/*

   *  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 <linux/hash.h>

  #include <linux/namei.h>

  #include <linux/smp_lock.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 active hierarchies */

  	struct list_head root_list;
  
  	/* Hierarchy-specific flags */
  	unsigned long flags;

  	/* The path to use for release notifications. */

  	char release_agent_path[PATH_MAX];

  };

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

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

  /* The list of hierarchy roots */
  
  static LIST_HEAD(roots);

  static int root_count;

  
  /* dummytop is a shorthand for the dummy hierarchy's top cgroup */
  #define dummytop (&rootnode.top_cgroup)
  
  /* This flag indicates whether tasks in the fork and exit paths should

   * check for fork/exit handlers to call. This avoids us having to do
   * extra work in the fork/exit path if none of the subsystems need to
   * be called.

   */

  static int need_forkexit_callback __read_mostly;

  /* convenient tests for these bits */

  inline int cgroup_is_removed(const struct cgroup *cgrp)

  {

  	return test_bit(CGRP_REMOVED, &cgrp->flags);

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

  static int cgroup_is_releasable(const struct cgroup *cgrp)

  {
  	const int bits =

  		(1 << CGRP_RELEASABLE) |
  		(1 << CGRP_NOTIFY_ON_RELEASE);
  	return (cgrp->flags & bits) == bits;

  }

  static int notify_on_release(const struct cgroup *cgrp)

  {

  	return test_bit(CGRP_NOTIFY_ON_RELEASE, &cgrp->flags);

  }

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

  /* for_each_active_root() allows you to iterate across the active hierarchies */
  #define for_each_active_root(_root) \

  list_for_each_entry(_root, &roots, root_list)

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

  static void check_for_release(struct cgroup *cgrp);

  /* Link structure for associating css_set objects with cgroups */
  struct cg_cgroup_link {
  	/*
  	 * List running through cg_cgroup_links associated with a
  	 * cgroup, anchored on cgroup->css_sets
  	 */

  	struct list_head cgrp_link_list;

  	/*
  	 * List running through cg_cgroup_links pointing at a
  	 * single css_set object, anchored on css_set->cg_links
  	 */
  	struct list_head cg_link_list;
  	struct css_set *cg;
  };
  
  /* The default css_set - used by init and its children prior to any
   * hierarchies being mounted. It contains a pointer to the root state
   * for each subsystem. Also used to anchor the list of css_sets. Not
   * reference-counted, to improve performance when child cgroups
   * haven't been created.
   */
  
  static struct css_set init_css_set;
  static struct cg_cgroup_link init_css_set_link;

  static int cgroup_subsys_init_idr(struct cgroup_subsys *ss);

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

  /* hash table for cgroup groups. This improves the performance to
   * find an existing css_set */
  #define CSS_SET_HASH_BITS	7
  #define CSS_SET_TABLE_SIZE	(1 << CSS_SET_HASH_BITS)
  static struct hlist_head css_set_table[CSS_SET_TABLE_SIZE];
  
  static struct hlist_head *css_set_hash(struct cgroup_subsys_state *css[])
  {
  	int i;
  	int index;
  	unsigned long tmp = 0UL;
  
  	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++)
  		tmp += (unsigned long)css[i];
  	tmp = (tmp >> 16) ^ tmp;
  
  	index = hash_long(tmp, CSS_SET_HASH_BITS);
  
  	return &css_set_table[index];
  }

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

  static int use_task_css_set_links __read_mostly;

  
  /* 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)

  {

  	struct cg_cgroup_link *link;
  	struct cg_cgroup_link *saved_link;

  	hlist_del(&cg->hlist);

  	css_set_count--;

  
  	list_for_each_entry_safe(link, saved_link, &cg->cg_links,
  				 cg_link_list) {

  		list_del(&link->cg_link_list);

  		list_del(&link->cgrp_link_list);

  		kfree(link);
  	}

  }

  static void __put_css_set(struct css_set *cg, int taskexit)

  {
  	int i;

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

  	unlink_css_set(cg);

  	write_unlock(&css_set_lock);

  
  	rcu_read_lock();
  	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {

  		struct cgroup *cgrp = rcu_dereference(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);

  }

  /*
   * refcounted get/put for css_set objects
   */
  static inline void get_css_set(struct css_set *cg)
  {

  	atomic_inc(&cg->refcount);

  }
  
  static inline void put_css_set(struct css_set *cg)
  {

  	__put_css_set(cg, 0);

  }

  static inline void put_css_set_taskexit(struct css_set *cg)
  {

  	__put_css_set(cg, 1);

  }

  /*
   * find_existing_css_set() is a helper for
   * find_css_set(), and checks to see whether an existing

   * css_set is suitable.

   *
   * oldcg: the cgroup group that we're using before the cgroup
   * transition
   *

   * cgrp: the cgroup that we're moving into

   *
   * template: location in which to build the desired set of subsystem
   * state objects for the new cgroup group
   */

  static struct css_set *find_existing_css_set(
  	struct css_set *oldcg,

  	struct cgroup *cgrp,

  	struct cgroup_subsys_state *template[])

  {
  	int i;

  	struct cgroupfs_root *root = cgrp->root;

  	struct hlist_head *hhead;
  	struct hlist_node *node;
  	struct css_set *cg;

  
  	/* Built the set of subsystem state objects that we want to
  	 * see in the new css_set */
  	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {

  		if (root->subsys_bits & (1UL << i)) {

  			/* Subsystem is in this hierarchy. So we want
  			 * the subsystem state from the new
  			 * cgroup */

  			template[i] = cgrp->subsys[i];

  		} else {
  			/* Subsystem is not in this hierarchy, so we
  			 * don't want to change the subsystem state */
  			template[i] = oldcg->subsys[i];
  		}
  	}

  	hhead = css_set_hash(template);
  	hlist_for_each_entry(cg, node, hhead, hlist) {

  		if (!memcmp(template, cg->subsys, sizeof(cg->subsys))) {
  			/* All subsystems matched */
  			return cg;
  		}

  	}

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

  static void free_cg_links(struct list_head *tmp)
  {
  	struct cg_cgroup_link *link;
  	struct cg_cgroup_link *saved_link;
  
  	list_for_each_entry_safe(link, saved_link, tmp, cgrp_link_list) {
  		list_del(&link->cgrp_link_list);
  		kfree(link);
  	}
  }

  /*
   * allocate_cg_links() allocates "count" cg_cgroup_link structures

   * and chains them on tmp through their cgrp_link_list fields. Returns 0 on

   * success or a negative error
   */

  static int allocate_cg_links(int count, struct list_head *tmp)
  {
  	struct cg_cgroup_link *link;
  	int i;
  	INIT_LIST_HEAD(tmp);
  	for (i = 0; i < count; i++) {
  		link = kmalloc(sizeof(*link), GFP_KERNEL);
  		if (!link) {

  			free_cg_links(tmp);

  			return -ENOMEM;
  		}

  		list_add(&link->cgrp_link_list, tmp);

  	}
  	return 0;
  }

  /**
   * link_css_set - a helper function to link a css_set to a cgroup
   * @tmp_cg_links: cg_cgroup_link objects allocated by allocate_cg_links()
   * @cg: the css_set to be linked
   * @cgrp: the destination cgroup
   */
  static void link_css_set(struct list_head *tmp_cg_links,
  			 struct css_set *cg, struct cgroup *cgrp)
  {
  	struct cg_cgroup_link *link;
  
  	BUG_ON(list_empty(tmp_cg_links));
  	link = list_first_entry(tmp_cg_links, struct cg_cgroup_link,
  				cgrp_link_list);
  	link->cg = cg;
  	list_move(&link->cgrp_link_list, &cgrp->css_sets);
  	list_add(&link->cg_link_list, &cg->cg_links);
  }

  /*
   * find_css_set() takes an existing cgroup group and a
   * cgroup object, and returns a css_set object that's
   * equivalent to the old group, but with the given cgroup
   * substituted into the appropriate hierarchy. Must be called with
   * cgroup_mutex held
   */

  static struct css_set *find_css_set(

  	struct css_set *oldcg, struct cgroup *cgrp)

  {
  	struct css_set *res;
  	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];
  	int i;
  
  	struct list_head tmp_cg_links;

  	struct hlist_head *hhead;

  	/* First see if we already have a cgroup group that matches
  	 * the desired set */

  	read_lock(&css_set_lock);

  	res = find_existing_css_set(oldcg, cgrp, template);

  	if (res)
  		get_css_set(res);

  	read_unlock(&css_set_lock);

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

  	atomic_set(&res->refcount, 1);

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

  	INIT_HLIST_NODE(&res->hlist);

  
  	/* Copy the set of subsystem state objects generated in
  	 * find_existing_css_set() */
  	memcpy(res->subsys, template, sizeof(res->subsys));
  
  	write_lock(&css_set_lock);
  	/* Add reference counts and links from the new css_set. */
  	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)
  			link_css_set(&tmp_cg_links, res, cgrp);

  	}

  	if (list_empty(&rootnode.subsys_list))
  		link_css_set(&tmp_cg_links, res, dummytop);

  
  	BUG_ON(!list_empty(&tmp_cg_links));

  	css_set_count++;

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

  	write_unlock(&css_set_lock);
  
  	return res;

  }

  /*
   * There is one global cgroup mutex. We also require taking
   * task_lock() when dereferencing a task's cgroup subsys pointers.
   * See "The task_lock() exception", at the end of this comment.
   *
   * A task must hold cgroup_mutex to modify cgroups.
   *
   * Any task can increment and decrement the count field without lock.
   * So in general, code holding cgroup_mutex can't rely on the count
   * field not changing.  However, if the count goes to zero, then only

   * cgroup_attach_task() can increment it again.  Because a count of zero

   * means that no tasks are currently attached, therefore there is no
   * way a task attached to that cgroup can fork (the other way to
   * increment the count).  So code holding cgroup_mutex can safely
   * assume that if the count is zero, it will stay zero. Similarly, if
   * a task holds cgroup_mutex on a cgroup with zero count, it
   * knows that the cgroup won't be removed, as cgroup_rmdir()
   * needs that mutex.
   *

   * The fork and exit callbacks cgroup_fork() and cgroup_exit(), don't
   * (usually) take cgroup_mutex.  These are the two most performance
   * critical pieces of code here.  The exception occurs on cgroup_exit(),
   * when a task in a notify_on_release cgroup exits.  Then cgroup_mutex
   * is taken, and if the cgroup count is zero, a usermode call made

   * to the release agent with the name of the cgroup (path relative to
   * the root of cgroup file system) as the argument.

   *
   * A cgroup can only be deleted if both its 'count' of using tasks
   * is zero, and its list of 'children' cgroups is empty.  Since all
   * tasks in the system use _some_ cgroup, and since there is always at
   * least one task in the system (init, pid == 1), therefore, top_cgroup
   * always has either children cgroups and/or using tasks.  So we don't
   * need a special hack to ensure that top_cgroup cannot be deleted.
   *
   *	The task_lock() exception
   *
   * The need for this exception arises from the action of

   * cgroup_attach_task(), which overwrites one tasks cgroup pointer with

   * another.  It does so using cgroup_mutex, however there are

   * several performance critical places that need to reference
   * task->cgroup without the expense of grabbing a system global
   * mutex.  Therefore except as noted below, when dereferencing or, as

   * in cgroup_attach_task(), modifying a task'ss cgroup pointer we use

   * task_lock(), which acts on a spinlock (task->alloc_lock) already in
   * the task_struct routinely used for such matters.
   *
   * P.S.  One more locking exception.  RCU is used to guard the

   * update of a tasks cgroup pointer by cgroup_attach_task()

   */

  /**
   * cgroup_lock - lock out any changes to cgroup structures
   *
   */

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

  void cgroup_unlock(void)
  {
  	mutex_unlock(&cgroup_mutex);
  }
  
  /*
   * A couple of forward declarations required, due to cyclic reference loop:
   * cgroup_mkdir -> cgroup_create -> cgroup_populate_dir ->
   * cgroup_add_file -> cgroup_create_file -> cgroup_dir_inode_operations
   * -> cgroup_mkdir.
   */
  
  static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, int mode);
  static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry);

  static int cgroup_populate_dir(struct cgroup *cgrp);

  static 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_AND_WRITEBACK,

  };

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

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

  
  	if (inode) {
  		inode->i_mode = mode;

  		inode->i_uid = current_fsuid();
  		inode->i_gid = current_fsgid();

  		inode->i_atime = inode->i_mtime = inode->i_ctime = CURRENT_TIME;
  		inode->i_mapping->backing_dev_info = &cgroup_backing_dev_info;
  	}
  	return inode;
  }

  /*
   * Call subsys's pre_destroy handler.
   * This is called before css refcnt check.
   */

  static int cgroup_call_pre_destroy(struct cgroup *cgrp)

  {
  	struct cgroup_subsys *ss;

  	int ret = 0;

  	for_each_subsys(cgrp->root, ss)

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

  }

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

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

  		struct cgroup *cgrp = dentry->d_fsdata;

  		struct cgroup_subsys *ss;

  		BUG_ON(!(cgroup_is_removed(cgrp)));

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

  
  		mutex_lock(&cgroup_mutex);
  		/*
  		 * Release the subsystem state objects.
  		 */

  		for_each_subsys(cgrp->root, ss)
  			ss->destroy(ss, cgrp);

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

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

  		deactivate_super(cgrp->root->sb);

  		call_rcu(&cgrp->rcu_head, free_cgroup_rcu);

  	}
  	iput(inode);
  }
  
  static void remove_dir(struct dentry *d)
  {
  	struct dentry *parent = dget(d->d_parent);
  
  	d_delete(d);
  	simple_rmdir(parent->d_inode, d);
  	dput(parent);
  }
  
  static void cgroup_clear_directory(struct dentry *dentry)
  {
  	struct list_head *node;
  
  	BUG_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));
  	spin_lock(&dcache_lock);
  	node = dentry->d_subdirs.next;
  	while (node != &dentry->d_subdirs) {
  		struct dentry *d = list_entry(node, struct dentry, d_u.d_child);
  		list_del_init(node);
  		if (d->d_inode) {
  			/* This should never be called on a cgroup
  			 * directory with child cgroups */
  			BUG_ON(d->d_inode->i_mode & S_IFDIR);
  			d = dget_locked(d);
  			spin_unlock(&dcache_lock);
  			d_delete(d);
  			simple_unlink(dentry->d_inode, d);
  			dput(d);
  			spin_lock(&dcache_lock);
  		}
  		node = dentry->d_subdirs.next;
  	}
  	spin_unlock(&dcache_lock);
  }
  
  /*
   * NOTE : the dentry must have been dget()'ed
   */
  static void cgroup_d_remove_dir(struct dentry *dentry)
  {
  	cgroup_clear_directory(dentry);
  
  	spin_lock(&dcache_lock);
  	list_del_init(&dentry->d_u.d_child);
  	spin_unlock(&dcache_lock);
  	remove_dir(dentry);
  }

  /*
   * A queue for waiters to do rmdir() cgroup. A tasks will sleep when
   * cgroup->count == 0 && list_empty(&cgroup->children) && subsys has some
   * reference to css->refcnt. In general, this refcnt is expected to goes down
   * to zero, soon.
   *
   * CGRP_WAIT_ON_RMDIR flag is modified under cgroup's inode->i_mutex;
   */
  DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);
  
  static void cgroup_wakeup_rmdir_waiters(const struct cgroup *cgrp)
  {
  	if (unlikely(test_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)))
  		wake_up_all(&cgroup_rmdir_waitq);
  }

  static int rebind_subsystems(struct cgroupfs_root *root,
  			      unsigned long final_bits)
  {
  	unsigned long added_bits, removed_bits;

  	struct cgroup *cgrp = &root->top_cgroup;

  	int i;
  
  	removed_bits = root->actual_subsys_bits & ~final_bits;
  	added_bits = final_bits & ~root->actual_subsys_bits;
  	/* Check that any added subsystems are currently free */
  	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {

  		unsigned long bit = 1UL << i;

  		struct cgroup_subsys *ss = subsys[i];
  		if (!(bit & added_bits))
  			continue;
  		if (ss->root != &rootnode) {
  			/* Subsystem isn't free */
  			return -EBUSY;
  		}
  	}
  
  	/* Currently we don't handle adding/removing subsystems when
  	 * any child cgroups exist. This is theoretically supportable
  	 * but involves complex error handling, so it's being left until
  	 * later */

  	if (root->number_of_cgroups > 1)

  		return -EBUSY;
  
  	/* Process each subsystem */
  	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
  		struct cgroup_subsys *ss = subsys[i];
  		unsigned long bit = 1UL << i;
  		if (bit & added_bits) {
  			/* We're binding this subsystem to this hierarchy */

  			BUG_ON(cgrp->subsys[i]);

  			BUG_ON(!dummytop->subsys[i]);
  			BUG_ON(dummytop->subsys[i]->cgroup != dummytop);

  			mutex_lock(&ss->hierarchy_mutex);

  			cgrp->subsys[i] = dummytop->subsys[i];
  			cgrp->subsys[i]->cgroup = cgrp;

  			list_move(&ss->sibling, &root->subsys_list);

  			ss->root = root;

  			if (ss->bind)

  				ss->bind(ss, cgrp);

  			mutex_unlock(&ss->hierarchy_mutex);

  		} else if (bit & removed_bits) {
  			/* We're removing this subsystem */

  			BUG_ON(cgrp->subsys[i] != dummytop->subsys[i]);
  			BUG_ON(cgrp->subsys[i]->cgroup != cgrp);

  			mutex_lock(&ss->hierarchy_mutex);

  			if (ss->bind)
  				ss->bind(ss, dummytop);
  			dummytop->subsys[i]->cgroup = dummytop;

  			cgrp->subsys[i] = NULL;

  			subsys[i]->root = &rootnode;

  			list_move(&ss->sibling, &rootnode.subsys_list);

  			mutex_unlock(&ss->hierarchy_mutex);

  		} else if (bit & final_bits) {
  			/* Subsystem state should already exist */

  			BUG_ON(!cgrp->subsys[i]);

  		} else {
  			/* Subsystem state shouldn't exist */

  			BUG_ON(cgrp->subsys[i]);

  		}
  	}
  	root->subsys_bits = root->actual_subsys_bits = final_bits;
  	synchronize_rcu();
  
  	return 0;
  }
  
  static int cgroup_show_options(struct seq_file *seq, struct vfsmount *vfs)
  {
  	struct cgroupfs_root *root = vfs->mnt_sb->s_fs_info;
  	struct cgroup_subsys *ss;
  
  	mutex_lock(&cgroup_mutex);
  	for_each_subsys(root, ss)
  		seq_printf(seq, ",%s", ss->name);
  	if (test_bit(ROOT_NOPREFIX, &root->flags))
  		seq_puts(seq, ",noprefix");

  	if (strlen(root->release_agent_path))
  		seq_printf(seq, ",release_agent=%s", root->release_agent_path);

  	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";

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

  
  	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;
  		}
  	}

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

  	/* 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;

  	lock_kernel();

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

  	mutex_lock(&cgroup_mutex);
  
  	/* See what subsystems are wanted */
  	ret = parse_cgroupfs_options(data, &opts);
  	if (ret)
  		goto out_unlock;
  
  	/* Don't allow flags to change at remount */
  	if (opts.flags != root->flags) {
  		ret = -EINVAL;
  		goto out_unlock;
  	}
  
  	ret = rebind_subsystems(root, opts.subsys_bits);

  	if (ret)
  		goto out_unlock;

  
  	/* (re)populate subsystem files */

  	cgroup_populate_dir(cgrp);

  	if (opts.release_agent)
  		strcpy(root->release_agent_path, opts.release_agent);

   out_unlock:

  	kfree(opts.release_agent);

  	mutex_unlock(&cgroup_mutex);

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

  	unlock_kernel();

  	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_housekeeping(struct cgroup *cgrp)
  {
  	INIT_LIST_HEAD(&cgrp->sibling);
  	INIT_LIST_HEAD(&cgrp->children);
  	INIT_LIST_HEAD(&cgrp->css_sets);
  	INIT_LIST_HEAD(&cgrp->release_list);
  	init_rwsem(&cgrp->pids_mutex);
  }

  static void init_cgroup_root(struct cgroupfs_root *root)
  {

  	struct cgroup *cgrp = &root->top_cgroup;

  	INIT_LIST_HEAD(&root->subsys_list);
  	INIT_LIST_HEAD(&root->root_list);
  	root->number_of_cgroups = 1;

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

  	init_cgroup_housekeeping(cgrp);

  }
  
  static 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;

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

  	if (ret) {

  		kfree(opts.release_agent);

  		return ret;

  	}

  
  	root = kzalloc(sizeof(*root), GFP_KERNEL);

  	if (!root) {

  		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 *root_cgrp = &root->top_cgroup;

  		struct inode *inode;

  		int i;

  
  		BUG_ON(sb->s_root != NULL);
  
  		ret = cgroup_get_rootdir(sb);
  		if (ret)
  			goto drop_new_super;

  		inode = sb->s_root->d_inode;

  		mutex_lock(&inode->i_mutex);

  		mutex_lock(&cgroup_mutex);

  		/*
  		 * 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 free_cg_links;

  		}
  
  		/* EBUSY should be the only error here */
  		BUG_ON(ret);
  
  		list_add(&root->root_list, &roots);

  		root_count++;

  		sb->s_root->d_fsdata = root_cgrp;

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

  		/* Link the top cgroup in this hierarchy into all
  		 * the css_set objects */
  		write_lock(&css_set_lock);

  		for (i = 0; i < CSS_SET_TABLE_SIZE; i++) {
  			struct hlist_head *hhead = &css_set_table[i];
  			struct hlist_node *node;

  			struct css_set *cg;

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

  		}

  		write_unlock(&css_set_lock);
  
  		free_cg_links(&tmp_cg_links);

  		BUG_ON(!list_empty(&root_cgrp->sibling));
  		BUG_ON(!list_empty(&root_cgrp->children));

  		BUG_ON(root->number_of_cgroups != 1);

  		cgroup_populate_dir(root_cgrp);

  		mutex_unlock(&inode->i_mutex);

  		mutex_unlock(&cgroup_mutex);
  	}

  	simple_set_mnt(mnt, sb);
  	return 0;

   free_cg_links:
  	free_cg_links(&tmp_cg_links);

   drop_new_super:

  	deactivate_locked_super(sb);

  	return ret;
  }
  
  static void cgroup_kill_sb(struct super_block *sb) {
  	struct cgroupfs_root *root = sb->s_fs_info;

  	struct cgroup *cgrp = &root->top_cgroup;

  	int ret;

  	struct cg_cgroup_link *link;
  	struct cg_cgroup_link *saved_link;

  
  	BUG_ON(!root);
  
  	BUG_ON(root->number_of_cgroups != 1);

  	BUG_ON(!list_empty(&cgrp->children));
  	BUG_ON(!list_empty(&cgrp->sibling));

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

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

  
  	list_for_each_entry_safe(link, saved_link, &cgrp->css_sets,
  				 cgrp_link_list) {

  		list_del(&link->cg_link_list);

  		list_del(&link->cgrp_link_list);

  		kfree(link);
  	}
  	write_unlock(&css_set_lock);

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

  	mutex_unlock(&cgroup_mutex);

  	kill_litter_super(sb);

  	kfree(root);

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

  static inline struct cgroup *__d_cgrp(struct dentry *dentry)

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

  /**
   * cgroup_path - generate the path of a cgroup
   * @cgrp: the cgroup in question
   * @buf: the buffer to write the path into
   * @buflen: the length of the buffer
   *

   * Called with cgroup_mutex held or else with an RCU-protected cgroup
   * reference.  Writes path of cgroup into buf.  Returns 0 on success,
   * -errno on error.

   */

  int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)

  {
  	char *start;

  	struct dentry *dentry = rcu_dereference(cgrp->dentry);

  	if (!dentry || cgrp == dummytop) {

  		/*
  		 * Inactive subsystems have no dentry for their root
  		 * cgroup
  		 */
  		strcpy(buf, "/");
  		return 0;
  	}
  
  	start = buf + buflen;
  
  	*--start = '\0';
  	for (;;) {

  		int len = dentry->d_name.len;

  		if ((start -= len) < buf)
  			return -ENAMETOOLONG;

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

  			break;

  		dentry = rcu_dereference(cgrp->dentry);

  		if (!cgrp->parent)

  			continue;
  		if (--start < buf)
  			return -ENAMETOOLONG;
  		*start = '/';
  	}
  	memmove(buf, start, buf + buflen - start);
  	return 0;
  }

  /*
   * 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;

  	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;

  		}
  	}

  	task_lock(tsk);
  	cg = tsk->cgroups;
  	get_css_set(cg);
  	task_unlock(tsk);

  	/*
  	 * Locate or allocate a new css_set for this task,
  	 * based on its final set of cgroups
  	 */

  	newcg = find_css_set(cg, cgrp);

  	put_css_set(cg);

  	if (!newcg)

  		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);

  
  	/*
  	 * wake up rmdir() waiter. the rmdir should fail since the cgroup
  	 * is no longer empty.
  	 */
  	cgroup_wakeup_rmdir_waiters(cgrp);

  	return 0;
  }
  
  /*

   * Attach task with pid 'pid' to cgroup 'cgrp'. Call with cgroup_mutex
   * held. May take task_lock of task

   */

  static int attach_task_by_pid(struct cgroup *cgrp, u64 pid)

  {

  	struct task_struct *tsk;

  	const struct cred *cred = current_cred(), *tcred;

  	int ret;

  	if (pid) {
  		rcu_read_lock();

  		tsk = find_task_by_vpid(pid);

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

  		tcred = __task_cred(tsk);
  		if (cred->euid &&
  		    cred->euid != tcred->uid &&
  		    cred->euid != tcred->suid) {
  			rcu_read_unlock();

  			return -EACCES;
  		}

  		get_task_struct(tsk);
  		rcu_read_unlock();

  	} else {
  		tsk = current;
  		get_task_struct(tsk);
  	}

  	ret = cgroup_attach_task(cgrp, tsk);

  	put_task_struct(tsk);
  	return ret;
  }

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

  /* 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_RELEASE_AGENT,

  };

  /**
   * cgroup_lock_live_group - take cgroup_mutex and check that cgrp is alive.
   * @cgrp: the cgroup to be checked for liveness
   *

   * On success, returns true; the lock should be later released with
   * cgroup_unlock(). On failure returns false with no lock held.

   */

  bool cgroup_lock_live_group(struct cgroup *cgrp)

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

  	cgroup_unlock();

  	return 0;
  }
  
  static int cgroup_release_agent_show(struct cgroup *cgrp, struct cftype *cft,
  				     struct seq_file *seq)
  {
  	if (!cgroup_lock_live_group(cgrp))
  		return -ENODEV;
  	seq_puts(seq, cgrp->root->release_agent_path);
  	seq_putc(seq, '
  ');

  	cgroup_unlock();

  	return 0;
  }

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

  static ssize_t cgroup_write_X64(struct cgroup *cgrp, struct cftype *cft,

  				struct file *file,
  				const char __user *userbuf,
  				size_t nbytes, loff_t *unused_ppos)

  {

  	char buffer[CGROUP_LOCAL_BUFFER_SIZE];

  	int retval = 0;

  	char *end;
  
  	if (!nbytes)
  		return -EINVAL;
  	if (nbytes >= sizeof(buffer))
  		return -E2BIG;
  	if (copy_from_user(buffer, userbuf, nbytes))
  		return -EFAULT;
  
  	buffer[nbytes] = 0;     /* nul-terminate */

  	strstrip(buffer);

  	if (cft->write_u64) {
  		u64 val = simple_strtoull(buffer, &end, 0);
  		if (*end)
  			return -EINVAL;
  		retval = cft->write_u64(cgrp, cft, val);
  	} else {
  		s64 val = simple_strtoll(buffer, &end, 0);
  		if (*end)
  			return -EINVAL;
  		retval = cft->write_s64(cgrp, cft, val);
  	}

  	if (!retval)
  		retval = nbytes;
  	return retval;
  }

  static ssize_t cgroup_write_string(struct cgroup *cgrp, struct cftype *cft,
  				   struct file *file,
  				   const char __user *userbuf,
  				   size_t nbytes, loff_t *unused_ppos)
  {

  	char local_buffer[CGROUP_LOCAL_BUFFER_SIZE];

  	int retval = 0;
  	size_t max_bytes = cft->max_write_len;
  	char *buffer = local_buffer;
  
  	if (!max_bytes)
  		max_bytes = sizeof(local_buffer) - 1;
  	if (nbytes >= max_bytes)
  		return -E2BIG;
  	/* Allocate a dynamic buffer if we need one */
  	if (nbytes >= sizeof(local_buffer)) {
  		buffer = kmalloc(nbytes + 1, GFP_KERNEL);
  		if (buffer == NULL)
  			return -ENOMEM;
  	}

  	if (nbytes && copy_from_user(buffer, userbuf, nbytes)) {
  		retval = -EFAULT;
  		goto out;
  	}

  
  	buffer[nbytes] = 0;     /* nul-terminate */
  	strstrip(buffer);
  	retval = cft->write_string(cgrp, cft, buffer);
  	if (!retval)
  		retval = nbytes;

  out:

  	if (buffer != local_buffer)
  		kfree(buffer);
  	return retval;
  }

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

  	struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);

  	if (cgroup_is_removed(cgrp))

  		return -ENODEV;

  	if (cft->write)

  		return cft->write(cgrp, cft, file, buf, nbytes, ppos);

  	if (cft->write_u64 || cft->write_s64)
  		return cgroup_write_X64(cgrp, cft, file, buf, nbytes, ppos);

  	if (cft->write_string)
  		return cgroup_write_string(cgrp, cft, file, buf, nbytes, ppos);

  	if (cft->trigger) {
  		int ret = cft->trigger(cgrp, (unsigned int)cft->private);
  		return ret ? ret : nbytes;
  	}

  	return -EINVAL;

  }

  static ssize_t cgroup_read_u64(struct cgroup *cgrp, struct cftype *cft,
  			       struct file *file,
  			       char __user *buf, size_t nbytes,
  			       loff_t *ppos)

  {

  	char tmp[CGROUP_LOCAL_BUFFER_SIZE];

  	u64 val = cft->read_u64(cgrp, cft);

  	int len = sprintf(tmp, "%llu
  ", (unsigned long long) val);
  
  	return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
  }

  static ssize_t cgroup_read_s64(struct cgroup *cgrp, struct cftype *cft,
  			       struct file *file,
  			       char __user *buf, size_t nbytes,
  			       loff_t *ppos)
  {

  	char tmp[CGROUP_LOCAL_BUFFER_SIZE];

  	s64 val = cft->read_s64(cgrp, cft);
  	int len = sprintf(tmp, "%lld
  ", (long long) val);
  
  	return simple_read_from_buffer(buf, nbytes, ppos, tmp, len);
  }

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

  	struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);

  	if (cgroup_is_removed(cgrp))

  		return -ENODEV;
  
  	if (cft->read)

  		return cft->read(cgrp, cft, file, buf, nbytes, ppos);

  	if (cft->read_u64)
  		return cgroup_read_u64(cgrp, cft, file, buf, nbytes, ppos);

  	if (cft->read_s64)
  		return cgroup_read_s64(cgrp, cft, file, buf, nbytes, ppos);

  	return -EINVAL;
  }

  /*
   * seqfile ops/methods for returning structured data. Currently just
   * supports string->u64 maps, but can be extended in future.
   */
  
  struct cgroup_seqfile_state {
  	struct cftype *cft;
  	struct cgroup *cgroup;
  };
  
  static int cgroup_map_add(struct cgroup_map_cb *cb, const char *key, u64 value)
  {
  	struct seq_file *sf = cb->state;
  	return seq_printf(sf, "%s %llu
  ", key, (unsigned long long)value);
  }
  
  static int cgroup_seqfile_show(struct seq_file *m, void *arg)
  {
  	struct cgroup_seqfile_state *state = m->private;
  	struct cftype *cft = state->cft;

  	if (cft->read_map) {
  		struct cgroup_map_cb cb = {
  			.fill = cgroup_map_add,
  			.state = m,
  		};
  		return cft->read_map(state->cgroup, cft, &cb);
  	}
  	return cft->read_seq_string(state->cgroup, cft, m);

  }

  static int cgroup_seqfile_release(struct inode *inode, struct file *file)

  {
  	struct seq_file *seq = file->private_data;
  	kfree(seq->private);
  	return single_release(inode, file);
  }
  
  static struct file_operations cgroup_seqfile_operations = {
  	.read = seq_read,

  	.write = cgroup_file_write,

  	.llseek = seq_lseek,
  	.release = cgroup_seqfile_release,
  };

  static int cgroup_file_open(struct inode *inode, struct file *file)
  {
  	int err;
  	struct cftype *cft;
  
  	err = generic_file_open(inode, file);
  	if (err)
  		return err;

  	cft = __d_cft(file->f_dentry);

  	if (cft->read_map || cft->read_seq_string) {

  		struct cgroup_seqfile_state *state =
  			kzalloc(sizeof(*state), GFP_USER);
  		if (!state)
  			return -ENOMEM;
  		state->cft = cft;
  		state->cgroup = __d_cgrp(file->f_dentry->d_parent);
  		file->f_op = &cgroup_seqfile_operations;
  		err = single_open(file, cgroup_seqfile_show, state);
  		if (err < 0)
  			kfree(state);
  	} else if (cft->open)

  		err = cft->open(inode, file);
  	else
  		err = 0;
  
  	return err;
  }
  
  static int cgroup_file_release(struct inode *inode, struct file *file)
  {
  	struct cftype *cft = __d_cft(file->f_dentry);
  	if (cft->release)
  		return cft->release(inode, file);
  	return 0;
  }
  
  /*
   * cgroup_rename - Only allow simple rename of directories in place.
   */
  static int cgroup_rename(struct inode *old_dir, struct dentry *old_dentry,
  			    struct inode *new_dir, struct dentry *new_dentry)
  {
  	if (!S_ISDIR(old_dentry->d_inode->i_mode))
  		return -ENOTDIR;
  	if (new_dentry->d_inode)
  		return -EEXIST;
  	if (old_dir != new_dir)
  		return -EIO;
  	return simple_rename(old_dir, old_dentry, new_dir, new_dentry);
  }
  
  static 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, mode_t mode,

  				struct super_block *sb)
  {

  	static const struct dentry_operations cgroup_dops = {

  		.d_iput = cgroup_diput,
  	};
  
  	struct inode *inode;
  
  	if (!dentry)
  		return -ENOENT;
  	if (dentry->d_inode)
  		return -EEXIST;
  
  	inode = cgroup_new_inode(mode, sb);
  	if (!inode)
  		return -ENOMEM;
  
  	if (S_ISDIR(mode)) {
  		inode->i_op = &cgroup_dir_inode_operations;
  		inode->i_fop = &simple_dir_operations;
  
  		/* start off with i_nlink == 2 (for "." entry) */
  		inc_nlink(inode);
  
  		/* start with the directory inode held, so that we can
  		 * populate it without racing with another mkdir */

  		mutex_lock_nested(&inode->i_mutex, I_MUTEX_CHILD);

  	} else if (S_ISREG(mode)) {
  		inode->i_size = 0;
  		inode->i_fop = &cgroup_file_operations;
  	}
  	dentry->d_op = &cgroup_dops;
  	d_instantiate(dentry, inode);
  	dget(dentry);	/* Extra count - pin the dentry in core */
  	return 0;
  }
  
  /*

   * cgroup_create_dir - create a directory for an object.
   * @cgrp: the cgroup we create the directory for. It must have a valid
   *        ->parent field. And we are going to fill its ->dentry field.
   * @dentry: dentry of the new cgroup
   * @mode: mode to set on new directory.

   */

  static int cgroup_create_dir(struct cgroup *cgrp, struct dentry *dentry,

  				mode_t mode)

  {
  	struct dentry *parent;
  	int error = 0;

  	parent = cgrp->parent->dentry;
  	error = cgroup_create_file(dentry, S_IFDIR | mode, cgrp->root->sb);

  	if (!error) {

  		dentry->d_fsdata = cgrp;

  		inc_nlink(parent->d_inode);

  		rcu_assign_pointer(cgrp->dentry, dentry);

  		dget(dentry);
  	}
  	dput(dentry);
  
  	return error;
  }

  /**
   * cgroup_file_mode - deduce file mode of a control file
   * @cft: the control file in question
   *
   * returns cft->mode if ->mode is not 0
   * returns S_IRUGO|S_IWUSR if it has both a read and a write handler
   * returns S_IRUGO if it has only a read handler
   * returns S_IWUSR if it has only a write hander
   */
  static mode_t cgroup_file_mode(const struct cftype *cft)
  {
  	mode_t mode = 0;
  
  	if (cft->mode)
  		return cft->mode;
  
  	if (cft->read || cft->read_u64 || cft->read_s64 ||
  	    cft->read_map || cft->read_seq_string)
  		mode |= S_IRUGO;
  
  	if (cft->write || cft->write_u64 || cft->write_s64 ||
  	    cft->write_string || cft->trigger)
  		mode |= S_IWUSR;
  
  	return mode;
  }

  int cgroup_add_file(struct cgroup *cgrp,

  		       struct cgroup_subsys *subsys,
  		       const struct cftype *cft)
  {

  	struct dentry *dir = cgrp->dentry;

  	struct dentry *dentry;
  	int error;

  	mode_t mode;

  
  	char name[MAX_CGROUP_TYPE_NAMELEN + MAX_CFTYPE_NAME + 2] = { 0 };

  	if (subsys && !test_bit(ROOT_NOPREFIX, &cgrp->root->flags)) {

  		strcpy(name, subsys->name);
  		strcat(name, ".");
  	}
  	strcat(name, cft->name);
  	BUG_ON(!mutex_is_locked(&dir->d_inode->i_mutex));
  	dentry = lookup_one_len(name, dir, strlen(name));
  	if (!IS_ERR(dentry)) {

  		mode = cgroup_file_mode(cft);
  		error = cgroup_create_file(dentry, mode | S_IFREG,

  						cgrp->root->sb);

  		if (!error)
  			dentry->d_fsdata = (void *)cft;
  		dput(dentry);
  	} else
  		error = PTR_ERR(dentry);
  	return error;
  }

  int cgroup_add_files(struct cgroup *cgrp,

  			struct cgroup_subsys *subsys,
  			const struct cftype cft[],
  			int count)
  {
  	int i, err;
  	for (i = 0; i < count; i++) {

  		err = cgroup_add_file(cgrp, subsys, &cft[i]);

  		if (err)
  			return err;
  	}
  	return 0;
  }

  /**
   * cgroup_task_count - count the number of tasks in a cgroup.
   * @cgrp: the cgroup in question
   *
   * Return the number of tasks in the cgroup.
   */

  int cgroup_task_count(const struct cgroup *cgrp)

  {
  	int count = 0;

  	struct cg_cgroup_link *link;

  
  	read_lock(&css_set_lock);

  	list_for_each_entry(link, &cgrp->css_sets, cgrp_link_list) {

  		count += atomic_read(&link->cg->refcount);

  	}
  	read_unlock(&css_set_lock);

  	return count;
  }
  
  /*

   * Advance a list_head iterator.  The iterator should be positioned at
   * the start of a css_set
   */

  static void cgroup_advance_iter(struct cgroup *cgrp,

  					  struct cgroup_iter *it)
  {
  	struct list_head *l = it->cg_link;
  	struct cg_cgroup_link *link;
  	struct css_set *cg;
  
  	/* Advance to the next non-empty css_set */
  	do {
  		l = l->next;

  		if (l == &cgrp->css_sets) {

  			it->cg_link = NULL;
  			return;
  		}

  		link = list_entry(l, struct cg_cgroup_link, cgrp_link_list);

  		cg = link->cg;
  	} while (list_empty(&cg->tasks));
  	it->cg_link = l;
  	it->task = cg->tasks.next;
  }

  /*
   * To reduce the fork() overhead for systems that are not actually
   * using their cgroups capability, we don't maintain the lists running
   * through each css_set to its tasks until we see the list actually
   * used - in other words after the first call to cgroup_iter_start().
   *
   * The tasklist_lock is not held here, as do_each_thread() and
   * while_each_thread() are protected by RCU.
   */

  static void cgroup_enable_task_cg_lists(void)

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

  		/*
  		 * We should check if the process is exiting, otherwise
  		 * it will race with cgroup_exit() in that the list
  		 * entry won't be deleted though the process has exited.
  		 */
  		if (!(p->flags & PF_EXITING) && list_empty(&p->cg_list))

  			list_add(&p->cg_list, &p->cgroups->tasks);
  		task_unlock(p);
  	} while_each_thread(g, p);
  	write_unlock(&css_set_lock);
  }

  void cgroup_iter_start(struct cgroup *cgrp, struct cgroup_iter *it)

  {
  	/*
  	 * The first time anyone tries to iterate across a cgroup,
  	 * we need to enable the list linking each css_set to its
  	 * tasks, and fix up all existing tasks.
  	 */

  	if (!use_task_css_set_links)
  		cgroup_enable_task_cg_lists();

  	read_lock(&css_set_lock);

  	it->cg_link = &cgrp->css_sets;
  	cgroup_advance_iter(cgrp, it);

  }

  struct task_struct *cgroup_iter_next(struct cgroup *cgrp,

  					struct cgroup_iter *it)
  {
  	struct task_struct *res;
  	struct list_head *l = it->task;

  	struct cg_cgroup_link *link;

  
  	/* If the iterator cg is NULL, we have no tasks */
  	if (!it->cg_link)
  		return NULL;
  	res = list_entry(l, struct task_struct, cg_list);
  	/* Advance iterator to find next entry */
  	l = l->next;

  	link = list_entry(it->cg_link, struct cg_cgroup_link, cgrp_link_list);
  	if (l == &link->cg->tasks) {

  		/* We reached the end of this task list - move on to
  		 * the next cg_cgroup_link */

  		cgroup_advance_iter(cgrp, it);

  	} else {
  		it->task = l;
  	}
  	return res;
  }

  void cgroup_iter_end(struct cgroup *cgrp, struct cgroup_iter *it)

  {
  	read_unlock(&css_set_lock);
  }

  static inline int started_after_time(struct task_struct *t1,
  				     struct timespec *time,
  				     struct task_struct *t2)
  {
  	int start_diff = timespec_compare(&t1->start_time, time);
  	if (start_diff > 0) {
  		return 1;
  	} else if (start_diff < 0) {
  		return 0;
  	} else {
  		/*
  		 * Arbitrarily, if two processes started at the same
  		 * time, we'll say that the lower pointer value
  		 * started first. Note that t2 may have exited by now
  		 * so this may not be a valid pointer any longer, but
  		 * that's fine - it still serves to distinguish
  		 * between two tasks started (effectively) simultaneously.
  		 */
  		return t1 > t2;
  	}
  }
  
  /*
   * This function is a callback from heap_insert() and is used to order
   * the heap.
   * In this case we order the heap in descending task start time.
   */
  static inline int started_after(void *p1, void *p2)
  {
  	struct task_struct *t1 = p1;
  	struct task_struct *t2 = p2;
  	return started_after_time(t1, &t2->start_time, t2);
  }
  
  /**
   * cgroup_scan_tasks - iterate though all the tasks in a cgroup
   * @scan: struct cgroup_scanner containing arguments for the scan
   *
   * Arguments include pointers to callback functions test_task() and
   * process_task().
   * Iterate through all the tasks in a cgroup, calling test_task() for each,
   * and if it returns true, call process_task() for it also.
   * The test_task pointer may be NULL, meaning always true (select all tasks).
   * Effectively duplicates cgroup_iter_{start,next,end}()
   * but does not lock css_set_lock for the call to process_task().
   * The struct cgroup_scanner may be embedded in any structure of the caller's
   * creation.
   * It is guaranteed that process_task() will act on every task that
   * is a member of the cgroup for the duration of this call. This
   * function may or may not call process_task() for tasks that exit
   * or move to a different cgroup during the call, or are forked or
   * move into the cgroup during the call.
   *
   * Note that test_task() may be called with locks held, and may in some
   * situations be called multiple times for the same task, so it should
   * be cheap.
   * If the heap pointer in the struct cgroup_scanner is non-NULL, a heap has been
   * pre-allocated and will be used for heap operations (and its "gt" member will
   * be overwritten), else a temporary heap will be used (allocation of which
   * may cause this function to fail).
   */
  int cgroup_scan_tasks(struct cgroup_scanner *scan)
  {
  	int retval, i;
  	struct cgroup_iter it;
  	struct task_struct *p, *dropped;
  	/* Never dereference latest_task, since it's not refcounted */
  	struct task_struct *latest_task = NULL;
  	struct ptr_heap tmp_heap;
  	struct ptr_heap *heap;
  	struct timespec latest_time = { 0, 0 };
  
  	if (scan->heap) {
  		/* The caller supplied our heap and pre-allocated its memory */
  		heap = scan->heap;
  		heap->gt = &started_after;
  	} else {
  		/* We need to allocate our own heap memory */
  		heap = &tmp_heap;
  		retval = heap_init(heap, PAGE_SIZE, GFP_KERNEL, &started_after);
  		if (retval)
  			/* cannot allocate the heap */
  			return retval;
  	}
  
   again:
  	/*
  	 * Scan tasks in the cgroup, using the scanner's "test_task" callback
  	 * to determine which are of interest, and using the scanner's
  	 * "process_task" callback to process any of them that need an update.
  	 * Since we don't want to hold any locks during the task updates,
  	 * gather tasks to be processed in a heap structure.
  	 * The heap is sorted by descending task start time.
  	 * If the statically-sized heap fills up, we overflow tasks that
  	 * started later, and in future iterations only consider tasks that
  	 * started after the latest task in the previous pass. This
  	 * guarantees forward progress and that we don't miss any tasks.
  	 */
  	heap->size = 0;
  	cgroup_iter_start(scan->cg, &it);
  	while ((p = cgroup_iter_next(scan->cg, &it))) {
  		/*
  		 * Only affect tasks that qualify per the caller's callback,
  		 * if he provided one
  		 */
  		if (scan->test_task && !scan->test_task(p, scan))
  			continue;
  		/*
  		 * Only process tasks that started after the last task
  		 * we processed
  		 */
  		if (!started_after_time(p, &latest_time, latest_task))
  			continue;
  		dropped = heap_insert(heap, p);
  		if (dropped == NULL) {
  			/*
  			 * The new task was inserted; the heap wasn't
  			 * previously full
  			 */
  			get_task_struct(p);
  		} else if (dropped != p) {
  			/*
  			 * The new task was inserted, and pushed out a
  			 * different task
  			 */
  			get_task_struct(p);
  			put_task_struct(dropped);
  		}
  		/*
  		 * Else the new task was newer than anything already in
  		 * the heap and wasn't inserted
  		 */
  	}
  	cgroup_iter_end(scan->cg, &it);
  
  	if (heap->size) {
  		for (i = 0; i < heap->size; i++) {

  			struct task_struct *q = heap->ptrs[i];

  			if (i == 0) {

  				latest_time = q->start_time;
  				latest_task = q;

  			}
  			/* Process the task per the caller's callback */

  			scan->process_task(q, scan);
  			put_task_struct(q);

  		}
  		/*
  		 * If we had to process any tasks at all, scan again
  		 * in case some of them were in the middle of forking
  		 * children that didn't get processed.
  		 * Not the most efficient way to do it, but it avoids
  		 * having to take callback_mutex in the fork path
  		 */
  		goto again;
  	}
  	if (heap == &tmp_heap)
  		heap_free(&tmp_heap);
  	return 0;
  }

  /*

   * Stuff for reading the 'tasks' 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.
   *

   */

  
  /*
   * 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, pid;

  	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;

  		pid = task_pid_vnr(tsk);
  		if (pid > 0)
  			pidarray[n++] = pid;

  	}

  	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,
  	 * and make sure it's a directory.

  	 */

  	if (dentry->d_sb->s_op != &cgroup_ops ||
  	    !S_ISDIR(dentry->d_inode->i_mode))

  		 goto err;
  
  	ret = 0;

  	cgrp = dentry->d_fsdata;

  	cgroup_iter_start(cgrp, &it);
  	while ((tsk = cgroup_iter_next(cgrp, &it))) {

  		switch (tsk->state) {
  		case TASK_RUNNING:
  			stats->nr_running++;
  			break;
  		case TASK_INTERRUPTIBLE:
  			stats->nr_sleeping++;
  			break;
  		case TASK_UNINTERRUPTIBLE:
  			stats->nr_uninterruptible++;
  			break;
  		case TASK_STOPPED:
  			stats->nr_stopped++;
  			break;
  		default:
  			if (delayacct_is_task_waiting_on_io(tsk))
  				stats->nr_io_wait++;
  			break;
  		}
  	}

  	cgroup_iter_end(cgrp, &it);

  err:
  	return ret;
  }

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

  /*

   * seq_file methods for the "tasks" file. The seq_file position is the
   * next pid to display; the seq_file iterator is a pointer to the pid
   * in the cgroup->tasks_pids array.

   */

  
  static void *cgroup_tasks_start(struct seq_file *s, loff_t *pos)

  {

  	/*
  	 * Initially we receive a position value that corresponds to
  	 * one more than the last pid shown (or 0 on the first call or
  	 * after a seek to the start). Use a binary-search to find the
  	 * next pid to display, if any
  	 */
  	struct cgroup *cgrp = s->private;
  	int index = 0, pid = *pos;
  	int *iter;
  
  	down_read(&cgrp->pids_mutex);
  	if (pid) {
  		int end = cgrp->pids_length;

  		while (index < end) {
  			int mid = (index + end) / 2;
  			if (cgrp->tasks_pids[mid] == pid) {
  				index = mid;
  				break;
  			} else if (cgrp->tasks_pids[mid] <= pid)
  				index = mid + 1;
  			else
  				end = mid;
  		}
  	}
  	/* If we're off the end of the array, we're done */
  	if (index >= cgrp->pids_length)
  		return NULL;
  	/* Update the abstract position to be the actual pid that we found */
  	iter = cgrp->tasks_pids + index;
  	*pos = *iter;
  	return iter;
  }
  
  static void cgroup_tasks_stop(struct seq_file *s, void *v)
  {
  	struct cgroup *cgrp = s->private;
  	up_read(&cgrp->pids_mutex);
  }
  
  static void *cgroup_tasks_next(struct seq_file *s, void *v, loff_t *pos)
  {
  	struct cgroup *cgrp = s->private;
  	int *p = v;
  	int *end = cgrp->tasks_pids + cgrp->pids_length;
  
  	/*
  	 * Advance to the next pid in the array. If this goes off the
  	 * end, we're done
  	 */
  	p++;
  	if (p >= end) {
  		return NULL;
  	} else {
  		*pos = *p;
  		return p;
  	}
  }
  
  static int cgroup_tasks_show(struct seq_file *s, void *v)
  {
  	return seq_printf(s, "%d
  ", *(int *)v);
  }

  static struct seq_operations cgroup_tasks_seq_operations = {
  	.start = cgroup_tasks_start,
  	.stop = cgroup_tasks_stop,
  	.next = cgroup_tasks_next,
  	.show = cgroup_tasks_show,
  };
  
  static void release_cgroup_pid_array(struct cgroup *cgrp)
  {
  	down_write(&cgrp->pids_mutex);
  	BUG_ON(!cgrp->pids_use_count);
  	if (!--cgrp->pids_use_count) {
  		kfree(cgrp->tasks_pids);
  		cgrp->tasks_pids = NULL;
  		cgrp->pids_length = 0;
  	}
  	up_write(&cgrp->pids_mutex);

  }

  static int cgroup_tasks_release(struct inode *inode, struct file *file)
  {
  	struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);
  
  	if (!(file->f_mode & FMODE_READ))
  		return 0;
  
  	release_cgroup_pid_array(cgrp);
  	return seq_release(inode, file);
  }
  
  static struct file_operations cgroup_tasks_operations = {
  	.read = seq_read,
  	.llseek = seq_lseek,
  	.write = cgroup_file_write,
  	.release = cgroup_tasks_release,
  };

  /*

   * Handle an open on 'tasks' file.  Prepare an array containing the

   * process id's of tasks currently attached to the cgroup being opened.

   */

  static int cgroup_tasks_open(struct inode *unused, struct file *file)
  {

  	struct cgroup *cgrp = __d_cgrp(file->f_dentry->d_parent);

  	pid_t *pidarray;
  	int npids;

  	int retval;

  	/* Nothing to do for write-only files */

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

  	/*
  	 * 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);

  	pidarray = kmalloc(npids * sizeof(pid_t), GFP_KERNEL);
  	if (!pidarray)
  		return -ENOMEM;
  	npids = pid_array_load(pidarray, npids, cgrp);
  	sort(pidarray, npids, sizeof(pid_t), cmppid, NULL);

  	/*
  	 * Store the array in the cgroup, freeing the old
  	 * array if necessary
  	 */
  	down_write(&cgrp->pids_mutex);
  	kfree(cgrp->tasks_pids);
  	cgrp->tasks_pids = pidarray;
  	cgrp->pids_length = npids;
  	cgrp->pids_use_count++;
  	up_write(&cgrp->pids_mutex);
  
  	file->f_op = &cgroup_tasks_operations;
  
  	retval = seq_open(file, &cgroup_tasks_seq_operations);
  	if (retval) {
  		release_cgroup_pid_array(cgrp);
  		return retval;

  	}

  	((struct seq_file *)file->private_data)->private = cgrp;

  	return 0;
  }

  static u64 cgroup_read_notify_on_release(struct cgroup *cgrp,

  					    struct cftype *cft)
  {

  	return notify_on_release(cgrp);

  }

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

  /*
   * for the common functions, 'private' gives the type of file
   */

  static struct cftype files[] = {
  	{
  		.name = "tasks",
  		.open = cgroup_tasks_open,

  		.write_u64 = cgroup_tasks_write,

  		.release = cgroup_tasks_release,
  		.private = FILE_TASKLIST,

  		.mode = S_IRUGO | S_IWUSR,

  	},
  
  	{
  		.name = "notify_on_release",

  		.read_u64 = cgroup_read_notify_on_release,

  		.write_u64 = cgroup_write_notify_on_release,

  		.private = FILE_NOTIFY_ON_RELEASE,
  	},

  };
  
  static struct cftype cft_release_agent = {
  	.name = "release_agent",

  	.read_seq_string = cgroup_release_agent_show,
  	.write_string = cgroup_release_agent_write,
  	.max_write_len = PATH_MAX,

  	.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;
  	}