cgroup_freezer.c

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/*
 * cgroup_freezer.c -  control group freezer subsystem
 *
 * Copyright IBM Corporation, 2007
 *
 * Author : Cedric Le Goater <[email protected]>
 *
 * 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,
};