/*

   *  Generic process-grouping system.
   *
   *  Based originally on the cpuset system, extracted by Paul Menage
   *  Copyright (C) 2006 Google, Inc
   *

   *  Notifications support
   *  Copyright (C) 2009 Nokia Corporation
   *  Author: Kirill A. Shutemov
   *

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

  #include <linux/ctype.h>

  #include <linux/errno.h>
  #include <linux/fs.h>
  #include <linux/kernel.h>
  #include <linux/list.h>
  #include <linux/mm.h>
  #include <linux/mutex.h>
  #include <linux/mount.h>
  #include <linux/pagemap.h>

  #include <linux/proc_fs.h>

  #include <linux/rcupdate.h>
  #include <linux/sched.h>

  #include <linux/backing-dev.h>

  #include <linux/seq_file.h>
  #include <linux/slab.h>
  #include <linux/magic.h>
  #include <linux/spinlock.h>
  #include <linux/string.h>

  #include <linux/sort.h>

  #include <linux/kmod.h>

  #include <linux/module.h>

  #include <linux/delayacct.h>
  #include <linux/cgroupstats.h>

  #include <linux/hash.h>

  #include <linux/namei.h>

  #include <linux/pid_namespace.h>

  #include <linux/idr.h>

  #include <linux/vmalloc.h> /* TODO: replace with more sophisticated array */

  #include <linux/eventfd.h>
  #include <linux/poll.h>

  #include <asm/atomic.h>

  static DEFINE_MUTEX(cgroup_mutex);

  /*
   * Generate an array of cgroup subsystem pointers. At boot time, this is
   * populated up to CGROUP_BUILTIN_SUBSYS_COUNT, and modular subsystems are
   * registered after that. The mutable section of this array is protected by
   * cgroup_mutex.
   */

  #define SUBSYS(_x) &_x ## _subsys,

  static struct cgroup_subsys *subsys[CGROUP_SUBSYS_COUNT] = {

  #include <linux/cgroup_subsys.h>
  };

  #define MAX_CGROUP_ROOT_NAMELEN 64

  /*
   * A cgroupfs_root represents the root of a cgroup hierarchy,
   * and may be associated with a superblock to form an active
   * hierarchy
   */
  struct cgroupfs_root {
  	struct super_block *sb;
  
  	/*
  	 * The bitmask of subsystems intended to be attached to this
  	 * hierarchy
  	 */
  	unsigned long subsys_bits;

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

  	/* The bitmask of subsystems currently attached to this hierarchy */
  	unsigned long actual_subsys_bits;
  
  	/* A list running through the attached subsystems */
  	struct list_head subsys_list;
  
  	/* The root cgroup for this hierarchy */
  	struct cgroup top_cgroup;
  
  	/* Tracks how many cgroups are currently defined in hierarchy.*/
  	int number_of_cgroups;

  	/* A list running through the active hierarchies */

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

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

  	char release_agent_path[PATH_MAX];

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

  };

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

  /*
   * CSS ID -- ID per subsys's Cgroup Subsys State(CSS). used only when
   * cgroup_subsys->use_id != 0.
   */
  #define CSS_ID_MAX	(65535)
  struct css_id {
  	/*
  	 * The css to which this ID points. This pointer is set to valid value
  	 * after cgroup is populated. If cgroup is removed, this will be NULL.
  	 * This pointer is expected to be RCU-safe because destroy()
  	 * is called after synchronize_rcu(). But for safe use, css_is_removed()
  	 * css_tryget() should be used for avoiding race.
  	 */

  	struct cgroup_subsys_state __rcu *css;

  	/*
  	 * ID of this css.
  	 */
  	unsigned short id;
  	/*
  	 * Depth in hierarchy which this ID belongs to.
  	 */
  	unsigned short depth;
  	/*
  	 * ID is freed by RCU. (and lookup routine is RCU safe.)
  	 */
  	struct rcu_head rcu_head;
  	/*
  	 * Hierarchy of CSS ID belongs to.
  	 */
  	unsigned short stack[0]; /* Array of Length (depth+1) */
  };

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

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

  static int root_count;

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

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

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

   */

  static int need_forkexit_callback __read_mostly;

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

  /* convenient tests for these bits */

  inline int cgroup_is_removed(const struct cgroup *cgrp)

  {

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

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

  static int cgroup_is_releasable(const struct cgroup *cgrp)

  {
  	const int bits =

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

  }

  static int notify_on_release(const struct cgroup *cgrp)

  {

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

  }

  static int clone_children(const struct cgroup *cgrp)
  {
  	return test_bit(CGRP_CLONE_CHILDREN, &cgrp->flags);
  }

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

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

  list_for_each_entry(_root, &roots, root_list)

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

  static void check_for_release(struct cgroup *cgrp);

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

  	struct list_head cgrp_link_list;

  	struct cgroup *cgrp;

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

  static int cgroup_init_idr(struct cgroup_subsys *ss,
  			   struct cgroup_subsys_state *css);

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

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

  #define CSS_SET_HASH_BITS	7
  #define CSS_SET_TABLE_SIZE	(1 << CSS_SET_HASH_BITS)
  static struct hlist_head css_set_table[CSS_SET_TABLE_SIZE];
  
  static struct hlist_head *css_set_hash(struct cgroup_subsys_state *css[])
  {
  	int i;
  	int index;
  	unsigned long tmp = 0UL;
  
  	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++)
  		tmp += (unsigned long)css[i];
  	tmp = (tmp >> 16) ^ tmp;
  
  	index = hash_long(tmp, CSS_SET_HASH_BITS);
  
  	return &css_set_table[index];
  }

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

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

  static int use_task_css_set_links __read_mostly;

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

  {

  	struct cg_cgroup_link *link;
  	struct cg_cgroup_link *saved_link;

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

  	/* This css_set is dead. unlink it and release cgroup refcounts */
  	hlist_del(&cg->hlist);
  	css_set_count--;
  
  	list_for_each_entry_safe(link, saved_link, &cg->cg_links,
  				 cg_link_list) {
  		struct cgroup *cgrp = link->cgrp;
  		list_del(&link->cg_link_list);
  		list_del(&link->cgrp_link_list);

  		if (atomic_dec_and_test(&cgrp->count) &&
  		    notify_on_release(cgrp)) {

  			if (taskexit)

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

  		}

  
  		kfree(link);

  	}

  
  	write_unlock(&css_set_lock);

  	call_rcu(&cg->rcu_head, free_css_set_rcu);

  }

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

  	atomic_inc(&cg->refcount);

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

  	__put_css_set(cg, 0);

  }

  static inline void put_css_set_taskexit(struct css_set *cg)
  {

  	__put_css_set(cg, 1);

  }

  /*

   * compare_css_sets - helper function for find_existing_css_set().
   * @cg: candidate css_set being tested
   * @old_cg: existing css_set for a task
   * @new_cgrp: cgroup that's being entered by the task
   * @template: desired set of css pointers in css_set (pre-calculated)
   *
   * Returns true if "cg" matches "old_cg" except for the hierarchy
   * which "new_cgrp" belongs to, for which it should match "new_cgrp".
   */
  static bool compare_css_sets(struct css_set *cg,
  			     struct css_set *old_cg,
  			     struct cgroup *new_cgrp,
  			     struct cgroup_subsys_state *template[])
  {
  	struct list_head *l1, *l2;
  
  	if (memcmp(template, cg->subsys, sizeof(cg->subsys))) {
  		/* Not all subsystems matched */
  		return false;
  	}
  
  	/*
  	 * Compare cgroup pointers in order to distinguish between
  	 * different cgroups in heirarchies with no subsystems. We
  	 * could get by with just this check alone (and skip the
  	 * memcmp above) but on most setups the memcmp check will
  	 * avoid the need for this more expensive check on almost all
  	 * candidates.
  	 */
  
  	l1 = &cg->cg_links;
  	l2 = &old_cg->cg_links;
  	while (1) {
  		struct cg_cgroup_link *cgl1, *cgl2;
  		struct cgroup *cg1, *cg2;
  
  		l1 = l1->next;
  		l2 = l2->next;
  		/* See if we reached the end - both lists are equal length. */
  		if (l1 == &cg->cg_links) {
  			BUG_ON(l2 != &old_cg->cg_links);
  			break;
  		} else {
  			BUG_ON(l2 == &old_cg->cg_links);
  		}
  		/* Locate the cgroups associated with these links. */
  		cgl1 = list_entry(l1, struct cg_cgroup_link, cg_link_list);
  		cgl2 = list_entry(l2, struct cg_cgroup_link, cg_link_list);
  		cg1 = cgl1->cgrp;
  		cg2 = cgl2->cgrp;
  		/* Hierarchies should be linked in the same order. */
  		BUG_ON(cg1->root != cg2->root);
  
  		/*
  		 * If this hierarchy is the hierarchy of the cgroup
  		 * that's changing, then we need to check that this
  		 * css_set points to the new cgroup; if it's any other
  		 * hierarchy, then this css_set should point to the
  		 * same cgroup as the old css_set.
  		 */
  		if (cg1->root == new_cgrp->root) {
  			if (cg1 != new_cgrp)
  				return false;
  		} else {
  			if (cg1 != cg2)
  				return false;
  		}
  	}
  	return true;
  }
  
  /*

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

   * css_set is suitable.

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

   * cgrp: the cgroup that we're moving into

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

  static struct css_set *find_existing_css_set(
  	struct css_set *oldcg,

  	struct cgroup *cgrp,

  	struct cgroup_subsys_state *template[])

  {
  	int i;

  	struct cgroupfs_root *root = cgrp->root;

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

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

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

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

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

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

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

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

  		if (!compare_css_sets(cg, oldcg, cgrp, template))
  			continue;
  
  		/* This css_set matches what we need */
  		return cg;

  	}

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

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

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

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

   * success or a negative error
   */

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

  			free_cg_links(tmp);

  			return -ENOMEM;
  		}

  		list_add(&link->cgrp_link_list, tmp);

  	}
  	return 0;
  }

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

  	link->cgrp = cgrp;

  	atomic_inc(&cgrp->count);

  	list_move(&link->cgrp_link_list, &cgrp->css_sets);

  	/*
  	 * Always add links to the tail of the list so that the list
  	 * is sorted by order of hierarchy creation
  	 */
  	list_add_tail(&link->cg_link_list, &cg->cg_links);

  }

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

  static struct css_set *find_css_set(

  	struct css_set *oldcg, struct cgroup *cgrp)

  {
  	struct css_set *res;
  	struct cgroup_subsys_state *template[CGROUP_SUBSYS_COUNT];

  
  	struct list_head tmp_cg_links;

  	struct hlist_head *hhead;

  	struct cg_cgroup_link *link;

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

  	read_lock(&css_set_lock);

  	res = find_existing_css_set(oldcg, cgrp, template);

  	if (res)
  		get_css_set(res);

  	read_unlock(&css_set_lock);

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

  	atomic_set(&res->refcount, 1);

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

  	INIT_HLIST_NODE(&res->hlist);

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

  	list_for_each_entry(link, &oldcg->cg_links, cg_link_list) {
  		struct cgroup *c = link->cgrp;
  		if (c->root == cgrp->root)
  			c = cgrp;
  		link_css_set(&tmp_cg_links, res, c);
  	}

  
  	BUG_ON(!list_empty(&tmp_cg_links));

  	css_set_count++;

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

  	write_unlock(&css_set_lock);
  
  	return res;

  }

  /*

   * Return the cgroup for "task" from the given hierarchy. Must be
   * called with cgroup_mutex held.
   */
  static struct cgroup *task_cgroup_from_root(struct task_struct *task,
  					    struct cgroupfs_root *root)
  {
  	struct css_set *css;
  	struct cgroup *res = NULL;
  
  	BUG_ON(!mutex_is_locked(&cgroup_mutex));
  	read_lock(&css_set_lock);
  	/*
  	 * No need to lock the task - since we hold cgroup_mutex the
  	 * task can't change groups, so the only thing that can happen
  	 * is that it exits and its css is set back to init_css_set.
  	 */
  	css = task->cgroups;
  	if (css == &init_css_set) {
  		res = &root->top_cgroup;
  	} else {
  		struct cg_cgroup_link *link;
  		list_for_each_entry(link, &css->cg_links, cg_link_list) {
  			struct cgroup *c = link->cgrp;
  			if (c->root == root) {
  				res = c;
  				break;
  			}
  		}
  	}
  	read_unlock(&css_set_lock);
  	BUG_ON(!res);
  	return res;
  }
  
  /*

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

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

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

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

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

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

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

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

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

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

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

   * update of a tasks cgroup pointer by cgroup_attach_task()

   */

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

  void cgroup_lock(void)
  {
  	mutex_lock(&cgroup_mutex);
  }

  EXPORT_SYMBOL_GPL(cgroup_lock);

  
  /**
   * cgroup_unlock - release lock on cgroup changes
   *
   * Undo the lock taken in a previous cgroup_lock() call.
   */

  void cgroup_unlock(void)
  {
  	mutex_unlock(&cgroup_mutex);
  }

  EXPORT_SYMBOL_GPL(cgroup_unlock);

  
  /*
   * A couple of forward declarations required, due to cyclic reference loop:
   * cgroup_mkdir -> cgroup_create -> cgroup_populate_dir ->
   * cgroup_add_file -> cgroup_create_file -> cgroup_dir_inode_operations
   * -> cgroup_mkdir.
   */
  
  static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, int mode);

  static struct dentry *cgroup_lookup(struct inode *, struct dentry *, struct nameidata *);

  static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry);

  static int cgroup_populate_dir(struct cgroup *cgrp);

  static const struct inode_operations cgroup_dir_inode_operations;

  static const struct file_operations proc_cgroupstats_operations;

  
  static struct backing_dev_info cgroup_backing_dev_info = {

  	.name		= "cgroup",

  	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK,

  };

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

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

  
  	if (inode) {

  		inode->i_ino = get_next_ino();

  		inode->i_mode = mode;

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

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

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

  static int cgroup_call_pre_destroy(struct cgroup *cgrp)

  {
  	struct cgroup_subsys *ss;

  	int ret = 0;

  	for_each_subsys(cgrp->root, ss)

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

  				break;

  		}

  	return ret;

  }

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

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

  		struct cgroup *cgrp = dentry->d_fsdata;

  		struct cgroup_subsys *ss;

  		BUG_ON(!(cgroup_is_removed(cgrp)));

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

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

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

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

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

  		deactivate_super(cgrp->root->sb);

  		/*
  		 * if we're getting rid of the cgroup, refcount should ensure
  		 * that there are no pidlists left.
  		 */
  		BUG_ON(!list_empty(&cgrp->pidlists));

  		call_rcu(&cgrp->rcu_head, free_cgroup_rcu);

  	}
  	iput(inode);
  }

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

  static void remove_dir(struct dentry *d)
  {
  	struct dentry *parent = dget(d->d_parent);
  
  	d_delete(d);
  	simple_rmdir(parent->d_inode, d);
  	dput(parent);
  }
  
  static void cgroup_clear_directory(struct dentry *dentry)
  {
  	struct list_head *node;
  
  	BUG_ON(!mutex_is_locked(&dentry->d_inode->i_mutex));

  	spin_lock(&dentry->d_lock);

  	node = dentry->d_subdirs.next;
  	while (node != &dentry->d_subdirs) {
  		struct dentry *d = list_entry(node, struct dentry, d_u.d_child);

  
  		spin_lock_nested(&d->d_lock, DENTRY_D_LOCK_NESTED);

  		list_del_init(node);
  		if (d->d_inode) {
  			/* This should never be called on a cgroup
  			 * directory with child cgroups */
  			BUG_ON(d->d_inode->i_mode & S_IFDIR);

  			dget_dlock(d);

  			spin_unlock(&d->d_lock);
  			spin_unlock(&dentry->d_lock);

  			d_delete(d);
  			simple_unlink(dentry->d_inode, d);
  			dput(d);

  			spin_lock(&dentry->d_lock);
  		} else
  			spin_unlock(&d->d_lock);

  		node = dentry->d_subdirs.next;
  	}

  	spin_unlock(&dentry->d_lock);

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

  	struct dentry *parent;

  	cgroup_clear_directory(dentry);

  	parent = dentry->d_parent;
  	spin_lock(&parent->d_lock);

  	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);

  	list_del_init(&dentry->d_u.d_child);

  	spin_unlock(&dentry->d_lock);
  	spin_unlock(&parent->d_lock);

  	remove_dir(dentry);
  }

  /*
   * A queue for waiters to do rmdir() cgroup. A tasks will sleep when
   * cgroup->count == 0 && list_empty(&cgroup->children) && subsys has some
   * reference to css->refcnt. In general, this refcnt is expected to goes down
   * to zero, soon.
   *

   * CGRP_WAIT_ON_RMDIR flag is set under cgroup's inode->i_mutex;

   */
  DECLARE_WAIT_QUEUE_HEAD(cgroup_rmdir_waitq);

  static void cgroup_wakeup_rmdir_waiter(struct cgroup *cgrp)

  {

  	if (unlikely(test_and_clear_bit(CGRP_WAIT_ON_RMDIR, &cgrp->flags)))

  		wake_up_all(&cgroup_rmdir_waitq);
  }

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

  /*

   * Call with cgroup_mutex held. Drops reference counts on modules, including
   * any duplicate ones that parse_cgroupfs_options took. If this function
   * returns an error, no reference counts are touched.

   */

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

  	struct cgroup *cgrp = &root->top_cgroup;

  	int i;

  	BUG_ON(!mutex_is_locked(&cgroup_mutex));

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

  		unsigned long bit = 1UL << i;

  		struct cgroup_subsys *ss = subsys[i];
  		if (!(bit & added_bits))
  			continue;

  		/*
  		 * Nobody should tell us to do a subsys that doesn't exist:
  		 * parse_cgroupfs_options should catch that case and refcounts
  		 * ensure that subsystems won't disappear once selected.
  		 */
  		BUG_ON(ss == NULL);

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

  	if (root->number_of_cgroups > 1)

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

  			BUG_ON(ss == NULL);

  			BUG_ON(cgrp->subsys[i]);

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

  			mutex_lock(&ss->hierarchy_mutex);

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

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

  			ss->root = root;

  			if (ss->bind)

  				ss->bind(ss, cgrp);

  			mutex_unlock(&ss->hierarchy_mutex);

  			/* refcount was already taken, and we're keeping it */

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

  			BUG_ON(ss == NULL);

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

  			mutex_lock(&ss->hierarchy_mutex);

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

  			cgrp->subsys[i] = NULL;

  			subsys[i]->root = &rootnode;

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

  			mutex_unlock(&ss->hierarchy_mutex);

  			/* subsystem is now free - drop reference on module */
  			module_put(ss->module);

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

  			BUG_ON(ss == NULL);

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

  			/*
  			 * a refcount was taken, but we already had one, so
  			 * drop the extra reference.
  			 */
  			module_put(ss->module);
  #ifdef CONFIG_MODULE_UNLOAD
  			BUG_ON(ss->module && !module_refcount(ss->module));
  #endif

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

  			BUG_ON(cgrp->subsys[i]);

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

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

  	if (clone_children(&root->top_cgroup))
  		seq_puts(seq, ",clone_children");

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

  	mutex_unlock(&cgroup_mutex);
  	return 0;
  }
  
  struct cgroup_sb_opts {
  	unsigned long subsys_bits;
  	unsigned long flags;

  	char *release_agent;

  	bool clone_children;

  	char *name;

  	/* User explicitly requested empty subsystem */
  	bool none;

  
  	struct cgroupfs_root *new_root;

  };

  /*
   * Convert a hierarchy specifier into a bitmask of subsystems and flags. Call

   * with cgroup_mutex held to protect the subsys[] array. This function takes
   * refcounts on subsystems to be used, unless it returns error, in which case
   * no refcounts are taken.

   */

  static int parse_cgroupfs_options(char *data, struct cgroup_sb_opts *opts)

  {

  	char *token, *o = data;
  	bool all_ss = false, one_ss = false;

  	unsigned long mask = (unsigned long)-1;

  	int i;
  	bool module_pin_failed = false;

  	BUG_ON(!mutex_is_locked(&cgroup_mutex));

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

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

  
  	while ((token = strsep(&o, ",")) != NULL) {
  		if (!*token)
  			return -EINVAL;

  		if (!strcmp(token, "none")) {

  			/* Explicitly have no subsystems */
  			opts->none = true;

  			continue;
  		}
  		if (!strcmp(token, "all")) {
  			/* Mutually exclusive option 'all' + subsystem name */
  			if (one_ss)
  				return -EINVAL;
  			all_ss = true;
  			continue;
  		}
  		if (!strcmp(token, "noprefix")) {

  			set_bit(ROOT_NOPREFIX, &opts->flags);

  			continue;
  		}
  		if (!strcmp(token, "clone_children")) {

  			opts->clone_children = true;

  			continue;
  		}
  		if (!strncmp(token, "release_agent=", 14)) {

  			/* Specifying two release agents is forbidden */
  			if (opts->release_agent)
  				return -EINVAL;

  			opts->release_agent =

  				kstrndup(token + 14, PATH_MAX - 1, GFP_KERNEL);

  			if (!opts->release_agent)
  				return -ENOMEM;

  			continue;
  		}
  		if (!strncmp(token, "name=", 5)) {

  			const char *name = token + 5;
  			/* Can't specify an empty name */
  			if (!strlen(name))
  				return -EINVAL;
  			/* Must match [\w.-]+ */
  			for (i = 0; i < strlen(name); i++) {
  				char c = name[i];
  				if (isalnum(c))
  					continue;
  				if ((c == '.') || (c == '-') || (c == '_'))
  					continue;
  				return -EINVAL;
  			}
  			/* Specifying two names is forbidden */
  			if (opts->name)
  				return -EINVAL;
  			opts->name = kstrndup(name,

  					      MAX_CGROUP_ROOT_NAMELEN - 1,

  					      GFP_KERNEL);
  			if (!opts->name)
  				return -ENOMEM;

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

  		}
  	}

  	/* Consistency checks */

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

  
  	/* Can't specify "none" and some subsystems */
  	if (opts->subsys_bits && opts->none)
  		return -EINVAL;
  
  	/*
  	 * We either have to specify by name or by subsystems. (So all
  	 * empty hierarchies must have a name).
  	 */

  	if (!opts->subsys_bits && !opts->name)

  		return -EINVAL;

  	/*
  	 * Grab references on all the modules we'll need, so the subsystems
  	 * don't dance around before rebind_subsystems attaches them. This may
  	 * take duplicate reference counts on a subsystem that's already used,
  	 * but rebind_subsystems handles this case.
  	 */
  	for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
  		unsigned long bit = 1UL << i;
  
  		if (!(bit & opts->subsys_bits))
  			continue;
  		if (!try_module_get(subsys[i]->module)) {
  			module_pin_failed = true;
  			break;
  		}
  	}
  	if (module_pin_failed) {
  		/*
  		 * oops, one of the modules was going away. this means that we
  		 * raced with a module_delete call, and to the user this is
  		 * essentially a "subsystem doesn't exist" case.
  		 */
  		for (i--; i >= CGROUP_BUILTIN_SUBSYS_COUNT; i--) {
  			/* drop refcounts only on the ones we took */
  			unsigned long bit = 1UL << i;
  
  			if (!(bit & opts->subsys_bits))
  				continue;
  			module_put(subsys[i]->module);
  		}
  		return -ENOENT;
  	}

  	return 0;
  }

  static void drop_parsed_module_refcounts(unsigned long subsys_bits)
  {
  	int i;
  	for (i = CGROUP_BUILTIN_SUBSYS_COUNT; i < CGROUP_SUBSYS_COUNT; i++) {
  		unsigned long bit = 1UL << i;
  
  		if (!(bit & subsys_bits))
  			continue;
  		module_put(subsys[i]->module);
  	}
  }

  static int cgroup_remount(struct super_block *sb, int *flags, char *data)
  {
  	int ret = 0;
  	struct cgroupfs_root *root = sb->s_fs_info;

  	struct cgroup *cgrp = &root->top_cgroup;

  	struct cgroup_sb_opts opts;

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

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

  	/* Don't allow flags or name to change at remount */
  	if (opts.flags != root->flags ||
  	    (opts.name && strcmp(opts.name, root->name))) {

  		ret = -EINVAL;

  		drop_parsed_module_refcounts(opts.subsys_bits);

  		goto out_unlock;
  	}

  	ret = rebind_subsystems(root, opts.subsys_bits);

  	if (ret) {
  		drop_parsed_module_refcounts(opts.subsys_bits);

  		goto out_unlock;

  	}

  
  	/* (re)populate subsystem files */

  	cgroup_populate_dir(cgrp);

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

   out_unlock:

  	kfree(opts.release_agent);

  	kfree(opts.name);

  	mutex_unlock(&cgroup_mutex);

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

  	return ret;
  }

  static const struct super_operations cgroup_ops = {

  	.statfs = simple_statfs,
  	.drop_inode = generic_delete_inode,
  	.show_options = cgroup_show_options,
  	.remount_fs = cgroup_remount,
  };

  static void init_cgroup_housekeeping(struct cgroup *cgrp)
  {
  	INIT_LIST_HEAD(&cgrp->sibling);
  	INIT_LIST_HEAD(&cgrp->children);
  	INIT_LIST_HEAD(&cgrp->css_sets);
  	INIT_LIST_HEAD(&cgrp->release_list);

  	INIT_LIST_HEAD(&cgrp->pidlists);
  	mutex_init(&cgrp->pidlist_mutex);

  	INIT_LIST_HEAD(&cgrp->event_list);
  	spin_lock_init(&cgrp->event_list_lock);

  }

  static void init_cgroup_root(struct cgroupfs_root *root)
  {

  	struct cgroup *cgrp = &root->top_cgroup;

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

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

  	init_cgroup_housekeeping(cgrp);

  }

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

  static int cgroup_test_super(struct super_block *sb, void *data)
  {

  	struct cgroup_sb_opts *opts = data;

  	struct cgroupfs_root *root = sb->s_fs_info;

  	/* If we asked for a name then it must match */
  	if (opts->name && strcmp(opts->name, root->name))
  		return 0;

  	/*
  	 * If we asked for subsystems (or explicitly for no
  	 * subsystems) then they must match
  	 */
  	if ((opts->subsys_bits || opts->none)
  	    && (opts->subsys_bits != root->subsys_bits))

  		return 0;
  
  	return 1;
  }

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

  	if (!opts->subsys_bits && !opts->none)

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

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

  	init_cgroup_root(root);

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

  	if (opts->clone_children)
  		set_bit(CGRP_CLONE_CHILDREN, &root->top_cgroup.flags);

  	return root;
  }

  static void cgroup_drop_root(struct cgroupfs_root *root)
  {
  	if (!root)
  		return;
  
  	BUG_ON(!root->hierarchy_id);
  	spin_lock(&hierarchy_id_lock);
  	ida_remove(&hierarchy_ida, root->hierarchy_id);
  	spin_unlock(&hierarchy_id_lock);
  	kfree(root);
  }

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

  	struct cgroup_sb_opts *opts = data;
  
  	/* If we don't have a new root, we can't set up a new sb */
  	if (!opts->new_root)
  		return -EINVAL;

  	BUG_ON(!opts->subsys_bits && !opts->none);

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

  	sb->s_fs_info = opts->new_root;
  	opts->new_root->sb = sb;

  
  	sb->s_blocksize = PAGE_CACHE_SIZE;
  	sb->s_blocksize_bits = PAGE_CACHE_SHIFT;
  	sb->s_magic = CGROUP_SUPER_MAGIC;
  	sb->s_op = &cgroup_ops;
  
  	return 0;
  }
  
  static int cgroup_get_rootdir(struct super_block *sb)
  {

  	static const struct dentry_operations cgroup_dops = {
  		.d_iput = cgroup_diput,

  		.d_delete = cgroup_delete,

  	};

  	struct inode *inode =
  		cgroup_new_inode(S_IFDIR | S_IRUGO | S_IXUGO | S_IWUSR, sb);
  	struct dentry *dentry;
  
  	if (!inode)
  		return -ENOMEM;

  	inode->i_fop = &simple_dir_operations;
  	inode->i_op = &cgroup_dir_inode_operations;
  	/* directories start off with i_nlink == 2 (for "." entry) */
  	inc_nlink(inode);
  	dentry = d_alloc_root(inode);
  	if (!dentry) {
  		iput(inode);
  		return -ENOMEM;
  	}
  	sb->s_root = dentry;

  	/* for everything else we want ->d_op set */
  	sb->s_d_op = &cgroup_dops;

  	return 0;
  }

  static struct dentry *cgroup_mount(struct file_system_type *fs_type,

  			 int flags, const char *unused_dev_name,

  			 void *data)

  {
  	struct cgroup_sb_opts opts;

  	struct cgroupfs_root *root;

  	int ret = 0;
  	struct super_block *sb;

  	struct cgroupfs_root *new_root;

  
  	/* First find the desired set of subsystems */

  	mutex_lock(&cgroup_mutex);

  	ret = parse_cgroupfs_options(data, &opts);

  	mutex_unlock(&cgroup_mutex);

  	if (ret)
  		goto out_err;

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

  		goto drop_modules;

  	}

  	opts.new_root = new_root;

  	/* Locate an existing or new sb for this hierarchy */
  	sb = sget(fs_type, cgroup_test_super, cgroup_set_super, &opts);

  	if (IS_ERR(sb)) {

  		ret = PTR_ERR(sb);

  		cgroup_drop_root(opts.new_root);

  		goto drop_modules;

  	}

  	root = sb->s_fs_info;
  	BUG_ON(!root);
  	if (root == opts.new_root) {
  		/* We used the new root structure, so this is a new hierarchy */
  		struct list_head tmp_cg_links;

  		struct cgroup *root_cgrp = &root->top_cgroup;

  		struct inode *inode;

  		struct cgroupfs_root *existing_root;

  		int i;

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

  		inode = sb->s_root->d_inode;

  		mutex_lock(&inode->i_mutex);

  		mutex_lock(&cgroup_mutex);

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

  		/*
  		 * We're accessing css_set_count without locking
  		 * css_set_lock here, but that's OK - it can only be
  		 * increased by someone holding cgroup_lock, and
  		 * that's us. The worst that can happen is that we
  		 * have some link structures left over
  		 */
  		ret = allocate_cg_links(css_set_count, &tmp_cg_links);
  		if (ret) {
  			mutex_unlock(&cgroup_mutex);
  			mutex_unlock(&inode->i_mutex);
  			goto drop_new_super;
  		}

  		ret = rebind_subsystems(root, root->subsys_bits);
  		if (ret == -EBUSY) {
  			mutex_unlock(&cgroup_mutex);

  			mutex_unlock(&inode->i_mutex);

  			free_cg_links(&tmp_cg_links);
  			goto drop_new_super;

  		}

  		/*
  		 * There must be no failure case after here, since rebinding
  		 * takes care of subsystems' refcounts, which are explicitly
  		 * dropped in the failure exit path.
  		 */

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

  		root_count++;

  		sb->s_root->d_fsdata = root_cgrp;

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

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

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

  			struct css_set *cg;

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

  		}

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

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

  		BUG_ON(root->number_of_cgroups != 1);

  		cgroup_populate_dir(root_cgrp);

  		mutex_unlock(&cgroup_mutex);

  		mutex_unlock(&inode->i_mutex);

  	} else {
  		/*
  		 * We re-used an existing hierarchy - the new root (if
  		 * any) is not needed
  		 */

  		cgroup_drop_root(opts.new_root);

  		/* no subsys rebinding, so refcounts don't change */
  		drop_parsed_module_refcounts(opts.subsys_bits);

  	}

  	kfree(opts.release_agent);
  	kfree(opts.name);

  	return dget(sb->s_root);

  
   drop_new_super:

  	deactivate_locked_super(sb);

   drop_modules:
  	drop_parsed_module_refcounts(opts.subsys_bits);

   out_err:
  	kfree(opts.release_agent);
  	kfree(opts.name);

  	return ERR_PTR(ret);

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

  	struct cgroup *cgrp = &root->top_cgroup;

  	int ret;

  	struct cg_cgroup_link *link;
  	struct cg_cgroup_link *saved_link;

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

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

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

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

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

  		list_del(&link->cg_link_list);

  		list_del(&link->cgrp_link_list);

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

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

  	mutex_unlock(&cgroup_mutex);

  	kill_litter_super(sb);

  	cgroup_drop_root(root);

  }
  
  static struct file_system_type cgroup_fs_type = {
  	.name = "cgroup",

  	.mount = cgroup_mount,

  	.kill_sb = cgroup_kill_sb,
  };

  static struct kobject *cgroup_kobj;

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

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

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

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

   */

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

  {
  	char *start;

  	struct dentry *dentry = rcu_dereference_check(cgrp->dentry,
  						      rcu_read_lock_held() ||
  						      cgroup_lock_is_held());

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

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

  		int len = dentry->d_name.len;

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

  		memcpy(start, dentry->d_name.name, len);

  		cgrp = cgrp->parent;
  		if (!cgrp)

  			break;

  
  		dentry = rcu_dereference_check(cgrp->dentry,
  					       rcu_read_lock_held() ||
  					       cgroup_lock_is_held());

  		if (!cgrp->parent)

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

  EXPORT_SYMBOL_GPL(cgroup_path);

  /**
   * cgroup_attach_task - attach task 'tsk' to cgroup 'cgrp'
   * @cgrp: the cgroup the task is attaching to
   * @tsk: the task to be attached

   *

   * Call holding cgroup_mutex. May take task_lock of
   * the task 'tsk' during call.

   */

  int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)

  {
  	int retval = 0;

  	struct cgroup_subsys *ss, *failed_ss = NULL;

  	struct cgroup *oldcgrp;

  	struct css_set *cg;

  	struct css_set *newcg;

  	struct cgroupfs_root *root = cgrp->root;

  
  	/* Nothing to do if the task is already in that cgroup */

  	oldcgrp = task_cgroup_from_root(tsk, root);

  	if (cgrp == oldcgrp)

  		return 0;
  
  	for_each_subsys(root, ss) {
  		if (ss->can_attach) {

  			retval = ss->can_attach(ss, cgrp, tsk, false);

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

  		}
  	}

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

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

  	newcg = find_css_set(cg, cgrp);

  	put_css_set(cg);

  	if (!newcg) {
  		retval = -ENOMEM;
  		goto out;
  	}

  	task_lock(tsk);
  	if (tsk->flags & PF_EXITING) {
  		task_unlock(tsk);

  		put_css_set(newcg);

  		retval = -ESRCH;
  		goto out;

  	}

  	rcu_assign_pointer(tsk->cgroups, newcg);

  	task_unlock(tsk);

  	/* Update the css_set linked lists if we're using them */
  	write_lock(&css_set_lock);
  	if (!list_empty(&tsk->cg_list)) {
  		list_del(&tsk->cg_list);
  		list_add(&tsk->cg_list, &newcg->tasks);
  	}
  	write_unlock(&css_set_lock);

  	for_each_subsys(root, ss) {

  		if (ss->attach)

  			ss->attach(ss, cgrp, oldcgrp, tsk, false);

  	}

  	set_bit(CGRP_RELEASABLE, &oldcgrp->flags);

  	synchronize_rcu();

  	put_css_set(cg);

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

  	cgroup_wakeup_rmdir_waiter(cgrp);

  out:
  	if (retval) {
  		for_each_subsys(root, ss) {
  			if (ss == failed_ss)
  				/*
  				 * This subsystem was the one that failed the
  				 * can_attach() check earlier, so we don't need
  				 * to call cancel_attach() against it or any
  				 * remaining subsystems.
  				 */
  				break;
  			if (ss->cancel_attach)
  				ss->cancel_attach(ss, cgrp, tsk, false);
  		}
  	}
  	return retval;

  }

  /**

   * cgroup_attach_task_all - attach task 'tsk' to all cgroups of task 'from'
   * @from: attach to all cgroups of a given task

   * @tsk: the task to be attached
   */

  int cgroup_attach_task_all(struct task_struct *from, struct task_struct *tsk)

  {
  	struct cgroupfs_root *root;

  	int retval = 0;
  
  	cgroup_lock();
  	for_each_active_root(root) {

  		struct cgroup *from_cg = task_cgroup_from_root(from, root);
  
  		retval = cgroup_attach_task(from_cg, tsk);

  		if (retval)
  			break;
  	}
  	cgroup_unlock();
  
  	return retval;
  }

  EXPORT_SYMBOL_GPL(cgroup_attach_task_all);

  /*

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

   */

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

  {

  	struct task_struct *tsk;

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

  	int ret;

  	if (pid) {
  		rcu_read_lock();

  		tsk = find_task_by_vpid(pid);

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

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

  			return -EACCES;
  		}

  		get_task_struct(tsk);
  		rcu_read_unlock();

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

  	ret = cgroup_attach_task(cgrp, tsk);

  	put_task_struct(tsk);
  	return ret;
  }

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

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

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

   */

  bool cgroup_lock_live_group(struct cgroup *cgrp)

  {
  	mutex_lock(&cgroup_mutex);
  	if (cgroup_is_removed(cgrp)) {
  		mutex_unlock(&cgroup_mutex);
  		return false;
  	}
  	return true;
  }

  EXPORT_SYMBOL_GPL(cgroup_lock_live_group);

  
  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 (strlen(buffer) >= PATH_MAX)
  		return -EINVAL;

  	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 */

  	if (cft->write_u64) {

  		u64 val = simple_strtoull(strstrip(buffer), &end, 0);

  		if (*end)
  			return -EINVAL;
  		retval = cft->write_u64(cgrp, cft, val);
  	} else {

  		s64 val = simple_strtoll(strstrip(buffer), &end, 0);

  		if (*end)
  			return -EINVAL;
  		retval = cft->write_s64(cgrp, cft, val);
  	}

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

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

  	char local_buffer[CGROUP_LOCAL_BUFFER_SIZE];

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

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

  
  	buffer[nbytes] = 0;     /* nul-terminate */

  	retval = cft->write_string(cgrp, cft, strstrip(buffer));

  	if (!retval)
  		retval = nbytes;

  out:

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

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

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

  	if (cgroup_is_removed(cgrp))

  		return -ENODEV;

  	if (cft->write)

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

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

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

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

  	return -EINVAL;

  }

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

  {

  	char tmp[CGROUP_LOCAL_BUFFER_SIZE];

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

  	int len = sprintf(tmp, "%llu
  ", (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);
  }