kernel/cgroup_freezer.c - arm/linux-arm64-legacy - Git at Google

 /*
  * cgroup_freezer.c -  control group freezer subsystem
  *
  * Copyright IBM Corporation, 2007
  *
  * Author : Cedric Le Goater <clg@fr.ibm.com>
  *
  * This program is free software; you can redistribute it and/or modify it
  * under the terms of version 2.1 of the GNU Lesser General Public License
  * as published by the Free Software Foundation.
  *
  * This program is distributed in the hope that it would be useful, but
  * WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  */

 #include <linux/export.h>
 #include <linux/slab.h>
 #include <linux/cgroup.h>
 #include <linux/fs.h>
 #include <linux/uaccess.h>
 #include <linux/freezer.h>
 #include <linux/seq_file.h>

 /*
  * A cgroup is freezing if any FREEZING flags are set.  FREEZING_SELF is
  * set if "FROZEN" is written to freezer.state cgroupfs file, and cleared
  * for "THAWED".  FREEZING_PARENT is set if the parent freezer is FREEZING
  * for whatever reason.  IOW, a cgroup has FREEZING_PARENT set if one of
  * its ancestors has FREEZING_SELF set.
  */
 enum freezer_state_flags {
 	CGROUP_FREEZER_ONLINE	= (1 << 0), /* freezer is fully online */
 	CGROUP_FREEZING_SELF	= (1 << 1), /* this freezer is freezing */
 	CGROUP_FREEZING_PARENT	= (1 << 2), /* the parent freezer is freezing */
 	CGROUP_FROZEN		= (1 << 3), /* this and its descendants frozen */

 	/* mask for all FREEZING flags */
 	CGROUP_FREEZING		= CGROUP_FREEZING_SELF | CGROUP_FREEZING_PARENT,
 };

 struct freezer {
 	struct cgroup_subsys_state	css;
 	unsigned int			state;
 	spinlock_t			lock;
 };

 static inline struct freezer *cgroup_freezer(struct cgroup *cgroup)
 {
 	return container_of(cgroup_subsys_state(cgroup, freezer_subsys_id),
 			    struct freezer, css);
 }

 static inline struct freezer *task_freezer(struct task_struct *task)
 {
 	return container_of(task_subsys_state(task, freezer_subsys_id),
 			    struct freezer, css);
 }

 static struct freezer *parent_freezer(struct freezer *freezer)
 {
 	struct cgroup *pcg = freezer->css.cgroup->parent;

 	if (pcg)
 		return cgroup_freezer(pcg);
 	return NULL;
 }

 bool cgroup_freezing(struct task_struct *task)
 {
 	bool ret;

 	rcu_read_lock();
 	ret = task_freezer(task)->state & CGROUP_FREEZING;
 	rcu_read_unlock();

 	return ret;
 }

 /*
  * cgroups_write_string() limits the size of freezer state strings to
  * CGROUP_LOCAL_BUFFER_SIZE
  */
 static const char *freezer_state_strs(unsigned int state)
 {
 	if (state & CGROUP_FROZEN)
 		return "FROZEN";
 	if (state & CGROUP_FREEZING)
 		return "FREEZING";
 	return "THAWED";
 };

 struct cgroup_subsys freezer_subsys;

 static struct cgroup_subsys_state *freezer_css_alloc(struct cgroup *cgroup)
 {
 	struct freezer *freezer;

 	freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
 	if (!freezer)
 		return ERR_PTR(-ENOMEM);

 	spin_lock_init(&freezer->lock);
 	return &freezer->css;
 }

 /**
  * freezer_css_online - commit creation of a freezer cgroup
  * @cgroup: cgroup being created
  *
  * We're committing to creation of @cgroup.  Mark it online and inherit
  * parent's freezing state while holding both parent's and our
  * freezer->lock.
  */
 static int freezer_css_online(struct cgroup *cgroup)
 {
 	struct freezer *freezer = cgroup_freezer(cgroup);
 	struct freezer *parent = parent_freezer(freezer);

 	/*
 	 * The following double locking and freezing state inheritance
 	 * guarantee that @cgroup can never escape ancestors' freezing
 	 * states.  See cgroup_for_each_descendant_pre() for details.
 	 */
 	if (parent)
 		spin_lock_irq(&parent->lock);
 	spin_lock_nested(&freezer->lock, SINGLE_DEPTH_NESTING);

 	freezer->state |= CGROUP_FREEZER_ONLINE;

 	if (parent && (parent->state & CGROUP_FREEZING)) {
 		freezer->state |= CGROUP_FREEZING_PARENT | CGROUP_FROZEN;
 		atomic_inc(&system_freezing_cnt);
 	}

 	spin_unlock(&freezer->lock);
 	if (parent)
 		spin_unlock_irq(&parent->lock);

 	return 0;
 }

 /**
  * freezer_css_offline - initiate destruction of @cgroup
  * @cgroup: cgroup being destroyed
  *
  * @cgroup is going away.  Mark it dead and decrement system_freezing_count
  * if it was holding one.
  */
 static void freezer_css_offline(struct cgroup *cgroup)
 {
 	struct freezer *freezer = cgroup_freezer(cgroup);

 	spin_lock_irq(&freezer->lock);

 	if (freezer->state & CGROUP_FREEZING)
 		atomic_dec(&system_freezing_cnt);

 	freezer->state = 0;

 	spin_unlock_irq(&freezer->lock);
 }

 static void freezer_css_free(struct cgroup *cgroup)
 {
 	kfree(cgroup_freezer(cgroup));
 }

 /*
  * Tasks can be migrated into a different freezer anytime regardless of its
  * current state.  freezer_attach() is responsible for making new tasks
  * conform to the current state.
  *
  * Freezer state changes and task migration are synchronized via
  * @freezer->lock.  freezer_attach() makes the new tasks conform to the
  * current state and all following state changes can see the new tasks.
  */
 static void freezer_attach(struct cgroup *new_cgrp, struct cgroup_taskset *tset)
 {
 	struct freezer *freezer = cgroup_freezer(new_cgrp);
 	struct task_struct *task;
 	bool clear_frozen = false;

 	spin_lock_irq(&freezer->lock);

 	/*
 	 * Make the new tasks conform to the current state of @new_cgrp.
 	 * For simplicity, when migrating any task to a FROZEN cgroup, we
 	 * revert it to FREEZING and let update_if_frozen() determine the
 	 * correct state later.
 	 *
 	 * Tasks in @tset are on @new_cgrp but may not conform to its
 	 * current state before executing the following - !frozen tasks may
 	 * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
 	 */
 	cgroup_taskset_for_each(task, new_cgrp, tset) {
 		if (!(freezer->state & CGROUP_FREEZING)) {
 			__thaw_task(task);
 		} else {
 			freeze_task(task);
 			freezer->state &= ~CGROUP_FROZEN;
 			clear_frozen = true;
 		}
 	}

 	spin_unlock_irq(&freezer->lock);

 	/*
 	 * Propagate FROZEN clearing upwards.  We may race with
 	 * update_if_frozen(), but as long as both work bottom-up, either
 	 * update_if_frozen() sees child's FROZEN cleared or we clear the
 	 * parent's FROZEN later.  No parent w/ !FROZEN children can be
 	 * left FROZEN.
 	 */
 	while (clear_frozen && (freezer = parent_freezer(freezer))) {
 		spin_lock_irq(&freezer->lock);
 		freezer->state &= ~CGROUP_FROZEN;
 		clear_frozen = freezer->state & CGROUP_FREEZING;
 		spin_unlock_irq(&freezer->lock);
 	}
 }

 static void freezer_fork(struct task_struct *task)
 {
 	struct freezer *freezer;

 	rcu_read_lock();
 	freezer = task_freezer(task);

 	/*
 	 * The root cgroup is non-freezable, so we can skip the
 	 * following check.
 	 */
 	if (!freezer->css.cgroup->parent)
 		goto out;

 	spin_lock_irq(&freezer->lock);
 	if (freezer->state & CGROUP_FREEZING)
 		freeze_task(task);
 	spin_unlock_irq(&freezer->lock);
 out:
 	rcu_read_unlock();
 }

 /**
  * update_if_frozen - update whether a cgroup finished freezing
  * @cgroup: cgroup of interest
  *
  * Once FREEZING is initiated, transition to FROZEN is lazily updated by
  * calling this function.  If the current state is FREEZING but not FROZEN,
  * this function checks whether all tasks of this cgroup and the descendant
  * cgroups finished freezing and, if so, sets FROZEN.
  *
  * The caller is responsible for grabbing RCU read lock and calling
  * update_if_frozen() on all descendants prior to invoking this function.
  *
  * Task states and freezer state might disagree while tasks are being
  * migrated into or out of @cgroup, so we can't verify task states against
  * @freezer state here.  See freezer_attach() for details.
  */
 static void update_if_frozen(struct cgroup *cgroup)
 {
 	struct freezer *freezer = cgroup_freezer(cgroup);
 	struct cgroup *pos;
 	struct cgroup_iter it;
 	struct task_struct *task;

 	WARN_ON_ONCE(!rcu_read_lock_held());

 	spin_lock_irq(&freezer->lock);

 	if (!(freezer->state & CGROUP_FREEZING) ||
 	    (freezer->state & CGROUP_FROZEN))
 		goto out_unlock;

 	/* are all (live) children frozen? */
 	cgroup_for_each_child(pos, cgroup) {
 		struct freezer *child = cgroup_freezer(pos);

 		if ((child->state & CGROUP_FREEZER_ONLINE) &&
 		    !(child->state & CGROUP_FROZEN))
 			goto out_unlock;
 	}

 	/* are all tasks frozen? */
 	cgroup_iter_start(cgroup, &it);

 	while ((task = cgroup_iter_next(cgroup, &it))) {
 		if (freezing(task)) {
 			/*
 			 * freezer_should_skip() indicates that the task
 			 * should be skipped when determining freezing
 			 * completion.  Consider it frozen in addition to
 			 * the usual frozen condition.
 			 */
 			if (!frozen(task) && !freezer_should_skip(task))
 				goto out_iter_end;
 		}
 	}

 	freezer->state |= CGROUP_FROZEN;
 out_iter_end:
 	cgroup_iter_end(cgroup, &it);
 out_unlock:
 	spin_unlock_irq(&freezer->lock);
 }

 static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
 			struct seq_file *m)
 {
 	struct cgroup *pos;

 	rcu_read_lock();

 	/* update states bottom-up */
 	cgroup_for_each_descendant_post(pos, cgroup)
 		update_if_frozen(pos);
 	update_if_frozen(cgroup);

 	rcu_read_unlock();

 	seq_puts(m, freezer_state_strs(cgroup_freezer(cgroup)->state));
 	seq_putc(m, '\n');
 	return 0;
 }

 static void freeze_cgroup(struct freezer *freezer)
 {
 	struct cgroup *cgroup = freezer->css.cgroup;
 	struct cgroup_iter it;
 	struct task_struct *task;

 	cgroup_iter_start(cgroup, &it);
 	while ((task = cgroup_iter_next(cgroup, &it)))
 		freeze_task(task);
 	cgroup_iter_end(cgroup, &it);
 }

 static void unfreeze_cgroup(struct freezer *freezer)
 {
 	struct cgroup *cgroup = freezer->css.cgroup;
 	struct cgroup_iter it;
 	struct task_struct *task;

 	cgroup_iter_start(cgroup, &it);
 	while ((task = cgroup_iter_next(cgroup, &it)))
 		__thaw_task(task);
 	cgroup_iter_end(cgroup, &it);
 }

 /**
  * freezer_apply_state - apply state change to a single cgroup_freezer
  * @freezer: freezer to apply state change to
  * @freeze: whether to freeze or unfreeze
  * @state: CGROUP_FREEZING_* flag to set or clear
  *
  * Set or clear @state on @cgroup according to @freeze, and perform
  * freezing or thawing as necessary.
  */
 static void freezer_apply_state(struct freezer *freezer, bool freeze,
 				unsigned int state)
 {
 	/* also synchronizes against task migration, see freezer_attach() */
 	lockdep_assert_held(&freezer->lock);

 	if (!(freezer->state & CGROUP_FREEZER_ONLINE))
 		return;

 	if (freeze) {
 		if (!(freezer->state & CGROUP_FREEZING))
 			atomic_inc(&system_freezing_cnt);
 		freezer->state |= state;
 		freeze_cgroup(freezer);
 	} else {
 		bool was_freezing = freezer->state & CGROUP_FREEZING;

 		freezer->state &= ~state;

 		if (!(freezer->state & CGROUP_FREEZING)) {
 			if (was_freezing)
 				atomic_dec(&system_freezing_cnt);
 			freezer->state &= ~CGROUP_FROZEN;
 			unfreeze_cgroup(freezer);
 		}
 	}
 }

 /**
  * freezer_change_state - change the freezing state of a cgroup_freezer
  * @freezer: freezer of interest
  * @freeze: whether to freeze or thaw
  *
  * Freeze or thaw @freezer according to @freeze.  The operations are
  * recursive - all descendants of @freezer will be affected.
  */
 static void freezer_change_state(struct freezer *freezer, bool freeze)
 {
 	struct cgroup *pos;

 	/* update @freezer */
 	spin_lock_irq(&freezer->lock);
 	freezer_apply_state(freezer, freeze, CGROUP_FREEZING_SELF);
 	spin_unlock_irq(&freezer->lock);

 	/*
 	 * Update all its descendants in pre-order traversal.  Each
 	 * descendant will try to inherit its parent's FREEZING state as
 	 * CGROUP_FREEZING_PARENT.
 	 */
 	rcu_read_lock();
 	cgroup_for_each_descendant_pre(pos, freezer->css.cgroup) {
 		struct freezer *pos_f = cgroup_freezer(pos);
 		struct freezer *parent = parent_freezer(pos_f);

 		/*
 		 * Our update to @parent->state is already visible which is
 		 * all we need.  No need to lock @parent.  For more info on
 		 * synchronization, see freezer_post_create().
 		 */
 		spin_lock_irq(&pos_f->lock);
 		freezer_apply_state(pos_f, parent->state & CGROUP_FREEZING,
 				    CGROUP_FREEZING_PARENT);
 		spin_unlock_irq(&pos_f->lock);
 	}
 	rcu_read_unlock();
 }

 static int freezer_write(struct cgroup *cgroup, struct cftype *cft,
 			 const char *buffer)
 {
 	bool freeze;

 	if (strcmp(buffer, freezer_state_strs(0)) == 0)
 		freeze = false;
 	else if (strcmp(buffer, freezer_state_strs(CGROUP_FROZEN)) == 0)
 		freeze = true;
 	else
 		return -EINVAL;

 	freezer_change_state(cgroup_freezer(cgroup), freeze);
 	return 0;
 }

 static u64 freezer_self_freezing_read(struct cgroup *cgroup, struct cftype *cft)
 {
 	struct freezer *freezer = cgroup_freezer(cgroup);

 	return (bool)(freezer->state & CGROUP_FREEZING_SELF);
 }

 static u64 freezer_parent_freezing_read(struct cgroup *cgroup, struct cftype *cft)
 {
 	struct freezer *freezer = cgroup_freezer(cgroup);

 	return (bool)(freezer->state & CGROUP_FREEZING_PARENT);
 }

 static struct cftype files[] = {
 	{
 		.name = "state",
 		.flags = CFTYPE_NOT_ON_ROOT,
 		.read_seq_string = freezer_read,
 		.write_string = freezer_write,
 	},
 	{
 		.name = "self_freezing",
 		.flags = CFTYPE_NOT_ON_ROOT,
 		.read_u64 = freezer_self_freezing_read,
 	},
 	{
 		.name = "parent_freezing",
 		.flags = CFTYPE_NOT_ON_ROOT,
 		.read_u64 = freezer_parent_freezing_read,
 	},
 	{ }	/* terminate */
 };

 struct cgroup_subsys freezer_subsys = {
 	.name		= "freezer",
 	.css_alloc	= freezer_css_alloc,
 	.css_online	= freezer_css_online,
 	.css_offline	= freezer_css_offline,
 	.css_free	= freezer_css_free,
 	.subsys_id	= freezer_subsys_id,
 	.attach		= freezer_attach,
 	.fork		= freezer_fork,
 	.base_cftypes	= files,
 };
	/*
	* cgroup_freezer.c - control group freezer subsystem
	*
	* Copyright IBM Corporation, 2007
	*
	* Author : Cedric Le Goater <clg@fr.ibm.com>
	*
	* This program is free software; you can redistribute it and/or modify it
	* under the terms of version 2.1 of the GNU Lesser General Public License
	* as published by the Free Software Foundation.
	*
	* This program is distributed in the hope that it would be useful, but
	* WITHOUT ANY WARRANTY; without even the implied warranty of
	* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
	*/

	#include <linux/export.h>
	#include <linux/slab.h>
	#include <linux/cgroup.h>
	#include <linux/fs.h>
	#include <linux/uaccess.h>
	#include <linux/freezer.h>
	#include <linux/seq_file.h>

	/*
	* A cgroup is freezing if any FREEZING flags are set. FREEZING_SELF is
	* set if "FROZEN" is written to freezer.state cgroupfs file, and cleared
	* for "THAWED". FREEZING_PARENT is set if the parent freezer is FREEZING
	* for whatever reason. IOW, a cgroup has FREEZING_PARENT set if one of
	* its ancestors has FREEZING_SELF set.
	*/
	enum freezer_state_flags {
	CGROUP_FREEZER_ONLINE = (1 << 0), /* freezer is fully online */
	CGROUP_FREEZING_SELF = (1 << 1), /* this freezer is freezing */
	CGROUP_FREEZING_PARENT = (1 << 2), /* the parent freezer is freezing */
	CGROUP_FROZEN = (1 << 3), /* this and its descendants frozen */

	/* mask for all FREEZING flags */
	CGROUP_FREEZING = CGROUP_FREEZING_SELF \| CGROUP_FREEZING_PARENT,
	};

	struct freezer {
	struct cgroup_subsys_state css;
	unsigned int state;
	spinlock_t lock;
	};

	static inline struct freezer cgroup_freezer(struct cgroup cgroup)
	{
	return container_of(cgroup_subsys_state(cgroup, freezer_subsys_id),
	struct freezer, css);
	}

	static inline struct freezer task_freezer(struct task_struct task)
	{
	return container_of(task_subsys_state(task, freezer_subsys_id),
	struct freezer, css);
	}

	static struct freezer parent_freezer(struct freezer freezer)
	{
	struct cgroup *pcg = freezer->css.cgroup->parent;

	if (pcg)
	return cgroup_freezer(pcg);
	return NULL;
	}

	bool cgroup_freezing(struct task_struct *task)
	{
	bool ret;

	rcu_read_lock();
	ret = task_freezer(task)->state & CGROUP_FREEZING;
	rcu_read_unlock();

	return ret;
	}

	/*
	* cgroups_write_string() limits the size of freezer state strings to
	* CGROUP_LOCAL_BUFFER_SIZE
	*/
	static const char *freezer_state_strs(unsigned int state)
	{
	if (state & CGROUP_FROZEN)
	return "FROZEN";
	if (state & CGROUP_FREEZING)
	return "FREEZING";
	return "THAWED";
	};

	struct cgroup_subsys freezer_subsys;

	static struct cgroup_subsys_state freezer_css_alloc(struct cgroup cgroup)
	{
	struct freezer *freezer;

	freezer = kzalloc(sizeof(struct freezer), GFP_KERNEL);
	if (!freezer)
	return ERR_PTR(-ENOMEM);

	spin_lock_init(&freezer->lock);
	return &freezer->css;
	}

	/**
	* freezer_css_online - commit creation of a freezer cgroup
	* @cgroup: cgroup being created
	*
	* We're committing to creation of @cgroup. Mark it online and inherit
	* parent's freezing state while holding both parent's and our
	* freezer->lock.
	*/
	static int freezer_css_online(struct cgroup *cgroup)
	{
	struct freezer *freezer = cgroup_freezer(cgroup);
	struct freezer *parent = parent_freezer(freezer);

	/*
	* The following double locking and freezing state inheritance
	* guarantee that @cgroup can never escape ancestors' freezing
	* states. See cgroup_for_each_descendant_pre() for details.
	*/
	if (parent)
	spin_lock_irq(&parent->lock);
	spin_lock_nested(&freezer->lock, SINGLE_DEPTH_NESTING);

	freezer->state \|= CGROUP_FREEZER_ONLINE;

	if (parent && (parent->state & CGROUP_FREEZING)) {
	freezer->state \|= CGROUP_FREEZING_PARENT \| CGROUP_FROZEN;
	atomic_inc(&system_freezing_cnt);
	}

	spin_unlock(&freezer->lock);
	if (parent)
	spin_unlock_irq(&parent->lock);

	return 0;
	}

	/**
	* freezer_css_offline - initiate destruction of @cgroup
	* @cgroup: cgroup being destroyed
	*
	* @cgroup is going away. Mark it dead and decrement system_freezing_count
	* if it was holding one.
	*/
	static void freezer_css_offline(struct cgroup *cgroup)
	{
	struct freezer *freezer = cgroup_freezer(cgroup);

	spin_lock_irq(&freezer->lock);

	if (freezer->state & CGROUP_FREEZING)
	atomic_dec(&system_freezing_cnt);

	freezer->state = 0;

	spin_unlock_irq(&freezer->lock);
	}

	static void freezer_css_free(struct cgroup *cgroup)
	{
	kfree(cgroup_freezer(cgroup));
	}

	/*
	* Tasks can be migrated into a different freezer anytime regardless of its
	* current state. freezer_attach() is responsible for making new tasks
	* conform to the current state.
	*
	* Freezer state changes and task migration are synchronized via
	* @freezer->lock. freezer_attach() makes the new tasks conform to the
	* current state and all following state changes can see the new tasks.
	*/
	static void freezer_attach(struct cgroup new_cgrp, struct cgroup_taskset tset)
	{
	struct freezer *freezer = cgroup_freezer(new_cgrp);
	struct task_struct *task;
	bool clear_frozen = false;

	spin_lock_irq(&freezer->lock);

	/*
	* Make the new tasks conform to the current state of @new_cgrp.
	* For simplicity, when migrating any task to a FROZEN cgroup, we
	* revert it to FREEZING and let update_if_frozen() determine the
	* correct state later.
	*
	* Tasks in @tset are on @new_cgrp but may not conform to its
	* current state before executing the following - !frozen tasks may
	* be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
	*/
	cgroup_taskset_for_each(task, new_cgrp, tset) {
	if (!(freezer->state & CGROUP_FREEZING)) {
	__thaw_task(task);
	} else {
	freeze_task(task);
	freezer->state &= ~CGROUP_FROZEN;
	clear_frozen = true;
	}
	}

	spin_unlock_irq(&freezer->lock);

	/*
	* Propagate FROZEN clearing upwards. We may race with
	* update_if_frozen(), but as long as both work bottom-up, either
	* update_if_frozen() sees child's FROZEN cleared or we clear the
	* parent's FROZEN later. No parent w/ !FROZEN children can be
	* left FROZEN.
	*/
	while (clear_frozen && (freezer = parent_freezer(freezer))) {
	spin_lock_irq(&freezer->lock);
	freezer->state &= ~CGROUP_FROZEN;
	clear_frozen = freezer->state & CGROUP_FREEZING;
	spin_unlock_irq(&freezer->lock);
	}
	}

	static void freezer_fork(struct task_struct *task)
	{
	struct freezer *freezer;

	rcu_read_lock();
	freezer = task_freezer(task);

	/*
	* The root cgroup is non-freezable, so we can skip the
	* following check.
	*/
	if (!freezer->css.cgroup->parent)
	goto out;

	spin_lock_irq(&freezer->lock);
	if (freezer->state & CGROUP_FREEZING)
	freeze_task(task);
	spin_unlock_irq(&freezer->lock);
	out:
	rcu_read_unlock();
	}

	/**
	* update_if_frozen - update whether a cgroup finished freezing
	* @cgroup: cgroup of interest
	*
	* Once FREEZING is initiated, transition to FROZEN is lazily updated by
	* calling this function. If the current state is FREEZING but not FROZEN,
	* this function checks whether all tasks of this cgroup and the descendant
	* cgroups finished freezing and, if so, sets FROZEN.
	*
	* The caller is responsible for grabbing RCU read lock and calling
	* update_if_frozen() on all descendants prior to invoking this function.
	*
	* Task states and freezer state might disagree while tasks are being
	* migrated into or out of @cgroup, so we can't verify task states against
	* @freezer state here. See freezer_attach() for details.
	*/
	static void update_if_frozen(struct cgroup *cgroup)
	{
	struct freezer *freezer = cgroup_freezer(cgroup);
	struct cgroup *pos;
	struct cgroup_iter it;
	struct task_struct *task;

	WARN_ON_ONCE(!rcu_read_lock_held());

	spin_lock_irq(&freezer->lock);

	if (!(freezer->state & CGROUP_FREEZING) \|\|
	(freezer->state & CGROUP_FROZEN))
	goto out_unlock;

	/* are all (live) children frozen? */
	cgroup_for_each_child(pos, cgroup) {
	struct freezer *child = cgroup_freezer(pos);

	if ((child->state & CGROUP_FREEZER_ONLINE) &&
	!(child->state & CGROUP_FROZEN))
	goto out_unlock;
	}

	/* are all tasks frozen? */
	cgroup_iter_start(cgroup, &it);

	while ((task = cgroup_iter_next(cgroup, &it))) {
	if (freezing(task)) {
	/*
	* freezer_should_skip() indicates that the task
	* should be skipped when determining freezing
	* completion. Consider it frozen in addition to
	* the usual frozen condition.
	*/
	if (!frozen(task) && !freezer_should_skip(task))
	goto out_iter_end;
	}
	}

	freezer->state \|= CGROUP_FROZEN;
	out_iter_end:
	cgroup_iter_end(cgroup, &it);
	out_unlock:
	spin_unlock_irq(&freezer->lock);
	}

	static int freezer_read(struct cgroup cgroup, struct cftype cft,
	struct seq_file *m)
	{
	struct cgroup *pos;

	rcu_read_lock();

	/* update states bottom-up */
	cgroup_for_each_descendant_post(pos, cgroup)
	update_if_frozen(pos);
	update_if_frozen(cgroup);

	rcu_read_unlock();

	seq_puts(m, freezer_state_strs(cgroup_freezer(cgroup)->state));
	seq_putc(m, '\n');
	return 0;
	}

	static void freeze_cgroup(struct freezer *freezer)
	{
	struct cgroup *cgroup = freezer->css.cgroup;
	struct cgroup_iter it;
	struct task_struct *task;

	cgroup_iter_start(cgroup, &it);
	while ((task = cgroup_iter_next(cgroup, &it)))
	freeze_task(task);
	cgroup_iter_end(cgroup, &it);
	}

	static void unfreeze_cgroup(struct freezer *freezer)
	{
	struct cgroup *cgroup = freezer->css.cgroup;
	struct cgroup_iter it;
	struct task_struct *task;

	cgroup_iter_start(cgroup, &it);
	while ((task = cgroup_iter_next(cgroup, &it)))
	__thaw_task(task);
	cgroup_iter_end(cgroup, &it);
	}

	/**
	* freezer_apply_state - apply state change to a single cgroup_freezer
	* @freezer: freezer to apply state change to
	* @freeze: whether to freeze or unfreeze
	* @state: CGROUP_FREEZING_* flag to set or clear
	*
	* Set or clear @state on @cgroup according to @freeze, and perform
	* freezing or thawing as necessary.
	*/
	static void freezer_apply_state(struct freezer *freezer, bool freeze,
	unsigned int state)
	{
	/* also synchronizes against task migration, see freezer_attach() */
	lockdep_assert_held(&freezer->lock);

	if (!(freezer->state & CGROUP_FREEZER_ONLINE))
	return;

	if (freeze) {
	if (!(freezer->state & CGROUP_FREEZING))
	atomic_inc(&system_freezing_cnt);
	freezer->state \|= state;
	freeze_cgroup(freezer);
	} else {
	bool was_freezing = freezer->state & CGROUP_FREEZING;

	freezer->state &= ~state;

	if (!(freezer->state & CGROUP_FREEZING)) {
	if (was_freezing)
	atomic_dec(&system_freezing_cnt);
	freezer->state &= ~CGROUP_FROZEN;
	unfreeze_cgroup(freezer);
	}
	}
	}

	/**
	* freezer_change_state - change the freezing state of a cgroup_freezer
	* @freezer: freezer of interest
	* @freeze: whether to freeze or thaw
	*
	* Freeze or thaw @freezer according to @freeze. The operations are
	* recursive - all descendants of @freezer will be affected.
	*/
	static void freezer_change_state(struct freezer *freezer, bool freeze)
	{
	struct cgroup *pos;

	/* update @freezer */
	spin_lock_irq(&freezer->lock);
	freezer_apply_state(freezer, freeze, CGROUP_FREEZING_SELF);
	spin_unlock_irq(&freezer->lock);

	/*
	* Update all its descendants in pre-order traversal. Each
	* descendant will try to inherit its parent's FREEZING state as
	* CGROUP_FREEZING_PARENT.
	*/
	rcu_read_lock();
	cgroup_for_each_descendant_pre(pos, freezer->css.cgroup) {
	struct freezer *pos_f = cgroup_freezer(pos);
	struct freezer *parent = parent_freezer(pos_f);

	/*
	* Our update to @parent->state is already visible which is
	* all we need. No need to lock @parent. For more info on
	* synchronization, see freezer_post_create().
	*/
	spin_lock_irq(&pos_f->lock);
	freezer_apply_state(pos_f, parent->state & CGROUP_FREEZING,
	CGROUP_FREEZING_PARENT);
	spin_unlock_irq(&pos_f->lock);
	}
	rcu_read_unlock();
	}

	static int freezer_write(struct cgroup cgroup, struct cftype cft,
	const char *buffer)
	{
	bool freeze;

	if (strcmp(buffer, freezer_state_strs(0)) == 0)
	freeze = false;
	else if (strcmp(buffer, freezer_state_strs(CGROUP_FROZEN)) == 0)
	freeze = true;
	else
	return -EINVAL;

	freezer_change_state(cgroup_freezer(cgroup), freeze);
	return 0;
	}

	static u64 freezer_self_freezing_read(struct cgroup cgroup, struct cftype cft)
	{
	struct freezer *freezer = cgroup_freezer(cgroup);

	return (bool)(freezer->state & CGROUP_FREEZING_SELF);
	}

	static u64 freezer_parent_freezing_read(struct cgroup cgroup, struct cftype cft)
	{
	struct freezer *freezer = cgroup_freezer(cgroup);

	return (bool)(freezer->state & CGROUP_FREEZING_PARENT);
	}

	static struct cftype files[] = {
	{
	.name = "state",
	.flags = CFTYPE_NOT_ON_ROOT,
	.read_seq_string = freezer_read,
	.write_string = freezer_write,
	},
	{
	.name = "self_freezing",
	.flags = CFTYPE_NOT_ON_ROOT,
	.read_u64 = freezer_self_freezing_read,
	},
	{
	.name = "parent_freezing",
	.flags = CFTYPE_NOT_ON_ROOT,
	.read_u64 = freezer_parent_freezing_read,
	},
	{ } /* terminate */
	};

	struct cgroup_subsys freezer_subsys = {
	.name = "freezer",
	.css_alloc = freezer_css_alloc,
	.css_online = freezer_css_online,
	.css_offline = freezer_css_offline,
	.css_free = freezer_css_free,
	.subsys_id = freezer_subsys_id,
	.attach = freezer_attach,
	.fork = freezer_fork,
	.base_cftypes = files,
	};