Merge branch 'devel'

[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/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 <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>
};

/*
 * 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;

        /* 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 mounted hierarchies */
        struct list_head root_list;

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

        /* The path to use for release notifications. No locking
         * between setting and use - so if userspace updates this
         * while child cgroups exist, you could miss a
         * notification. We ensure that it's always a valid
         * NUL-terminated string */
        char release_agent_path[PATH_MAX];
};


/*
 * 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;

/* The list of hierarchy roots */

static LIST_HEAD(roots);
static int root_count;

/* 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;

/* bits in struct cgroup flags field */
enum {
        /* Control Group is dead */
        CGRP_REMOVED,
        /* Control Group has previously had a child cgroup or a task,
         * but no longer (only if CGRP_NOTIFY_ON_RELEASE is set) */
        CGRP_RELEASABLE,
        /* Control Group requires release notifications to userspace */
        CGRP_NOTIFY_ON_RELEASE,
};

/* 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_root() allows you to iterate across the active hierarchies */
#define for_each_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;
        /*
         * 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;

/* 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;

/* 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;

/* When we create or destroy a css_set, the operation simply
 * takes/releases a reference count on all the cgroups referenced
 * by subsystems in this css_set. This can end up multiple-counting
 * some cgroups, but that's OK - the ref-count is just a
 * busy/not-busy indicator; ensuring that we only count each cgroup
 * once would require taking a global lock to ensure that no
 * subsystems moved between hierarchies while we were doing so.
 *
 * Possible TODO: decide at boot time based on the number of
 * registered subsystems and the number of CPUs or NUMA nodes whether
 * it's better for performance to ref-count every subsystem, or to
 * take a global lock and only add one ref count to each hierarchy.
 */

/*
 * unlink a css_set from the list and free it
 */
static void unlink_css_set(struct css_set *cg)
{
        write_lock(&css_set_lock);
        list_del(&cg->list);
        css_set_count--;
        while (!list_empty(&cg->cg_links)) {
                struct cg_cgroup_link *link;
                link = list_entry(cg->cg_links.next,
                                  struct cg_cgroup_link, cg_link_list);
                list_del(&link->cg_link_list);
                list_del(&link->cgrp_link_list);
                kfree(link);
        }
        write_unlock(&css_set_lock);
}

static void __release_css_set(struct kref *k, int taskexit)
{
        int i;
        struct css_set *cg = container_of(k, struct css_set, ref);

        unlink_css_set(cg);

        rcu_read_lock();
        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup *cgrp = cg->subsys[i]->cgroup;
                if (atomic_dec_and_test(&cgrp->count) &&
                    notify_on_release(cgrp)) {
                        if (taskexit)
                                set_bit(CGRP_RELEASABLE, &cgrp->flags);
                        check_for_release(cgrp);
                }
        }
        rcu_read_unlock();
        kfree(cg);
}

static void release_css_set(struct kref *k)
{
        __release_css_set(k, 0);
}

static void release_css_set_taskexit(struct kref *k)
{
        __release_css_set(k, 1);
}

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

static inline void put_css_set(struct css_set *cg)
{
        kref_put(&cg->ref, release_css_set);
}

static inline void put_css_set_taskexit(struct css_set *cg)
{
        kref_put(&cg->ref, release_css_set_taskexit);
}

/*
 * find_existing_css_set() is a helper for
 * find_css_set(), and checks to see whether an existing
 * css_set is suitable. This currently walks a linked-list for
 * simplicity; a later patch will use a hash table for better
 * performance
 *
 * 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 list_head *l = &init_css_set.list;

        /* 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];
                }
        }

        /* Look through existing cgroup groups to find one to reuse */
        do {
                struct css_set *cg =
                        list_entry(l, struct css_set, list);

                if (!memcmp(template, cg->subsys, sizeof(cg->subsys))) {
                        /* All subsystems matched */
                        return cg;
                }
                /* Try the next cgroup group */
                l = l->next;
        } while (l != &init_css_set.list);

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

/*
 * 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) {
                        while (!list_empty(tmp)) {
                                link = list_entry(tmp->next,
                                                  struct cg_cgroup_link,
                                                  cgrp_link_list);
                                list_del(&link->cgrp_link_list);
                                kfree(link);
                        }
                        return -ENOMEM;
                }
                list_add(&link->cgrp_link_list, tmp);
        }
        return 0;
}

static void free_cg_links(struct list_head *tmp)
{
        while (!list_empty(tmp)) {
                struct cg_cgroup_link *link;
                link = list_entry(tmp->next,
                                  struct cg_cgroup_link,
                                  cgrp_link_list);
                list_del(&link->cgrp_link_list);
                kfree(link);
        }
}

/*
 * 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];
        int i;

        struct list_head tmp_cg_links;
        struct cg_cgroup_link *link;

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

        kref_init(&res->ref);
        INIT_LIST_HEAD(&res->cg_links);
        INIT_LIST_HEAD(&res->tasks);

        /* 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. */
        for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
                struct cgroup *cgrp = res->subsys[i]->cgroup;
                struct cgroup_subsys *ss = subsys[i];
                atomic_inc(&cgrp->count);
                /*
                 * We want to add a link once per cgroup, so we
                 * only do it for the first subsystem in each
                 * hierarchy
                 */
                if (ss->root->subsys_list.next == &ss->sibling) {
                        BUG_ON(list_empty(&tmp_cg_links));
                        link = list_entry(tmp_cg_links.next,
                                          struct cg_cgroup_link,
                                          cgrp_link_list);
                        list_del(&link->cgrp_link_list);
                        list_add(&link->cgrp_link_list, &cgrp->css_sets);
                        link->cg = res;
                        list_add(&link->cg_link_list, &res->cg_links);
                }
        }
        if (list_empty(&rootnode.subsys_list)) {
                link = list_entry(tmp_cg_links.next,
                                  struct cg_cgroup_link,
                                  cgrp_link_list);
                list_del(&link->cgrp_link_list);
                list_add(&link->cgrp_link_list, &dummytop->css_sets);
                link->cg = res;
                list_add(&link->cg_link_list, &res->cg_links);
        }

        BUG_ON(!list_empty(&tmp_cg_links));

        /* Link this cgroup group into the list */
        list_add(&res->list, &init_css_set.list);
        css_set_count++;
        write_unlock(&css_set_lock);

        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 cgroup_common_file_write handler for operations that modify
 * the cgroup hierarchy holds cgroup_mutex across the entire operation,
 * single threading all such cgroup modifications across the system.
 *
 * 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 struct inode_operations cgroup_dir_inode_operations;
static struct file_operations proc_cgroupstats_operations;

static struct backing_dev_info cgroup_backing_dev_info = {
        .capabilities   = BDI_CAP_NO_ACCT_DIRTY | BDI_CAP_NO_WRITEBACK,
};

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_blocks = 0;
                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 void cgroup_call_pre_destroy(struct cgroup *cgrp)
{
        struct cgroup_subsys *ss;
        for_each_subsys(cgrp->root, ss)
                if (ss->pre_destroy && cgrp->subsys[ss->subsys_id])
                        ss->pre_destroy(ss, cgrp);
        return;
}

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) {
                        if (cgrp->subsys[ss->subsys_id])
                                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);

                kfree(cgrp);
        }
        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);
}

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 (!list_empty(&cgrp->children))
                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);
                        cgrp->subsys[i] = dummytop->subsys[i];
                        cgrp->subsys[i]->cgroup = cgrp;
                        list_add(&ss->sibling, &root->subsys_list);
                        rcu_assign_pointer(ss->root, root);
                        if (ss->bind)
                                ss->bind(ss, cgrp);

                } 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);
                        if (ss->bind)
                                ss->bind(ss, dummytop);
                        dummytop->subsys[i]->cgroup = dummytop;
                        cgrp->subsys[i] = NULL;
                        rcu_assign_pointer(subsys[i]->root, &rootnode);
                        list_del(&ss->sibling);
                } 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);
        mutex_unlock(&cgroup_mutex);
        return 0;
}

struct cgroup_sb_opts {
        unsigned long subsys_bits;
        unsigned long flags;
        char *release_agent;
};

/* 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";

        opts->subsys_bits = 0;
        opts->flags = 0;
        opts->release_agent = NULL;

        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, "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 = kzalloc(PATH_MAX, GFP_KERNEL);
                        if (!opts->release_agent)
                                return -ENOMEM;
                        strncpy(opts->release_agent, token + 14, PATH_MAX - 1);
                        opts->release_agent[PATH_MAX - 1] = 0;
                } 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;
                }
        }

        /* We can't have an empty hierarchy */
        if (!opts->subsys_bits)
                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;

        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;
        }

        ret = rebind_subsystems(root, opts.subsys_bits);

        /* (re)populate subsystem files */
        if (!ret)
                cgroup_populate_dir(cgrp);

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

static 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_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_LIST_HEAD(&cgrp->sibling);
        INIT_LIST_HEAD(&cgrp->children);
        INIT_LIST_HEAD(&cgrp->css_sets);
        INIT_LIST_HEAD(&cgrp->release_list);
}

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

        /* First check subsystems */
        if (new->subsys_bits != root->subsys_bits)
            return 0;

        /* Next check flags */
        if (new->flags != root->flags)
                return 0;

        return 1;
}

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

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

        sb->s_fs_info = root;
        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;
        int ret = 0;
        struct super_block *sb;
        struct cgroupfs_root *root;
        struct list_head tmp_cg_links, *l;
        INIT_LIST_HEAD(&tmp_cg_links);

        /* First find the desired set of subsystems */
        ret = parse_cgroupfs_options(data, &opts);
        if (ret) {
                if (opts.release_agent)
                        kfree(opts.release_agent);
                return ret;
        }

        root = kzalloc(sizeof(*root), GFP_KERNEL);
        if (!root) {
                if (opts.release_agent)
                        kfree(opts.release_agent);
                return -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);
                kfree(opts.release_agent);
        }

        sb = sget(fs_type, cgroup_test_super, cgroup_set_super, root);

        if (IS_ERR(sb)) {
                kfree(root);
                return PTR_ERR(sb);
        }

        if (sb->s_fs_info != root) {
                /* Reusing an existing superblock */
                BUG_ON(sb->s_root == NULL);
                kfree(root);
                root = NULL;
        } else {
                /* New superblock */
                struct cgroup *cgrp = &root->top_cgroup;
                struct inode *inode;

                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);

                /*
                 * 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);
                        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->top_cgroup;
                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);
                l = &init_css_set.list;
                do {
                        struct css_set *cg;
                        struct cg_cgroup_link *link;
                        cg = list_entry(l, struct css_set, list);
                        BUG_ON(list_empty(&tmp_cg_links));
                        link = list_entry(tmp_cg_links.next,
                                          struct cg_cgroup_link,
                                          cgrp_link_list);
                        list_del(&link->cgrp_link_list);
                        link->cg = cg;
                        list_add(&link->cgrp_link_list,
                                 &root->top_cgroup.css_sets);
                        list_add(&link->cg_link_list, &cg->cg_links);
                        l = l->next;
                } while (l != &init_css_set.list);
                write_unlock(&css_set_lock);

                free_cg_links(&tmp_cg_links);

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

                cgroup_populate_dir(cgrp);
                mutex_unlock(&inode->i_mutex);
                mutex_unlock(&cgroup_mutex);
        }

        return simple_set_mnt(mnt, sb);

 drop_new_super:
        up_write(&sb->s_umount);
        deactivate_super(sb);
        free_cg_links(&tmp_cg_links);
        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;

        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);
        while (!list_empty(&cgrp->css_sets)) {
                struct cg_cgroup_link *link;
                link = list_entry(cgrp->css_sets.next,
                                  struct cg_cgroup_link, 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);

        kfree(root);
        kill_litter_super(sb);
}

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. 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;

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

        start = buf + buflen;

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

/*
 * Return the first subsystem attached to a cgroup's hierarchy, and
 * its subsystem id.
 */

static void get_first_subsys(const struct cgroup *cgrp,
                        struct cgroup_subsys_state **css, int *subsys_id)
{
        const struct cgroupfs_root *root = cgrp->root;
        const struct cgroup_subsys *test_ss;
        BUG_ON(list_empty(&root->subsys_list));
        test_ss = list_entry(root->subsys_list.next,
                             struct cgroup_subsys, sibling);
        if (css) {
                *css = cgrp->subsys[test_ss->subsys_id];
                BUG_ON(!*css);
        }
        if (subsys_id)
                *subsys_id = test_ss->subsys_id;
}

/**
 * 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;
        struct cgroup *oldcgrp;
        struct css_set *cg = tsk->cgroups;
        struct css_set *newcg;
        struct cgroupfs_root *root = cgrp->root;
        int subsys_id;

        get_first_subsys(cgrp, NULL, &subsys_id);

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

        for_each_subsys(root, ss) {
                if (ss->can_attach) {
                        retval = ss->can_attach(ss, cgrp, tsk);
                        if (retval)
                                return retval;
                }
        }

        /*
         * Locate or allocate a new css_set for this task,
         * based on its final set of cgroups
         */
        newcg = find_css_set(cg, cgrp);
        if (!newcg)
                return -ENOMEM;

        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_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);
        }
        set_bit(CGRP_RELEASABLE, &oldcgrp->flags);
        synchronize_rcu();
        put_css_set(cg);
        return 0;
}

/*
 * Attach task with pid 'pid' to cgroup 'cgrp'. Call with
 * cgroup_mutex, may take task_lock of task
 */
static int attach_task_by_pid(struct cgroup *cgrp, char *pidbuf)
{
        pid_t pid;
        struct task_struct *tsk;
        int ret;

        if (sscanf(pidbuf, "%d", &pid) != 1)
                return -EIO;

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

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

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

/* The various types of files and directories in a cgroup file system */
enum cgroup_filetype {
        FILE_ROOT,
        FILE_DIR,
        FILE_TASKLIST,
        FILE_NOTIFY_ON_RELEASE,
        FILE_RELEASABLE,
        FILE_RELEASE_AGENT,
};

static ssize_t cgroup_write_uint(struct cgroup *cgrp, struct cftype *cft,
                                 struct file *file,
                                 const char __user *userbuf,
                                 size_t nbytes, loff_t *unused_ppos)
{
        char buffer[64];
        int retval = 0;
        u64 val;
        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 */

        /* strip newline if necessary */
        if (nbytes && (buffer[nbytes-1] == '\n'))
                buffer[nbytes-1] = 0;
        val = simple_strtoull(buffer, &end, 0);
        if (*end)
                return -EINVAL;

        /* Pass to subsystem */
        retval = cft->write_uint(cgrp, cft, val);
        if (!retval)
                retval = nbytes;
        return retval;
}

static ssize_t cgroup_common_file_write(struct cgroup *cgrp,
                                           struct cftype *cft,
                                           struct file *file,
                                           const char __user *userbuf,
                                           size_t nbytes, loff_t *unused_ppos)
{
        enum cgroup_filetype type = cft->private;
        char *buffer;
        int retval = 0;

        if (nbytes >= PATH_MAX)
                return -E2BIG;

        /* +1 for nul-terminator */
        buffer = kmalloc(nbytes + 1, GFP_KERNEL);
        if (buffer == NULL)
                return -ENOMEM;

        if (copy_from_user(buffer, userbuf, nbytes)) {
                retval = -EFAULT;
                goto out1;
        }
        buffer[nbytes] = 0;     /* nul-terminate */
        strstrip(buffer);       /* strip -just- trailing whitespace */

        mutex_lock(&cgroup_mutex);

        /*
         * This was already checked for in cgroup_file_write(), but
         * check again now we're holding cgroup_mutex.
         */
        if (cgroup_is_removed(cgrp)) {
                retval = -ENODEV;
                goto out2;
        }

        switch (type) {
        case FILE_TASKLIST:
                retval = attach_task_by_pid(cgrp, buffer);
                break;
        case FILE_NOTIFY_ON_RELEASE:
                clear_bit(CGRP_RELEASABLE, &cgrp->flags);
                if (simple_strtoul(buffer, NULL, 10) != 0)
                        set_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
                else
                        clear_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);
                break;
        case FILE_RELEASE_AGENT:
                BUILD_BUG_ON(sizeof(cgrp->root->release_agent_path) < PATH_MAX);
                strcpy(cgrp->root->release_agent_path, buffer);
                break;
        default:
                retval = -EINVAL;
                goto out2;
        }

        if (retval == 0)
                retval = nbytes;
out2:
        mutex_unlock(&cgroup_mutex);
out1:
        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 (!cft || cgroup_is_removed(cgrp))
                return -ENODEV;
        if (cft->write)
                return cft->write(cgrp, cft, file, buf, nbytes, ppos);
        if (cft->write_uint)
                return cgroup_write_uint(cgrp, cft, file, buf, nbytes, ppos);
        return -EINVAL;
}

static ssize_t cgroup_read_uint(struct cgroup *cgrp, struct cftype *cft,
                                   struct file *file,
                                   char __user *buf, size_t nbytes,
                                   loff_t *ppos)
{
        char tmp[64];
        u64 val = cft->read_uint(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_common_file_read(struct cgroup *cgrp,
                                          struct cftype *cft,
                                          struct file *file,
                                          char __user *buf,
                                          size_t nbytes, loff_t *ppos)
{
        enum cgroup_filetype type = cft->private;
        char *page;
        ssize_t retval = 0;
        char *s;

        if (!(page = (char *)__get_free_page(GFP_KERNEL)))
                return -ENOMEM;

        s = page;

        switch (type) {
        case FILE_RELEASE_AGENT:
        {
                struct cgroupfs_root *root;
                size_t n;
                mutex_lock(&cgroup_mutex);
                root = cgrp->root;
                n = strnlen(root->release_agent_path,
                            sizeof(root->release_agent_path));
                n = min(n, (size_t) PAGE_SIZE);
                strncpy(s, root->release_agent_path, n);
                mutex_unlock(&cgroup_mutex);
                s += n;
                break;
        }
        default:
                retval = -EINVAL;
                goto out;
        }
        *s++ = '\n';

        retval = simple_read_from_buffer(buf, nbytes, ppos, page, s - page);
out:
        free_page((unsigned long)page);
        return retval;
}

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 (!cft || cgroup_is_removed(cgrp))
                return -ENODEV;

        if (cft->read)
                return cft->read(cgrp, cft, file, buf, nbytes, ppos);
        if (cft->read_uint)
                return cgroup_read_uint(cgrp, cft, file, buf, nbytes, ppos);
        return -EINVAL;
}

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)
                return -ENODEV;
        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 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 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, int mode,
                                struct super_block *sb)
{
        static 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,
                                int 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);
                cgrp->dentry = dentry;
                dget(dentry);
        }
        dput(dentry);

        return error;
}

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;

        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)) {
                error = cgroup_create_file(dentry, 0644 | 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 list_head *l;

        read_lock(&css_set_lock);
        l = cgrp->css_sets.next;
        while (l != &cgrp->css_sets) {
                struct cg_cgroup_link *link =
                        list_entry(l, struct cg_cgroup_link, cgrp_link_list);
                count += atomic_read(&link->cg->ref.refcount);
                l = l->next;
        }
        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.
 */
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;

        /* 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;
        if (l == &res->cgroups->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 *p = heap->ptrs[i];
                        if (i == 0) {
                                latest_time = p->start_time;
                                latest_task = p;
                        }
                        /* Process the task per the caller's callback */
                        scan->process_task(p, scan);
                        put_task_struct(p);
                }
                /*
                 * 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' file.
 *
 * 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.
 *
 * Upon tasks file open(), a struct ctr_struct is allocated, that
 * will have a pointer to an array (also allocated here).  The struct
 * ctr_struct * is stored in file->private_data.  Its resources will
 * be freed by release() when the file is closed.  The array is used
 * to sprintf the PIDs and then used by read().
 */
struct ctr_struct {
        char *buf;
        int bufsz;
};

/*
 * Load into 'pidarray' up to 'npids' of the tasks using cgroup
 * 'cgrp'.  Return actual number of pids loaded.  No need to
 * task_lock(p) when reading out p->cgroup, since we're in an RCU
 * read section, so the css_set can't go away, and is
 * immutable after creation.
 */
static int pid_array_load(pid_t *pidarray, int npids, struct cgroup *cgrp)
{
        int n = 0;
        struct cgroup_iter it;
        struct task_struct *tsk;
        cgroup_iter_start(cgrp, &it);
        while ((tsk = cgroup_iter_next(cgrp, &it))) {
                if (unlikely(n == npids))
                        break;
                pidarray[n++] = task_pid_vnr(tsk);
        }
        cgroup_iter_end(cgrp, &it);
        return n;
}

/**
 * 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
         */
        if (dentry->d_sb->s_op != &cgroup_ops)
                 goto err;

        ret = 0;
        cgrp = dentry->d_fsdata;
        rcu_read_lock();

        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);

        rcu_read_unlock();
err:
        return ret;
}

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

/*
 * Convert array 'a' of 'npids' pid_t's to a string of newline separated
 * decimal pids in 'buf'.  Don't write more than 'sz' chars, but return
 * count 'cnt' of how many chars would be written if buf were large enough.
 */
static int pid_array_to_buf(char *buf, int sz, pid_t *a, int npids)
{
        int cnt = 0;
        int i;

        for (i = 0; i < npids; i++)
                cnt += snprintf(buf + cnt, max(sz - cnt, 0), "%d\n", a[i]);
        return cnt;
}

/*
 * Handle an open on 'tasks' file.  Prepare a buffer listing the
 * process id's of tasks currently attached to the cgroup being opened.
 *
 * Does not require any specific cgroup mutexes, and does not take any.
 */
static int cgroup_tasks_open(struct inode *unused, struct file *file)
{
        struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
        struct ctr_struct *ctr;
        pid_t *pidarray;
        int npids;
        char c;

        if (!(file->f_mode & FMODE_READ))
                return 0;

        ctr = kmalloc(sizeof(*ctr), GFP_KERNEL);
        if (!ctr)
                goto err0;

        /*
         * 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.
         */
        npids = cgroup_task_count(cgrp);
        if (npids) {
                pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
                if (!pidarray)
                        goto err1;

                npids = pid_array_load(pidarray, npids, cgrp);
                sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);

                /* Call pid_array_to_buf() twice, first just to get bufsz */
                ctr->bufsz = pid_array_to_buf(&c, sizeof(c), pidarray, npids) + 1;
                ctr->buf = kmalloc(ctr->bufsz, GFP_KERNEL);
                if (!ctr->buf)
                        goto err2;
                ctr->bufsz = pid_array_to_buf(ctr->buf, ctr->bufsz, pidarray, npids);

                kfree(pidarray);
        } else {
                ctr->buf = NULL;
                ctr->bufsz = 0;
        }
        file->private_data = ctr;
        return 0;

err2:
        kfree(pidarray);
err1:
        kfree(ctr);
err0:
        return -ENOMEM;
}

static ssize_t cgroup_tasks_read(struct cgroup *cgrp,
                                    struct cftype *cft,
                                    struct file *file, char __user *buf,
                                    size_t nbytes, loff_t *ppos)
{
        struct ctr_struct *ctr = file->private_data;

        return simple_read_from_buffer(buf, nbytes, ppos, ctr->buf, ctr->bufsz);
}

static int cgroup_tasks_release(struct inode *unused_inode,
                                        struct file *file)
{
        struct ctr_struct *ctr;

        if (file->f_mode & FMODE_READ) {
                ctr = file->private_data;
                kfree(ctr->buf);
                kfree(ctr);
        }
        return 0;
}

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

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

/*
 * for the common functions, 'private' gives the type of file
 */
static struct cftype files[] = {
        {
                .name = "tasks",
                .open = cgroup_tasks_open,
                .read = cgroup_tasks_read,
                .write = cgroup_common_file_write,
                .release = cgroup_tasks_release,
                .private = FILE_TASKLIST,
        },

        {
                .name = "notify_on_release",
                .read_uint = cgroup_read_notify_on_release,
                .write = cgroup_common_file_write,
                .private = FILE_NOTIFY_ON_RELEASE,
        },

        {
                .name = "releasable",
                .read_uint = cgroup_read_releasable,
                .private = FILE_RELEASABLE,
        }
};

static struct cftype cft_release_agent = {
        .name = "release_agent",
        .read = cgroup_common_file_read,
        .write = cgroup_common_file_write,
        .private = FILE_RELEASE_AGENT,
};

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;
        }

        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, 0);
        css->flags = 0;
        if (cgrp == dummytop)
                set_bit(CSS_ROOT, &css->flags);
        BUG_ON(cgrp->subsys[ss->subsys_id]);
        cgrp->subsys[ss->subsys_id] = css;
}

/*
 * 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,
                             int 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_LIST_HEAD(&cgrp->sibling);
        INIT_LIST_HEAD(&cgrp->children);
        INIT_LIST_HEAD(&cgrp->css_sets);
        INIT_LIST_HEAD(&cgrp->release_list);

        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);
        }

        list_add(&cgrp->sibling, &cgrp->parent->children);
        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:

        list_del(&cgrp->sibling);
        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 inline 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 0, 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))
                        return 1;
        }
        return 0;
}

static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)
{
        struct cgroup *cgrp = dentry->d_fsdata;
        struct dentry *d;
        struct cgroup *parent;
        struct super_block *sb;
        struct cgroupfs_root *root;

        /* the vfs holds both inode->i_mutex already */

        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;
        }

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

        /*
         * Call pre_destroy handlers of subsys. Notify subsystems
         * that rmdir() request comes.
         */
        cgroup_call_pre_destroy(cgrp);

        if (cgroup_has_css_refs(cgrp)) {
                mutex_unlock(&cgroup_mutex);
                return -EBUSY;
        }

        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);
        /* delete my sibling from parent->children */
        list_del(&cgrp->sibling);
        spin_lock(&cgrp->dentry->d_lock);
        d = dget(cgrp->dentry);
        cgrp->dentry = NULL;
        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 cgroup_init_subsys(struct cgroup_subsys *ss)
{
        struct cgroup_subsys_state *css;
        struct list_head *l;

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

        /* Create the top cgroup state for this subsystem */
        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 all cgroup groups to contain a subsys
         * pointer to this state - since the subsystem is
         * newly registered, all tasks and hence all cgroup
         * groups are in the subsystem's top cgroup. */
        write_lock(&css_set_lock);
        l = &init_css_set.list;
        do {
                struct css_set *cg =
                        list_entry(l, struct css_set, list);
                cg->subsys[ss->subsys_id] = dummytop->subsys[ss->subsys_id];
                l = l->next;
        } while (l != &init_css_set.list);
        write_unlock(&css_set_lock);

        /* If this subsystem requested that it be notified with fork
         * events, we should send it one now for every process in the
         * system */
        if (ss->fork) {
                struct task_struct *g, *p;

                read_lock(&tasklist_lock);
                do_each_thread(g, p) {
                        ss->fork(ss, p);
                } while_each_thread(g, p);
                read_unlock(&tasklist_lock);
        }

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

        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;
        kref_init(&init_css_set.ref);
        kref_get(&init_css_set.ref);
        INIT_LIST_HEAD(&init_css_set.list);
        INIT_LIST_HEAD(&init_css_set.cg_links);
        INIT_LIST_HEAD(&init_css_set.tasks);
        css_set_count = 1;
        init_cgroup_root(&rootnode);
        list_add(&rootnode.root_list, &roots);
        root_count = 1;
        init_task.cgroups = &init_css_set;

        init_css_set_link.cg = &init_css_set;
        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 < 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 proc_dir_entry *entry;

        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);
        }

        err = register_filesystem(&cgroup_fs_type);
        if (err < 0)
                goto out;

        entry = create_proc_entry("cgroups", 0, NULL);
        if (entry)
                entry->proc_fops = &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_root(root) {
                struct cgroup_subsys *ss;
                struct cgroup *cgrp;
                int subsys_id;
                int count = 0;

                /* Skip this hierarchy if it has no active subsystems */
                if (!root->actual_subsys_bits)
                        continue;
                seq_printf(m, "%lu:", root->subsys_bits);
                for_each_subsys(root, ss)
                        seq_printf(m, "%s%s", count++ ? "," : "", ss->name);
                seq_putc(m, ':');
                get_first_subsys(&root->top_cgroup, NULL, &subsys_id);
                cgrp = task_cgroup(tsk, subsys_id);
                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);
}

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%lu\t%d\t%d\n",
                           ss->name, ss->root->subsys_bits,
                           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 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);
                if (list_empty(&child->cg_list))
                        list_add(&child->cg_list, &child->cgroups->tasks);
                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
 *
 * 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)
{
        struct dentry *dentry;
        int ret = 0;
        char nodename[MAX_CGROUP_TYPE_NAMELEN];
        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) {
                printk(KERN_INFO
                       "Not cloning cgroup for unused subsystem %s\n",
                       subsys->name);
                mutex_unlock(&cgroup_mutex);
                return 0;
        }
        cg = tsk->cgroups;
        parent = task_cgroup(tsk, subsys->subsys_id);

        snprintf(nodename, MAX_CGROUP_TYPE_NAMELEN, "node_%d", tsk->pid);

        /* Pin the hierarchy */
        atomic_inc(&parent->root->sb->s_active);

        /* Keep the cgroup alive */
        get_css_set(cg);
        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, S_IFDIR | 0755);
        child = __d_cgrp(dentry);
        dput(dentry);
        if (ret) {
                printk(KERN_INFO
                       "Failed to create cgroup %s: %d\n", nodename,
                       ret);
                goto out_release;
        }

        if (!child) {
                printk(KERN_INFO
                       "Couldn't find new cgroup %s\n", nodename);
                ret = -ENOMEM;
                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(parent->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(parent->root->sb);
        return ret;
}

/**
 * cgroup_is_descendant - see if @cgrp is a descendant of current task's cgrp
 * @cgrp: the cgroup in question
 *
 * See if @cgrp is a descendant of the current 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)
{
        int ret;
        struct cgroup *target;
        int subsys_id;

        if (cgrp == dummytop)
                return 1;

        get_first_subsys(cgrp, NULL, &subsys_id);
        target = task_cgroup(current, subsys_id);
        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;
        rcu_read_lock();
        if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cgrp)) {
                set_bit(CGRP_RELEASABLE, &cgrp->flags);
                check_for_release(cgrp);
        }
        rcu_read_unlock();
}

/*
 * 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;
                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) {
                        spin_lock(&release_list_lock);
                        continue;
                }

                if (cgroup_path(cgrp, pathbuf, PAGE_SIZE) < 0) {
                        kfree(pathbuf);
                        spin_lock(&release_list_lock);
                        continue;
                }

                i = 0;
                argv[i++] = cgrp->root->release_agent_path;
                argv[i++] = (char *)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);
                kfree(pathbuf);
                mutex_lock(&cgroup_mutex);
                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);