/*
 * 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_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)
{
        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_subsys *ss,
                              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 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.  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",
                .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,
        .fork           = freezer_fork,
};