linux/kernel/cgroup_freezer.c
Matt Helsley 5a7aadfe2f Freezer: Fix buggy resume test for tasks frozen with cgroup freezer
When the cgroup freezer is used to freeze tasks we do not want to thaw
those tasks during resume. Currently we test the cgroup freezer
state of the resuming tasks to see if the cgroup is FROZEN.  If so
then we don't thaw the task. However, the FREEZING state also indicates
that the task should remain frozen.

This also avoids a problem pointed out by Oren Ladaan: the freezer state
transition from FREEZING to FROZEN is updated lazily when userspace reads
or writes the freezer.state file in the cgroup filesystem. This means that
resume will thaw tasks in cgroups which should be in the FROZEN state if
there is no read/write of the freezer.state file to trigger this
transition before suspend.

NOTE: Another "simple" solution would be to always update the cgroup
freezer state during resume. However it's a bad choice for several reasons:
Updating the cgroup freezer state is somewhat expensive because it requires
walking all the tasks in the cgroup and checking if they are each frozen.
Worse, this could easily make resume run in N^2 time where N is the number
of tasks in the cgroup. Finally, updating the freezer state from this code
path requires trickier locking because of the way locks must be ordered.

Instead of updating the freezer state we rely on the fact that lazy
updates only manage the transition from FREEZING to FROZEN. We know that
a cgroup with the FREEZING state may actually be FROZEN so test for that
state too. This makes sense in the resume path even for partially-frozen
cgroups -- those that really are FREEZING but not FROZEN.

Reported-by: Oren Ladaan <orenl@cs.columbia.edu>
Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Cc: stable@kernel.org
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
2010-03-26 23:51:44 +01:00

396 lines
9.3 KiB
C

/*
* 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/module.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);
}
int cgroup_freezing_or_frozen(struct task_struct *task)
{
struct freezer *freezer;
enum freezer_state state;
task_lock(task);
freezer = task_freezer(task);
if (!freezer->css.cgroup->parent)
state = CGROUP_THAWED; /* root cgroup can't be frozen */
else
state = freezer->state;
task_unlock(task);
return (state == CGROUP_FREEZING) || (state == CGROUP_FROZEN);
}
/*
* 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
* ------------------------------
* css_set_lock
* cgroup_mutex (AKA cgroup_lock)
* task->alloc_lock (AKA task_lock)
* freezer->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 ]
*
* can_attach():
* cgroup_mutex
*
* cgroup_frozen():
* task->alloc_lock (to get task's cgroup)
*
* freezer_fork() (preserving fork() performance means can't take cgroup_mutex):
* task->alloc_lock (to get task's cgroup)
* freezer->lock
* sighand->siglock (if the cgroup is freezing)
*
* freezer_read():
* cgroup_mutex
* freezer->lock
* read_lock css_set_lock (cgroup iterator start)
*
* freezer_write() (freeze):
* cgroup_mutex
* freezer->lock
* read_lock css_set_lock (cgroup iterator start)
* sighand->siglock
*
* freezer_write() (unfreeze):
* cgroup_mutex
* freezer->lock
* read_lock css_set_lock (cgroup iterator start)
* task->alloc_lock (to prevent races with freeze_task())
* sighand->siglock
*/
static struct cgroup_subsys_state *freezer_create(struct cgroup_subsys *ss,
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_subsys *ss,
struct cgroup *cgroup)
{
kfree(cgroup_freezer(cgroup));
}
/* 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_subsys *ss,
struct cgroup *new_cgroup,
struct task_struct *task, bool threadgroup)
{
struct freezer *freezer;
/*
* Anything frozen can't move or be moved to/from.
*
* Since orig_freezer->state == FROZEN means that @task has been
* frozen, so it's sufficient to check the latter condition.
*/
if (is_task_frozen_enough(task))
return -EBUSY;
freezer = cgroup_freezer(new_cgroup);
if (freezer->state == CGROUP_FROZEN)
return -EBUSY;
if (threadgroup) {
struct task_struct *c;
rcu_read_lock();
list_for_each_entry_rcu(c, &task->thread_group, thread_group) {
if (is_task_frozen_enough(c)) {
rcu_read_unlock();
return -EBUSY;
}
}
rcu_read_unlock();
}
return 0;
}
static void freezer_fork(struct cgroup_subsys *ss, 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.
*/
freezer = task_freezer(task);
/*
* 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, true);
spin_unlock_irq(&freezer->lock);
}
/*
* caller must hold freezer->lock
*/
static void update_freezer_state(struct cgroup *cgroup,
struct freezer *freezer)
{
struct cgroup_iter it;
struct task_struct *task;
unsigned int nfrozen = 0, ntotal = 0;
cgroup_iter_start(cgroup, &it);
while ((task = cgroup_iter_next(cgroup, &it))) {
ntotal++;
if (is_task_frozen_enough(task))
nfrozen++;
}
/*
* Transition to FROZEN when no new tasks can be added ensures
* that we never exist in the FROZEN state while there are unfrozen
* tasks.
*/
if (nfrozen == ntotal)
freezer->state = CGROUP_FROZEN;
else if (nfrozen > 0)
freezer->state = CGROUP_FREEZING;
else
freezer->state = CGROUP_THAWED;
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_freezer_state(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;
freezer->state = CGROUP_FREEZING;
cgroup_iter_start(cgroup, &it);
while ((task = cgroup_iter_next(cgroup, &it))) {
if (!freeze_task(task, true))
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_process(task);
}
cgroup_iter_end(cgroup, &it);
freezer->state = CGROUP_THAWED;
}
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_freezer_state(cgroup, freezer);
if (goal_state == freezer->state)
goto out;
switch (goal_state) {
case CGROUP_THAWED:
unfreeze_cgroup(cgroup, freezer);
break;
case CGROUP_FROZEN:
retval = try_to_freeze_cgroup(cgroup, freezer);
break;
default:
BUG();
}
out:
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",
.read_seq_string = freezer_read,
.write_string = freezer_write,
},
};
static int freezer_populate(struct cgroup_subsys *ss, struct cgroup *cgroup)
{
if (!cgroup->parent)
return 0;
return cgroup_add_files(cgroup, ss, files, ARRAY_SIZE(files));
}
struct cgroup_subsys freezer_subsys = {
.name = "freezer",
.create = freezer_create,
.destroy = freezer_destroy,
.populate = freezer_populate,
.subsys_id = freezer_subsys_id,
.can_attach = freezer_can_attach,
.attach = NULL,
.fork = freezer_fork,
.exit = NULL,
};