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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
 * take callback_mutex and 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 */
};

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

inline 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 & (1ull << 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++;
      INIT_LIST_HEAD(&res->tasks);
      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
 * 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 /sbin/cgroup_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
 * attach_task(), which overwrites one tasks cgroup pointer with
 * another.  It does so using cgroup_mutexe, 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 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 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;
}

static void cgroup_diput(struct dentry *dentry, struct inode *inode)
{
      /* is dentry a directory ? if so, kfree() associated cgroup */
      if (S_ISDIR(inode->i_mode)) {
            struct cgroup *cgrp = dentry->d_fsdata;
            BUG_ON(!(cgroup_is_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();
            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 long bit = 1ull << 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")) {
                  opts->subsys_bits = (1 << CGROUP_SUBSYS_COUNT) - 1;
            } 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)) {
                              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_op = &simple_dir_inode_operations;
      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)
            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;
}

/*
 * 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;
}

/*
 * Attach task 'tsk' to cgroup 'cgrp'
 *
 * Call holding cgroup_mutex.  May take task_lock of
 * the task 'pid' during call.
 */
static int 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_pid(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 = 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 */

      mutex_lock(&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:
      {
            struct cgroupfs_root *root = cgrp->root;
            /* Strip trailing newline */
            if (nbytes && (buffer[nbytes-1] == '\n')) {
                  buffer[nbytes-1] = 0;
            }
            if (nbytes < sizeof(root->release_agent_path)) {
                  /* We never write anything other than '\0'
                   * into the last char of release_agent_path,
                   * so it always remains a NUL-terminated
                   * string */
                  strncpy(root->release_agent_path, buffer, nbytes);
                  root->release_agent_path[nbytes] = 0;
            } else {
                  retval = -ENOSPC;
            }
            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)
            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)
            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;
}

/* Count the number of tasks in a 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;
}

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) {
            struct task_struct *p, *g;
            write_lock(&css_set_lock);
            use_task_css_set_links = 1;
            do_each_thread(g, p) {
                  task_lock(p);
                  if (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);
      }
      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);
}

/*
 * 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_nr(tsk);
      }
      cgroup_iter_end(cgrp, &it);
      return n;
}

/**
 * Build and fill cgroupstats so that taskstats can export it to user
 * space.
 *
 * @stats: cgroupstats to fill information into
 * @dentry: A dentry entry belonging to the cgroup for which stats have
 * been requested.
 */
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 = 0;
            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.
 *    name:       name of the new cgroup. Will be strcpy'ed.
 *    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);

      cgrp->flags = 0;
      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;

      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 cgroup_subsys *ss;
      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;

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

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

      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);
      root->number_of_cgroups--;

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

      mutex_unlock(&cgroup_mutex);
      /* Drop the active superblock reference that we took when we
       * created the cgroup */
      deactivate_super(sb);
      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 - 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 - 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 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;
            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\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\n",
                     ss->name, ss->root->subsys_bits,
                     ss->root->number_of_cgroups);
      }
      mutex_unlock(&cgroup_mutex);
      return 0;
}

static int cgroupstats_open(struct inode *inode, struct file *file)
{
      return single_open(file, proc_cgroupstats_show, 0);
}

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.
 * @tsk: 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.  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 - 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. 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
 *
 * 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 attach_task() attempts, or task is a failed
 *    fork, never visible to 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 - 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 = 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;
}

/*
 * 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);
}

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