/*
 *  Generic process-grouping system.
 *
 *  Based originally on the cpuset system, extracted by Paul Menage
 *  Copyright (C) 2006 Google, Inc
 *
 *  Notifications support
 *  Copyright (C) 2009 Nokia Corporation
 *  Author: Kirill A. Shutemov
 *
 *  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/cred.h>
#include <linux/ctype.h>
#include <linux/errno.h>
#include <linux/fs.h>
#include <linux/init_task.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/module.h>
#include <linux/delayacct.h>
#include <linux/cgroupstats.h>
#include <linux/hash.h>
#include <linux/namei.h>
#include <linux/pid_namespace.h>
#include <linux/idr.h>
#include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */
#include <linux/eventfd.h>
#include <linux/poll.h>
#include <linux/flex_array.h> /* used in cgroup_attach_proc */

#include <linux/atomic.h>

static DEFINE_MUTEX(cgroup_mutex);

/*
 * Generate an array of cgroup subsystem pointers. At boot time, this is
 * populated up to CGROUP_BUILTIN_SUBSYS_COUNT, and modular subsystems are
 * registered after that. The mutable section of this array is protected by
 * cgroup_mutex.
 */
#define SUBSYS(_x) &_x ## _subsys,
static struct cgroup_subsys *subsys[CGROUP_SUBSYS_COUNT] = {
#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 __rcu *css;
        /*
         * ID of this css.
         */
        unsigned short id;
        /*
         * Depth in hierarchy which this ID belongs to.
         */
        unsigned short depth;
        /*
         * ID is freed by RCU. (and lookup routine is RCU safe.)
         */
        struct rcu_head rcu_head;
        /*
         * Hierarchy of CSS ID belongs to.
         */
        unsigned short stack[0]; /* Array of Length (depth+1) */
};

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

/* The list of hierarchy roots */

static LIST_HEAD(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);
}

static int clone_children(const struct cgroup *cgrp)
{
        return test_bit(CGRP_CLONE_CHILDREN, &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_init_idr(struct cgroup_subsys *ss,
                           struct cgroup_subsys_state *css);

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

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

/* 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);
        kfree_rcu(cg, rcu_head);
}

/*
 * 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;

        /*
         * Build the set of subsystem state objects that we want to see in the
         * new css_set. while subsystems can change globally, the entries here
         * won't change, so no need for locking.
         */
        for (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);
}
EXPORT_SYMBOL_GPL(cgroup_lock);

/**
 * 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);
}
EXPORT_SYMBOL_GPL(cgroup_unlock);

/*
 * 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 struct dentry *cgroup_lookup(struct inode *, struct dentry *, struct nameidata *);
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_ino = get_next_ino();
                inode->i_mode = mode;
                inode->i_uid = current_fsuid();
                inode->i_gid = current_fsgid();
                inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
                inode->i_mapping->backing_dev_info = &cgroup_backing_dev_info;
        }
        return inode;
}

/*
 * 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 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));

                kfree_rcu(cgrp, rcu_head);
        }
        iput(inode);
}

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

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

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

static void cgroup_clear_directory(struct dentry *dentry)
{
        struct list_head *node;

        BUG_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
        spin_lock(&dentry->d_lock);
        node = dentry->d_subdirs.next;
        while (node != &dentry->d_subdirs) {
                struct dentry *d = list_entry(node, struct dentry, d_u.d_child);

                spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);
                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);
                        dget_dlock(d);
                        spin_unlock(&d->d_lock);
                        spin_unlock(&dentry->d_lock);
                        d_delete(d);
                        simple_unlink(dentry->d_inode, d);
                        dput(d);
                        spin_lock(&dentry->d_lock);
                } else
                        spin_unlock(&d->d_lock);
                node = dentry->d_subdirs.next;
        }
        spin_unlock(&dentry->d_lock);
}

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

        cgroup_clear_directory(dentry);

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

/*
 * 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);
}

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

        BUG_ON(!mutex_is_locked(&cgroup_mutex));

        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;
                /*
                 * Nobody should tell us to do a subsys that doesn't exist:
                 * parse_cgroupfs_options should catch that case and refcounts
                 * ensure that subsystems won't disappear once selected.
                 */
                BUG_ON(ss == NULL);
                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(ss == NULL);
                        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);
                        /* refcount was already taken, and we're keeping it */
                } else if (bit & removed_bits) {
                        /* We're removing this subsystem */
                        BUG_ON(ss == NULL);
                        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);
                        /* subsystem is now free - drop reference on module */
                        module_put(ss->module);
                } else if (bit & final_bits) {
                        /* Subsystem state should already exist */
                        BUG_ON(ss == NULL);
                        BUG_ON(!cgrp->subsys[i]);
                        /*
                         * a refcount was taken, but we already had one, so
                         * drop the extra reference.
                         */
                        module_put(ss->module);
#ifdef CONFIG_MODULE_UNLOAD
                        BUG_ON(ss->module && !module_refcount(ss->module));
#endif
                } else {
                        /* Subsystem state shouldn't exist */
                        BUG_ON(cgrp->subsys[i]);
                }
        }
        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 (clone_children(&root->top_cgroup))
                seq_puts(seq, ",clone_children");
        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;
        bool clone_children;
        char *name;
        /* User explicitly requested empty subsystem */
        bool none;

        struct cgroupfs_root *new_root;

};

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

        BUG_ON(!mutex_is_locked(&cgroup_mutex));

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

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

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

                        continue;
                }

                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                        struct cgroup_subsys *ss = subsys[i];
                        if (ss == NULL)
                                continue;
                        if (strcmp(token, ss->name))
                                continue;
                        if (ss->disabled)
                                continue;

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

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

        /*
         * If the 'all' option was specified select all the subsystems,
         * otherwise 'all, 'none' and a subsystem name options were not
         * specified, let's default to 'all'
         */
        if (all_ss || (!all_ss && !one_ss && !opts->none)) {
                for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                        struct cgroup_subsys *ss = subsys[i];
                        if (ss == NULL)
                                continue;
                        if (ss->disabled)
                                continue;
                        set_bit(i, &opts->subsys_bits);
                }
        }

        /* 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;

        /*
         * Grab references on all the modules we'll need, so the subsystems
         * don't dance around before rebind_subsystems attaches them. This may
         * take duplicate reference counts on a subsystem that's already used,
         * but rebind_subsystems handles this case.
         */
        for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
                unsigned long bit = 1UL << i;

                if (!(bit & opts->subsys_bits))
                        continue;
                if (!try_module_get(subsys[i]->module)) {
                        module_pin_failed = true;
                        break;
                }
        }
        if (module_pin_failed) {
                /*
                 * oops, one of the modules was going away. this means that we
                 * raced with a module_delete call, and to the user this is
                 * essentially a "subsystem doesn't exist" case.
                 */
                for (i--; i >= CGROUP_BUILTIN_SUBSYS_COUNT; i--) {
                        /* drop refcounts only on the ones we took */
                        unsigned long bit = 1UL << i;

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

        return 0;
}

static void drop_parsed_module_refcounts(unsigned long subsys_bits)
{
        int i;
        for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
                unsigned long bit = 1UL << i;

                if (!(bit & subsys_bits))
                        continue;
                module_put(subsys[i]->module);
        }
}

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;

        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 or name to change at remount */
        if (opts.flags != root->flags ||
            (opts.name && strcmp(opts.name, root->name))) {
                ret = -EINVAL;
                drop_parsed_module_refcounts(opts.subsys_bits);
                goto out_unlock;
        }

        ret = rebind_subsystems(root, opts.subsys_bits);
        if (ret) {
                drop_parsed_module_refcounts(opts.subsys_bits);
                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);
        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);
        INIT_LIST_HEAD(&cgrp->event_list);
        spin_lock_init(&cgrp->event_list_lock);
}

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);
        if (opts->clone_children)
                set_bit(CGRP_CLONE_CHILDREN, &root->top_cgroup.flags);
        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)
{
        static const struct dentry_operations cgroup_dops = {
                .d_iput = cgroup_diput,
                .d_delete = cgroup_delete,
        };

        struct inode *inode =
                cgroup_new_inode(S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR, sb);
        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;
        /* for everything else we want ->d_op set */
        sb->s_d_op = &cgroup_dops;
        return 0;
}

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

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

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

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

        root = sb->s_fs_info;
        BUG_ON(!root);
        if (root == opts.new_root) {
                /* We used the new root structure, so this is a new hierarchy */
                struct list_head tmp_cg_links;
                struct cgroup *root_cgrp = &root->top_cgroup;
                struct inode *inode;
                struct cgroupfs_root *existing_root;
                const struct cred *cred;
                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;
                }
                /*
                 * There must be no failure case after here, since rebinding
                 * takes care of subsystems' refcounts, which are explicitly
                 * dropped in the failure exit path.
                 */

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

                list_add(&root->root_list, &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);

                cred = override_creds(&init_cred);
                cgroup_populate_dir(root_cgrp);
                revert_creds(cred);
                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);
                /* no subsys rebinding, so refcounts don't change */
                drop_parsed_module_refcounts(opts.subsys_bits);
        }

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

 drop_new_super:
        deactivate_locked_super(sb);
 drop_modules:
        drop_parsed_module_refcounts(opts.subsys_bits);
 out_err:
        kfree(opts.release_agent);
        kfree(opts.name);
        return ERR_PTR(ret);
}

static void cgroup_kill_sb(struct super_block *sb) {
        struct cgroupfs_root *root = sb->s_fs_info;
        struct cgroup *cgrp = &root->top_cgroup;
        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",
        .mount = cgroup_mount,
        .kill_sb = cgroup_kill_sb,
};

static struct kobject *cgroup_kobj;

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_check(cgrp->dentry,
                                                      cgroup_lock_is_held());

        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, dentry->d_name.name, len);
                cgrp = cgrp->parent;
                if (!cgrp)
                        break;

                dentry = rcu_dereference_check(cgrp->dentry,
                                               cgroup_lock_is_held());
                if (!cgrp->parent)
                        continue;
                if (--start < buf)
                        return -ENAMETOOLONG;
                *start = '/';
        }
        memmove(buf, start, buf + buflen - start);
        return 0;
}
EXPORT_SYMBOL_GPL(cgroup_path);

/*
 * cgroup_task_migrate - move a task from one cgroup to another.
 *
 * 'guarantee' is set if the caller promises that a new css_set for the task
 * will already exist. If not set, this function might sleep, and can fail with
 * -ENOMEM. Otherwise, it can only fail with -ESRCH.
 */
static int cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
                               struct task_struct *tsk, bool guarantee)
{
        struct css_set *oldcg;
        struct css_set *newcg;

        /*
         * get old css_set. we need to take task_lock and refcount it, because
         * an exiting task can change its css_set to init_css_set and drop its
         * old one without taking cgroup_mutex.
         */
        task_lock(tsk);
        oldcg = tsk->cgroups;
        get_css_set(oldcg);
        task_unlock(tsk);

        /* locate or allocate a new css_set for this task. */
        if (guarantee) {
                /* we know the css_set we want already exists. */
                struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
                read_lock(&css_set_lock);
                newcg = find_existing_css_set(oldcg, cgrp, template);
                BUG_ON(!newcg);
                get_css_set(newcg);
                read_unlock(&css_set_lock);
        } else {
                might_sleep();
                /* find_css_set will give us newcg already referenced. */
                newcg = find_css_set(oldcg, cgrp);
                if (!newcg) {
                        put_css_set(oldcg);
                        return -ENOMEM;
                }
        }
        put_css_set(oldcg);

        /* if PF_EXITING is set, the tsk->cgroups pointer is no longer safe. */
        task_lock(tsk);
        if (tsk->flags & PF_EXITING) {
                task_unlock(tsk);
                put_css_set(newcg);
                return -ESRCH;
        }
        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_move(&tsk->cg_list, &newcg->tasks);
        write_unlock(&css_set_lock);

        /*
         * We just gained a reference on oldcg by taking it from the task. As
         * trading it for newcg is protected by cgroup_mutex, we're safe to drop
         * it here; it will be freed under RCU.
         */
        put_css_set(oldcg);

        set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
        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;
        struct cgroup_subsys *ss, *failed_ss = NULL;
        struct cgroup *oldcgrp;
        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);
                        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;
                        }
                }
                if (ss->can_attach_task) {
                        retval = ss->can_attach_task(cgrp, tsk);
                        if (retval) {
                                failed_ss = ss;
                                goto out;
                        }
                }
        }

        retval = cgroup_task_migrate(cgrp, oldcgrp, tsk, false);
        if (retval)
                goto out;

        for_each_subsys(root, ss) {
                if (ss->pre_attach)
                        ss->pre_attach(cgrp);
                if (ss->attach_task)
                        ss->attach_task(cgrp, tsk);
                if (ss->attach)
                        ss->attach(ss, cgrp, oldcgrp, tsk);
        }

        synchronize_rcu();

        /*
         * 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);
                }
        }
        return retval;
}

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

        cgroup_lock();
        for_each_active_root(root) {
                struct cgroup *from_cg = task_cgroup_from_root(from, root);

                retval = cgroup_attach_task(from_cg, tsk);
                if (retval)
                        break;
        }
        cgroup_unlock();

        return retval;
}
EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

/*
 * cgroup_attach_proc works in two stages, the first of which prefetches all
 * new css_sets needed (to make sure we have enough memory before committing
 * to the move) and stores them in a list of entries of the following type.
 * TODO: possible optimization: use css_set->rcu_head for chaining instead
 */
struct cg_list_entry {
        struct css_set *cg;
        struct list_head links;
};

static bool css_set_check_fetched(struct cgroup *cgrp,
                                  struct task_struct *tsk, struct css_set *cg,
                                  struct list_head *newcg_list)
{
        struct css_set *newcg;
        struct cg_list_entry *cg_entry;
        struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];

        read_lock(&css_set_lock);
        newcg = find_existing_css_set(cg, cgrp, template);
        if (newcg)
                get_css_set(newcg);
        read_unlock(&css_set_lock);

        /* doesn't exist at all? */
        if (!newcg)
                return false;
        /* see if it's already in the list */
        list_for_each_entry(cg_entry, newcg_list, links) {
                if (cg_entry->cg == newcg) {
                        put_css_set(newcg);
                        return true;
                }
        }

        /* not found */
        put_css_set(newcg);
        return false;
}

/*
 * Find the new css_set and store it in the list in preparation for moving the
 * given task to the given cgroup. Returns 0 or -ENOMEM.
 */
static int css_set_prefetch(struct cgroup *cgrp, struct css_set *cg,
                            struct list_head *newcg_list)
{
        struct css_set *newcg;
        struct cg_list_entry *cg_entry;

        /* ensure a new css_set will exist for this thread */
        newcg = find_css_set(cg, cgrp);
        if (!newcg)
                return -ENOMEM;
        /* add it to the list */
        cg_entry = kmalloc(sizeof(struct cg_list_entry), GFP_KERNEL);
        if (!cg_entry) {
                put_css_set(newcg);
                return -ENOMEM;
        }
        cg_entry->cg = newcg;
        list_add(&cg_entry->links, newcg_list);
        return 0;
}

/**
 * cgroup_attach_proc - attach all threads in a threadgroup to a cgroup
 * @cgrp: the cgroup to attach to
 * @leader: the threadgroup leader task_struct of the group to be attached
 *
 * Call holding cgroup_mutex and the threadgroup_fork_lock of the leader. Will
 * take task_lock of each thread in leader's threadgroup individually in turn.
 */
int cgroup_attach_proc(struct cgroup *cgrp, struct task_struct *leader)
{
        int retval, i, group_size;
        struct cgroup_subsys *ss, *failed_ss = NULL;
        bool cancel_failed_ss = false;
        /* guaranteed to be initialized later, but the compiler needs this */
        struct cgroup *oldcgrp = NULL;
        struct css_set *oldcg;
        struct cgroupfs_root *root = cgrp->root;
        /* threadgroup list cursor and array */