseek         = seq_lseek,
        .release        = single_release,
};

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

        seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
        mutex_lock(&cgroup_mutex);
        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup_subsys *ss = subsys[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);
        child->cgroups = current->cgroups;
        get_css_set(child->cgroups);
        task_unlock(current);
        INIT_LIST_HEAD(&child->cg_list);
}

/**
 * cgroup_fork_callbacks - run fork callbacks
 * @child: the new task
 *
 * Called on a new task very soon before adding it to the
 * tasklist. No need to take any locks since no-one can
 * be operating on this task.
 */
void cgroup_fork_callbacks(struct task_struct *child)
{
        if (need_forkexit_callback) {
                int i;
                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                        struct cgroup_subsys *ss = subsys[i];
                        if (ss->fork)
                                ss->fork(ss, child);
                }
        }
}

/**
 * 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.
 * 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)
{
        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, &child->cgroups->tasks);
                task_unlock(child);
                write_unlock(&css_set_lock);
        }
}
/**
 * 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)
{
        int i;
        struct css_set *cg;

        if (run_callbacks && need_forkexit_callback) {
                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                        struct cgroup_subsys *ss = subsys[i];
                        if (ss->exit)
                                ss->exit(ss, tsk);
                }
        }

        /*
         * 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(&tsk->cg_list);
                write_unlock(&css_set_lock);
        }

        /* Reassign the task to the init_css_set. */
        task_lock(tsk);
        cg = tsk->cgroups;
        tsk->cgroups = &init_css_set;
        task_unlock(tsk);
        if (cg)
                put_css_set_taskexit(cg);
}

/**
 * cgroup_clone - clone the cgroup the given subsystem is attached to
 * @tsk: the task to be moved
 * @subsys: the given subsystem
 * @nodename: the name for the new cgroup
 *
 * Duplicate the current cgroup in the hierarchy that the given
 * subsystem is attached to, and move this task into the new
 * child.
 */
int cgroup_clone(struct task_struct *tsk, struct cgroup_subsys *subsys,
                                                        char *nodename)
{
        struct dentry *dentry;
        int ret = 0;
        struct cgroup *parent, *child;
        struct inode *inode;
        struct css_set *cg;
        struct cgroupfs_root *root;
        struct cgroup_subsys *ss;

        /* We shouldn't be called by an unregistered subsystem */
        BUG_ON(!subsys->active);

        /* First figure out what hierarchy and cgroup we're dealing
         * with, and pin them so we can drop cgroup_mutex */
        mutex_lock(&cgroup_mutex);
 again:
        root = subsys->root;
        if (root == &rootnode) {
                mutex_unlock(&cgroup_mutex);
                return 0;
        }

        /* Pin the hierarchy */
        if (!atomic_inc_not_zero(&root->sb->s_active)) {
                /* We race with the final deactivate_super() */
                mutex_unlock(&cgroup_mutex);
                return 0;
        }

        /* Keep the cgroup alive */
        task_lock(tsk);
        parent = task_cgroup(tsk, subsys->subsys_id);
        cg = tsk->cgroups;
        get_css_set(cg);
        task_unlock(tsk);

        mutex_unlock(&cgroup_mutex);

        /* Now do the VFS work to create a cgroup */
        inode = parent->dentry->d_inode;

        /* Hold the parent directory mutex across this operation to
         * stop anyone else deleting the new cgroup */
        mutex_lock(&inode->i_mutex);
        dentry = lookup_one_len(nodename, parent->dentry, strlen(nodename));
        if (IS_ERR(dentry)) {
                printk(KERN_INFO
                       "cgroup: Couldn't allocate dentry for %s: %ld\n", nodename,
                       PTR_ERR(dentry));
                ret = PTR_ERR(dentry);
                goto out_release;
        }

        /* Create the cgroup directory, which also creates the cgroup */
        ret = vfs_mkdir(inode, dentry, 0755);
        child = __d_cgrp(dentry);
        dput(dentry);
        if (ret) {
                printk(KERN_INFO
                       "Failed to create cgroup %s: %d\n", nodename,
                       ret);
                goto out_release;
        }

        /* The cgroup now exists. Retake cgroup_mutex and check
         * that we're still in the same state that we thought we
         * were. */
        mutex_lock(&cgroup_mutex);
        if ((root != subsys->root) ||
            (parent != task_cgroup(tsk, subsys->subsys_id))) {
                /* Aargh, we raced ... */
                mutex_unlock(&inode->i_mutex);
                put_css_set(cg);

                deactivate_super(root->sb);
                /* The cgroup is still accessible in the VFS, but
                 * we're not going to try to rmdir() it at this
                 * point. */
                printk(KERN_INFO
                       "Race in cgroup_clone() - leaking cgroup %s\n",
                       nodename);
                goto again;
        }

        /* do any required auto-setup */
        for_each_subsys(root, ss) {
                if (ss->post_clone)
                        ss->post_clone(ss, child);
        }

        /* All seems fine. Finish by moving the task into the new cgroup */
        ret = cgroup_attach_task(child, tsk);
        mutex_unlock(&cgroup_mutex);

 out_release:
        mutex_unlock(&inode->i_mutex);

        mutex_lock(&cgroup_mutex);
        put_css_set(cg);
        mutex_unlock(&cgroup_mutex);
        deactivate_super(root->sb);
        return ret;
}

/**
 * cgroup_is_descendant - see if @cgrp is a descendant of @task's cgrp
 * @cgrp: the cgroup in question
 * @task: the task in question
 *
 * See if @cgrp is a descendant of @task's cgroup in the appropriate
 * hierarchy.
 *
 * If we are sending in dummytop, then presumably we are creating
 * the top cgroup in the subsystem.
 *
 * Called only by the ns (nsproxy) cgroup.
 */
int cgroup_is_descendant(const struct cgroup *cgrp, struct task_struct *task)
{
        int ret;
        struct cgroup *target;

        if (cgrp == dummytop)
                return 1;

        target = task_cgroup_from_root(task, cgrp->root);
        while (cgrp != target && cgrp!= cgrp->top_cgroup)
                cgrp = cgrp->parent;
        ret = (cgrp == target);
        return ret;
}

static void check_for_release(struct cgroup *cgrp)
{
        /* All of these checks rely on RCU to keep the cgroup
         * structure alive */
        if (cgroup_is_releasable(cgrp) && !atomic_read(&cgrp->count)
            && list_empty(&cgrp->children) && !cgroup_has_css_refs(cgrp)) {
                /* Control Group is currently removeable. If it's not
                 * already queued for a userspace notification, queue
                 * it now */
                int need_schedule_work = 0;
                spin_lock(&release_list_lock);
                if (!cgroup_is_removed(cgrp) &&
                    list_empty(&cgrp->release_list)) {
                        list_add(&cgrp->release_list, &release_list);
                        need_schedule_work = 1;
                }
                spin_unlock(&release_list_lock);
                if (need_schedule_work)
                        schedule_work(&release_agent_work);
        }
}

void __css_put(struct cgroup_subsys_state *css)
{
        struct cgroup *cgrp = css->cgroup;
        int val;
        rcu_read_lock();
        val = atomic_dec_return(&css->refcnt);
        if (val == 1) {
                if (notify_on_release(cgrp)) {
                        set_bit(CGRP_RELEASABLE, &cgrp->flags);
                        check_for_release(cgrp);
                }
                cgroup_wakeup_rmdir_waiter(cgrp);
        }
        rcu_read_unlock();
        WARN_ON_ONCE(val < 1);
}

/*
 * 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);
        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);
                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);
                spin_lock(&release_list_lock);
        }
        spin_unlock(&release_list_lock);
        mutex_unlock(&cgroup_mutex);
}

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

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

                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                        struct cgroup_subsys *ss = subsys[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_removed().
 */
unsigned short css_id(struct cgroup_subsys_state *css)
{
        struct css_id *cssid = rcu_dereference(css->id);

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

unsigned short css_depth(struct cgroup_subsys_state *css)
{
        struct css_id *cssid = rcu_dereference(css->id);

        if (cssid)
                return cssid->depth;
        return 0;
}

bool css_is_ancestor(struct cgroup_subsys_state *child,
                    const struct cgroup_subsys_state *root)
{
        struct css_id *child_id = rcu_dereference(child->id);
        struct css_id *root_id = rcu_dereference(root->id);

        if (!child_id || !root_id || (child_id->depth < root_id->depth))
                return false;
        return child_id->stack[root_id->depth] == root_id->id;
}

static void __free_css_id_cb(struct rcu_head *head)
{
        struct css_id *id;

        id = container_of(head, struct css_id, rcu_head);
        kfree(id);
}

void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css)
{
        struct css_id *id = css->id;
        /* 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);
        call_rcu(&id->rcu_head, __free_css_id_cb);
}

/*
 * 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 myid, error, 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);
        /* get id */
        if (unlikely(!idr_pre_get(&ss->idr, GFP_KERNEL))) {
                error = -ENOMEM;
                goto err_out;
        }
        spin_lock(&ss->id_lock);
        /* Don't use 0. allocates an ID of 1-65535 */
        error = idr_get_new_above(&ss->idr, newid, 1, &myid);
        spin_unlock(&ss->id_lock);

        /* Returns error when there are no free spaces for new ID.*/
        if (error) {
                error = -ENOSPC;
                goto err_out;
        }
        if (myid > CSS_ID_MAX)
                goto remove_idr;

        newid->id = myid;
        newid->depth = depth;
        return newid;
remove_idr:
        error = -ENOSPC;
        spin_lock(&ss->id_lock);
        idr_remove(&ss->idr, myid);
        spin_unlock(&ss->id_lock);
err_out:
        kfree(newid);
        return ERR_PTR(error);

}

static int __init cgroup_subsys_init_idr(struct cgroup_subsys *ss)
{
        struct css_id *newid;
        struct cgroup_subsys_state *rootcss;

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

        rootcss = init_css_set.subsys[ss->subsys_id];
        newid = get_new_cssid(ss, 0);
        if (IS_ERR(newid))
                return PTR_ERR(newid);

        newid->stack[0] = newid->id;
        newid->css = rootcss;
        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 = NULL;

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

        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);
}

/**
 * css_get_next - lookup next cgroup under specified hierarchy.
 * @ss: pointer to subsystem
 * @id: current position of iteration.
 * @root: pointer to css. search tree under this.
 * @foundid: position of found object.
 *
 * Search next css under the specified hierarchy of rootid. Calling under
 * rcu_read_lock() is necessary. Returns NULL if it reaches the end.
 */
struct cgroup_subsys_state *
css_get_next(struct cgroup_subsys *ss, int id,
             struct cgroup_subsys_state *root, int *foundid)
{
        struct cgroup_subsys_state *ret = NULL;
        struct css_id *tmp;
        int tmpid;
        int rootid = css_id(root);
        int depth = css_depth(root);

        if (!rootid)
                return NULL;

        BUG_ON(!ss->use_id);
        /* fill start point for scan */
        tmpid = id;
        while (1) {
                /*
                 * scan next entry from bitmap(tree), tmpid is updated after
                 * idr_get_next().
                 */
                spin_lock(&ss->id_lock);
                tmp = idr_get_next(&ss->idr, &tmpid);
                spin_unlock(&ss->id_lock);

                if (!tmp)
                        break;
                if (tmp->depth >= depth && tmp->stack[depth] == rootid) {
                        ret = rcu_dereference(tmp->css);
                        if (ret) {
                                *foundid = tmpid;
                                break;
                        }
                }
                /* continue to scan from next id */
                tmpid = tmpid + 1;
        }
        return ret;
}

#ifdef CONFIG_CGROUP_DEBUG
static struct cgroup_subsys_state *debug_create(struct cgroup_subsys *ss,
                                                   struct cgroup *cont)
{
        struct cgroup_subsys_state *css = kzalloc(sizeof(*css), GFP_KERNEL);

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

        return css;
}

static void debug_destroy(struct cgroup_subsys *ss, struct cgroup *cont)
{
        kfree(cont->subsys[debug_subsys_id]);
}

static u64 cgroup_refcount_read(struct cgroup *cont, struct cftype *cft)
{
        return atomic_read(&cont->count);
}

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

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

static u64 current_css_set_refcount_read(struct cgroup *cont,
                                           struct cftype *cft)
{
        u64 count;

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

static int current_css_set_cg_links_read(struct cgroup *cont,
                                         struct cftype *cft,
                                         struct seq_file *seq)
{
        struct cg_cgroup_link *link;
        struct css_set *cg;

        read_lock(&css_set_lock);
        rcu_read_lock();
        cg = rcu_dereference(current->cgroups);
        list_for_each_entry(link, &cg->cg_links, cg_link_list) {
                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 *cont,
                                 struct cftype *cft,
                                 struct seq_file *seq)
{
        struct cg_cgroup_link *link;

        read_lock(&css_set_lock);
        list_for_each_entry(link, &cont->css_sets, cgrp_link_list) {
                struct css_set *cg = link->cg;
                struct task_struct *task;
                int count = 0;
                seq_printf(seq, "css_set %p\n", cg);
                list_for_each_entry(task, &cg->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 = "cgroup_refcount",
                .read_u64 = cgroup_refcount_read,
        },
        {
                .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,
        },
};

static int debug_populate(struct cgroup_subsys *ss, struct cgroup *cont)
{
        return cgroup_add_files(cont, ss, debug_files,
                                ARRAY_SIZE(debug_files));
}

struct cgroup_subsys debug_subsys = {
        .name = "debug",
        .create = debug_create,
        .destroy = debug_destroy,
        .populate = debug_populate,
        .subsys_id = debug_subsys_id,
};
#endif /* CONFIG_CGROUP_DEBUG */