kernel/cgroup/legacy_freezer.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/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>
#include <linux/mutex.h>

/*
 * 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.
 */
enum freezer_state_flags {
    CGROUP_FREEZER_ONLINE = (1 << 0),  /* freezer is fully online */
    CGROUP_FREEZING_SELF = (1 << 1),   /* this freezer is freezing */
    CGROUP_FREEZING_PARENT = (1 << 2), /* the parent freezer is freezing */
    CGROUP_FROZEN = (1 << 3),          /* this and its descendants frozen */

    /* mask for all FREEZING flags */
    CGROUP_FREEZING = CGROUP_FREEZING_SELF | CGROUP_FREEZING_PARENT,
};

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

static DEFINE_MUTEX(freezer_mutex);

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

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

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

bool cgroup_freezing(struct task_struct *task)
{
    bool ret;

    rcu_read_lock();
    ret = task_freezer(task)->state & CGROUP_FREEZING;
    rcu_read_unlock();

    return ret;
}

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

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

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

    return &freezer->css;
}

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

    mutex_lock(&freezer_mutex);

    freezer->state |= CGROUP_FREEZER_ONLINE;

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

    mutex_unlock(&freezer_mutex);
    return 0;
}

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

    mutex_lock(&freezer_mutex);

    if (freezer->state & CGROUP_FREEZING) {
        atomic_dec(&system_freezing_cnt);
    }

    freezer->state = 0;

    mutex_unlock(&freezer_mutex);
}

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

/*
 * 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.
 */
static void freezer_attach(struct cgroup_taskset *tset)
{
    struct task_struct *task;
    struct cgroup_subsys_state *new_css;

    mutex_lock(&freezer_mutex);

    /*
     * Make the new tasks conform to the current state of @new_css.
     * 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.
     *
     * Tasks in @tset are on @new_css but may not conform to its
     * current state before executing the following - !frozen tasks may
     * be visible in a FROZEN cgroup and frozen tasks in a THAWED one.
     */
    cgroup_taskset_for_each(task, new_css, tset)
    {
        struct freezer *freezer = css_freezer(new_css);

        if (!(freezer->state & CGROUP_FREEZING)) {
            __thaw_task(task);
        } else {
            freeze_task(task);
            /* clear FROZEN and propagate upwards */
            while (freezer && (freezer->state & CGROUP_FROZEN)) {
                freezer->state &= ~CGROUP_FROZEN;
                freezer = parent_freezer(freezer);
            }
        }
    }

    mutex_unlock(&freezer_mutex);
}

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

    /*
     * 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.
     */
    if (task_css_is_root(task, freezer_cgrp_id)) {
        return;
    }

    mutex_lock(&freezer_mutex);
    rcu_read_lock();

    freezer = task_freezer(task);
    if (freezer->state & CGROUP_FREEZING) {
        freeze_task(task);
    }

    rcu_read_unlock();
    mutex_unlock(&freezer_mutex);
}

/**
 * update_if_frozen - update whether a cgroup finished freezing
 * @css: css of interest
 *
 * 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.
 *
 * Task states and freezer state might disagree while tasks are being
 * migrated into or out of @css, so we can't verify task states against
 * @freezer state here.  See freezer_attach() for details.
 */
static void update_if_frozen(struct cgroup_subsys_state *css)
{
    struct freezer *freezer = css_freezer(css);
    struct cgroup_subsys_state *pos;
    struct css_task_iter it;
    struct task_struct *task;

    lockdep_assert_held(&freezer_mutex);

    if (!(freezer->state & CGROUP_FREEZING) || (freezer->state & CGROUP_FROZEN)) {
        return;
    }

    /* are all (live) children frozen? */
    rcu_read_lock();
    css_for_each_child(pos, css)
    {
        struct freezer *child = css_freezer(pos);

        if ((child->state & CGROUP_FREEZER_ONLINE) && !(child->state & CGROUP_FROZEN)) {
            rcu_read_unlock();
            return;
        }
    }
    rcu_read_unlock();

    /* are all tasks frozen? */
    css_task_iter_start(css, 0, &it);

    while ((task = css_task_iter_next(&it))) {
        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.
             */
            if (!frozen(task) && !freezer_should_skip(task)) {
                goto out_iter_end;
            }
        }
    }

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

static int freezer_read(struct seq_file *m, void *v)
{
    struct cgroup_subsys_state *css = seq_css(m), *pos;

    mutex_lock(&freezer_mutex);
    rcu_read_lock();

    /* update states bottom-up */
    css_for_each_descendant_post(pos, css)
    {
        if (!css_tryget_online(pos)) {
            continue;
        }
        rcu_read_unlock();

        update_if_frozen(pos);

        rcu_read_lock();
        css_put(pos);
    }

    rcu_read_unlock();
    mutex_unlock(&freezer_mutex);

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

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

    css_task_iter_start(&freezer->css, 0, &it);
    while ((task = css_task_iter_next(&it))) {
        freeze_task(task);
    }
    css_task_iter_end(&it);
}

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

    css_task_iter_start(&freezer->css, 0, &it);
    while ((task = css_task_iter_next(&it))) {
        __thaw_task(task);
    }
    css_task_iter_end(&it);
}

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

    if (!(freezer->state & CGROUP_FREEZER_ONLINE)) {
        return;
    }

    if (freeze) {
        if (!(freezer->state & CGROUP_FREEZING)) {
            atomic_inc(&system_freezing_cnt);
        }
        freezer->state |= state;
        freeze_cgroup(freezer);
    } else {
        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);
        }
    }
}

/**
 * freezer_change_state - change the freezing state of a cgroup_freezer
 * @freezer: freezer of interest
 * @freeze: whether to freeze or thaw
 *
 * Freeze or thaw @freezer according to @freeze.  The operations are
 * recursive - all descendants of @freezer will be affected.
 */
static void freezer_change_state(struct freezer *freezer, bool freeze)
{
    struct cgroup_subsys_state *pos;

    /*
     * Update all its descendants in pre-order traversal.  Each
     * descendant will try to inherit its parent's FREEZING state as
     * CGROUP_FREEZING_PARENT.
     */
    mutex_lock(&freezer_mutex);
    rcu_read_lock();
    css_for_each_descendant_pre(pos, &freezer->css)
    {
        struct freezer *pos_f = css_freezer(pos);
        struct freezer *parent = parent_freezer(pos_f);

        if (!css_tryget_online(pos)) {
            continue;
        }
        rcu_read_unlock();

        if (pos_f == freezer) {
            freezer_apply_state(pos_f, freeze, CGROUP_FREEZING_SELF);
        } else {
            freezer_apply_state(pos_f, parent->state & CGROUP_FREEZING, CGROUP_FREEZING_PARENT);
        }

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

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

    buf = strstrip(buf);
    if (strcmp(buf, freezer_state_strs(0)) == 0) {
        freeze = false;
    } else if (strcmp(buf, freezer_state_strs(CGROUP_FROZEN)) == 0) {
        freeze = true;
    } else {
        return -EINVAL;
    }

    freezer_change_state(css_freezer(of_css(of)), freeze);
    return nbytes;
}

static u64 freezer_self_freezing_read(struct cgroup_subsys_state *css, struct cftype *cft)
{
    struct freezer *freezer = css_freezer(css);

    return (bool)(freezer->state & CGROUP_FREEZING_SELF);
}

static u64 freezer_parent_freezing_read(struct cgroup_subsys_state *css, struct cftype *cft)
{
    struct freezer *freezer = css_freezer(css);

    return (bool)(freezer->state & CGROUP_FREEZING_PARENT);
}

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

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