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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.
*/

#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>

enum freezer_state {
CGROUP_THAWED = 0,
CGROUP_FREEZING,
CGROUP_FROZEN,
};

struct freezer {
struct cgroup_subsys_state css;
enum freezer_state state;
spinlock_t lock; /* protects _writes_ to state */
};

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);
}

bool cgroup_freezing(struct task_struct *task)
{
enum freezer_state state;
bool ret;

rcu_read_lock();
state = task_freezer(task)->state;
ret = state == CGROUP_FREEZING || state == CGROUP_FROZEN;
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[] = {
"THAWED",
"FREEZING",
"FROZEN",
};

/*
* State diagram
* Transitions are caused by userspace writes to the freezer.state file.
* The values in parenthesis are state labels. The rest are edge labels.
*
* (THAWED) --FROZEN--> (FREEZING) --FROZEN--> (FROZEN)
* ^ ^ | |
* | \_______THAWED_______/ |
* \__________________________THAWED____________/
*/

struct cgroup_subsys freezer_subsys;

/* Locks taken and their ordering
* ------------------------------
* cgroup_mutex (AKA cgroup_lock)
* freezer->lock
* css_set_lock
* task->alloc_lock (AKA task_lock)
* task->sighand->siglock
*
* cgroup code forces css_set_lock to be taken before task->alloc_lock
*
* freezer_create(), freezer_destroy():
* cgroup_mutex [ by cgroup core ]
*
* freezer_can_attach():
* cgroup_mutex (held by caller of can_attach)
*
* freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
* freezer->lock
* sighand->siglock (if the cgroup is freezing)
*
* freezer_read():
* cgroup_mutex
* freezer->lock
* write_lock css_set_lock (cgroup iterator start)
* task->alloc_lock
* read_lock css_set_lock (cgroup iterator start)
*
* freezer_write() (freeze):
* cgroup_mutex
* freezer->lock
* write_lock css_set_lock (cgroup iterator start)
* task->alloc_lock
* read_lock css_set_lock (cgroup iterator start)
* sighand->siglock (fake signal delivery inside freeze_task())
*
* freezer_write() (unfreeze):
* cgroup_mutex
* freezer->lock
* write_lock css_set_lock (cgroup iterator start)
* task->alloc_lock
* read_lock css_set_lock (cgroup iterator start)
* task->alloc_lock (inside __thaw_task(), prevents race with refrigerator())
* sighand->siglock
*/
static struct cgroup_subsys_state *freezer_create(struct cgroup *cgroup)
{
struct freezer *freezer;

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

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

static void freezer_destroy(struct cgroup *cgroup)
{
struct freezer *freezer = cgroup_freezer(cgroup);

if (freezer->state != CGROUP_THAWED)
atomic_dec(&system_freezing_cnt);
kfree(freezer);
}

/* task is frozen or will freeze immediately when next it gets woken */
static bool is_task_frozen_enough(struct task_struct *task)
{
return frozen(task) ||
(task_is_stopped_or_traced(task) && freezing(task));
}

/*
* The call to cgroup_lock() in the freezer.state write method prevents
* a write to that file racing against an attach, and hence the
* can_attach() result will remain valid until the attach completes.
*/
static int freezer_can_attach(struct cgroup *new_cgroup,
struct cgroup_taskset *tset)
{
struct freezer *freezer;
struct task_struct *task;

/*
* Anything frozen can't move or be moved to/from.
*/
cgroup_taskset_for_each(task, new_cgroup, tset)
if (cgroup_freezing(task))
return -EBUSY;

freezer = cgroup_freezer(new_cgroup);
if (freezer->state != CGROUP_THAWED)
return -EBUSY;

return 0;
}

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

/*
* No lock is needed, since the task isn't on tasklist yet,
* so it can't be moved to another cgroup, which means the
* freezer won't be removed and will be valid during this
* function call. Nevertheless, apply RCU read-side critical
* section to suppress RCU lockdep false positives.
*/
rcu_read_lock();
freezer = task_freezer(task);
rcu_read_unlock();

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

spin_lock_irq(&freezer->lock);
BUG_ON(freezer->state == CGROUP_FROZEN);

/* Locking avoids race with FREEZING -> THAWED transitions. */
if (freezer->state == CGROUP_FREEZING)
freeze_task(task);
spin_unlock_irq(&freezer->lock);
}

/*
* caller must hold freezer->lock
*/
static void update_if_frozen(struct cgroup *cgroup,
struct freezer *freezer)
{
struct cgroup_iter it;
struct task_struct *task;
unsigned int nfrozen = 0, ntotal = 0;
enum freezer_state old_state = freezer->state;

cgroup_iter_start(cgroup, &it);
while ((task = cgroup_iter_next(cgroup, &it))) {
ntotal++;
if (freezing(task) && is_task_frozen_enough(task))
nfrozen++;
}

if (old_state == CGROUP_THAWED) {
BUG_ON(nfrozen > 0);
} else if (old_state == CGROUP_FREEZING) {
if (nfrozen == ntotal)
freezer->state = CGROUP_FROZEN;
} else { /* old_state == CGROUP_FROZEN */
BUG_ON(nfrozen != ntotal);
}

cgroup_iter_end(cgroup, &it);
}

static int freezer_read(struct cgroup *cgroup, struct cftype *cft,
struct seq_file *m)
{
struct freezer *freezer;
enum freezer_state state;

if (!cgroup_lock_live_group(cgroup))
return -ENODEV;

freezer = cgroup_freezer(cgroup);
spin_lock_irq(&freezer->lock);
state = freezer->state;
if (state == CGROUP_FREEZING) {
/* We change from FREEZING to FROZEN lazily if the cgroup was
* only partially frozen when we exitted write. */
update_if_frozen(cgroup, freezer);
state = freezer->state;
}
spin_unlock_irq(&freezer->lock);
cgroup_unlock();

seq_puts(m, freezer_state_strs[state]);
seq_putc(m, '\n');
return 0;
}

static int try_to_freeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
{
struct cgroup_iter it;
struct task_struct *task;
unsigned int num_cant_freeze_now = 0;

cgroup_iter_start(cgroup, &it);
while ((task = cgroup_iter_next(cgroup, &it))) {
if (!freeze_task(task))
continue;
if (is_task_frozen_enough(task))
continue;
if (!freezing(task) && !freezer_should_skip(task))
num_cant_freeze_now++;
}
cgroup_iter_end(cgroup, &it);

return num_cant_freeze_now ? -EBUSY : 0;
}

static void unfreeze_cgroup(struct cgroup *cgroup, struct freezer *freezer)
{
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);
}

static int freezer_change_state(struct cgroup *cgroup,
enum freezer_state goal_state)
{
struct freezer *freezer;
int retval = 0;

freezer = cgroup_freezer(cgroup);

spin_lock_irq(&freezer->lock);

update_if_frozen(cgroup, freezer);

switch (goal_state) {
case CGROUP_THAWED:
if (freezer->state != CGROUP_THAWED)
atomic_dec(&system_freezing_cnt);
freezer->state = CGROUP_THAWED;
unfreeze_cgroup(cgroup, freezer);
break;
case CGROUP_FROZEN:
if (freezer->state == CGROUP_THAWED)
atomic_inc(&system_freezing_cnt);
freezer->state = CGROUP_FREEZING;
retval = try_to_freeze_cgroup(cgroup, freezer);
break;
default:
BUG();
}

spin_unlock_irq(&freezer->lock);

return retval;
}

static int freezer_write(struct cgroup *cgroup,
struct cftype *cft,
const char *buffer)
{
int retval;
enum freezer_state goal_state;

if (strcmp(buffer, freezer_state_strs[CGROUP_THAWED]) == 0)
goal_state = CGROUP_THAWED;
else if (strcmp(buffer, freezer_state_strs[CGROUP_FROZEN]) == 0)
goal_state = CGROUP_FROZEN;
else
return -EINVAL;

if (!cgroup_lock_live_group(cgroup))
return -ENODEV;
retval = freezer_change_state(cgroup, goal_state);
cgroup_unlock();
return retval;
}

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

struct cgroup_subsys freezer_subsys = {
.name = "freezer",
.create = freezer_create,
.destroy = freezer_destroy,
.subsys_id = freezer_subsys_id,
.can_attach = freezer_can_attach,
.fork = freezer_fork,
.base_cftypes = files,

/*
* freezer subsys doesn't handle hierarchy at all. Frozen state
* should be inherited through the hierarchy - if a parent is
* frozen, all its children should be frozen. Fix it and remove
* the following.
*/
.broken_hierarchy = true,
};
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