cgroup_freezer.c 12.5 KB
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/*
 * 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.
 */

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#include <linux/export.h>
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#include <linux/slab.h>
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#include <linux/cgroup.h>
#include <linux/fs.h>
#include <linux/uaccess.h>
#include <linux/freezer.h>
#include <linux/seq_file.h>
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#include <linux/mutex.h>
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/*
 * 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.
 */
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enum freezer_state_flags {
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	CGROUP_FREEZER_ONLINE	= (1 << 0), /* freezer is fully online */
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	CGROUP_FREEZING_SELF	= (1 << 1), /* this freezer is freezing */
	CGROUP_FREEZING_PARENT	= (1 << 2), /* the parent freezer is freezing */
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	CGROUP_FROZEN		= (1 << 3), /* this and its descendants frozen */
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	/* mask for all FREEZING flags */
	CGROUP_FREEZING		= CGROUP_FREEZING_SELF | CGROUP_FREEZING_PARENT,
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};

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

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static DEFINE_MUTEX(freezer_mutex);

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static inline struct freezer *css_freezer(struct cgroup_subsys_state *css)
{
	return css ? container_of(css, struct freezer, css) : NULL;
}

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static inline struct freezer *task_freezer(struct task_struct *task)
{
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	return css_freezer(task_css(task, freezer_cgrp_id));
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}

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static struct freezer *parent_freezer(struct freezer *freezer)
{
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	return css_freezer(freezer->css.parent);
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}

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bool cgroup_freezing(struct task_struct *task)
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{
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	bool ret;
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	rcu_read_lock();
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	ret = task_freezer(task)->state & CGROUP_FREEZING;
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	rcu_read_unlock();

	return ret;
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}

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static const char *freezer_state_strs(unsigned int state)
{
	if (state & CGROUP_FROZEN)
		return "FROZEN";
	if (state & CGROUP_FREEZING)
		return "FREEZING";
	return "THAWED";
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};

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

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

	return &freezer->css;
}

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

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

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	mutex_unlock(&freezer_mutex);
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	return 0;
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}

/**
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 * freezer_css_offline - initiate destruction of a freezer css
 * @css: css being destroyed
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 *
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 * @css is going away.  Mark it dead and decrement system_freezing_count if
 * it was holding one.
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 */
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static void freezer_css_offline(struct cgroup_subsys_state *css)
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{
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	struct freezer *freezer = css_freezer(css);
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	mutex_lock(&freezer_mutex);
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	if (freezer->state & CGROUP_FREEZING)
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		atomic_dec(&system_freezing_cnt);
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	freezer->state = 0;

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	mutex_unlock(&freezer_mutex);
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}

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static void freezer_css_free(struct cgroup_subsys_state *css)
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{
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	kfree(css_freezer(css));
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}

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/*
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 * 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.
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 */
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static void freezer_attach(struct cgroup_subsys_state *new_css,
			   struct cgroup_taskset *tset)
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{
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	struct freezer *freezer = css_freezer(new_css);
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	struct task_struct *task;
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	bool clear_frozen = false;
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	mutex_lock(&freezer_mutex);
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	/*
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	 * Make the new tasks conform to the current state of @new_css.
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	 * 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.
	 *
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	 * Tasks in @tset are on @new_css but may not conform to its
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	 * current state before executing the following - !frozen tasks may
	 * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
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	 */
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	cgroup_taskset_for_each(task, tset) {
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		if (!(freezer->state & CGROUP_FREEZING)) {
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			__thaw_task(task);
		} else {
			freeze_task(task);
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			freezer->state &= ~CGROUP_FROZEN;
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			clear_frozen = true;
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		}
	}
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	/* propagate FROZEN clearing upwards */
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	while (clear_frozen && (freezer = parent_freezer(freezer))) {
		freezer->state &= ~CGROUP_FROZEN;
		clear_frozen = freezer->state & CGROUP_FREEZING;
	}
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	mutex_unlock(&freezer_mutex);
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}

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/**
 * freezer_fork - cgroup post fork callback
 * @task: a task which has just been forked
 *
 * @task has just been created and should conform to the current state of
 * the cgroup_freezer it belongs to.  This function may race against
 * freezer_attach().  Losing to freezer_attach() means that we don't have
 * to do anything as freezer_attach() will put @task into the appropriate
 * state.
 */
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static void freezer_fork(struct task_struct *task)
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{
	struct freezer *freezer;

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	/*
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	 * The root cgroup is non-freezable, so we can skip locking the
	 * freezer.  This is safe regardless of race with task migration.
	 * If we didn't race or won, skipping is obviously the right thing
	 * to do.  If we lost and root is the new cgroup, noop is still the
	 * right thing to do.
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	 */
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	if (task_css_is_root(task, freezer_cgrp_id))
		return;
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	mutex_lock(&freezer_mutex);
	rcu_read_lock();

	freezer = task_freezer(task);
	if (freezer->state & CGROUP_FREEZING)
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		freeze_task(task);
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	rcu_read_unlock();
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	mutex_unlock(&freezer_mutex);
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}

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/**
 * update_if_frozen - update whether a cgroup finished freezing
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 * @css: css of interest
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 *
 * 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.
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 *
 * Task states and freezer state might disagree while tasks are being
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 * migrated into or out of @css, so we can't verify task states against
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 * @freezer state here.  See freezer_attach() for details.
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 */
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static void update_if_frozen(struct cgroup_subsys_state *css)
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{
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	struct freezer *freezer = css_freezer(css);
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	struct cgroup_subsys_state *pos;
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	struct css_task_iter it;
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	struct task_struct *task;
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	lockdep_assert_held(&freezer_mutex);
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	if (!(freezer->state & CGROUP_FREEZING) ||
	    (freezer->state & CGROUP_FROZEN))
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		return;
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	/* are all (live) children frozen? */
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	rcu_read_lock();
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	css_for_each_child(pos, css) {
		struct freezer *child = css_freezer(pos);
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		if ((child->state & CGROUP_FREEZER_ONLINE) &&
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		    !(child->state & CGROUP_FROZEN)) {
			rcu_read_unlock();
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			return;
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		}
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	}
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	rcu_read_unlock();
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	/* are all tasks frozen? */
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	css_task_iter_start(css, &it);
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	while ((task = css_task_iter_next(&it))) {
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		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.
			 */
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			if (!frozen(task) && !freezer_should_skip(task))
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				goto out_iter_end;
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		}
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	}

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	freezer->state |= CGROUP_FROZEN;
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out_iter_end:
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	css_task_iter_end(&it);
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}

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static int freezer_read(struct seq_file *m, void *v)
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{
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	struct cgroup_subsys_state *css = seq_css(m), *pos;
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	mutex_lock(&freezer_mutex);
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	rcu_read_lock();
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	/* update states bottom-up */
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	css_for_each_descendant_post(pos, css) {
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		if (!css_tryget_online(pos))
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			continue;
		rcu_read_unlock();

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		update_if_frozen(pos);
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		rcu_read_lock();
		css_put(pos);
	}

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	rcu_read_unlock();
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	mutex_unlock(&freezer_mutex);
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	seq_puts(m, freezer_state_strs(css_freezer(css)->state));
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	seq_putc(m, '\n');
	return 0;
}

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static void freeze_cgroup(struct freezer *freezer)
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{
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	struct css_task_iter it;
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	struct task_struct *task;

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	css_task_iter_start(&freezer->css, &it);
	while ((task = css_task_iter_next(&it)))
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		freeze_task(task);
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	css_task_iter_end(&it);
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}

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static void unfreeze_cgroup(struct freezer *freezer)
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{
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	struct css_task_iter it;
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	struct task_struct *task;

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	css_task_iter_start(&freezer->css, &it);
	while ((task = css_task_iter_next(&it)))
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		__thaw_task(task);
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	css_task_iter_end(&it);
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}

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

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	if (freeze) {
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		if (!(freezer->state & CGROUP_FREEZING))
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			atomic_inc(&system_freezing_cnt);
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		freezer->state |= state;
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		freeze_cgroup(freezer);
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	} else {
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		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);
		}
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	}
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}
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/**
 * freezer_change_state - change the freezing state of a cgroup_freezer
 * @freezer: freezer of interest
 * @freeze: whether to freeze or thaw
 *
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 * Freeze or thaw @freezer according to @freeze.  The operations are
 * recursive - all descendants of @freezer will be affected.
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 */
static void freezer_change_state(struct freezer *freezer, bool freeze)
{
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	struct cgroup_subsys_state *pos;
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	/*
	 * Update all its descendants in pre-order traversal.  Each
	 * descendant will try to inherit its parent's FREEZING state as
	 * CGROUP_FREEZING_PARENT.
	 */
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	mutex_lock(&freezer_mutex);
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	rcu_read_lock();
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	css_for_each_descendant_pre(pos, &freezer->css) {
		struct freezer *pos_f = css_freezer(pos);
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		struct freezer *parent = parent_freezer(pos_f);

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		if (!css_tryget_online(pos))
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			continue;
		rcu_read_unlock();
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		if (pos_f == freezer)
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			freezer_apply_state(pos_f, freeze,
					    CGROUP_FREEZING_SELF);
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		else
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			freezer_apply_state(pos_f,
					    parent->state & CGROUP_FREEZING,
					    CGROUP_FREEZING_PARENT);

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		rcu_read_lock();
		css_put(pos);
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	}
	rcu_read_unlock();
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	mutex_unlock(&freezer_mutex);
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}

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static ssize_t freezer_write(struct kernfs_open_file *of,
			     char *buf, size_t nbytes, loff_t off)
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{
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	bool freeze;
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	buf = strstrip(buf);

	if (strcmp(buf, freezer_state_strs(0)) == 0)
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		freeze = false;
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	else if (strcmp(buf, freezer_state_strs(CGROUP_FROZEN)) == 0)
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		freeze = true;
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	else
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		return -EINVAL;
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	freezer_change_state(css_freezer(of_css(of)), freeze);
	return nbytes;
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}

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static u64 freezer_self_freezing_read(struct cgroup_subsys_state *css,
				      struct cftype *cft)
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{
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	struct freezer *freezer = css_freezer(css);
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	return (bool)(freezer->state & CGROUP_FREEZING_SELF);
}

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static u64 freezer_parent_freezing_read(struct cgroup_subsys_state *css,
					struct cftype *cft)
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{
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	struct freezer *freezer = css_freezer(css);
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	return (bool)(freezer->state & CGROUP_FREEZING_PARENT);
}

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static struct cftype files[] = {
	{
		.name = "state",
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		.flags = CFTYPE_NOT_ON_ROOT,
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		.seq_show = freezer_read,
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		.write = freezer_write,
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	},
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	{
		.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,
	},
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	{ }	/* terminate */
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};

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struct cgroup_subsys freezer_cgrp_subsys = {
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	.css_alloc	= freezer_css_alloc,
	.css_online	= freezer_css_online,
	.css_offline	= freezer_css_offline,
	.css_free	= freezer_css_free,
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	.attach		= freezer_attach,
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	.fork		= freezer_fork,
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	.legacy_cftypes	= files,
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};