blk-cgroup.c

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/*
 * Common Block IO controller cgroup interface
 *
 * Based on ideas and code from CFQ, CFS and BFQ:
 * Copyright (C) 2003 Jens Axboe <[email protected]>
 *
 * Copyright (C) 2008 Fabio Checconi <[email protected]>
 *            Paolo Valente <[email protected]>
 *
 * Copyright (C) 2009 Vivek Goyal <[email protected]>
 *                Nauman Rafique <[email protected]>
 */
#include <linux/ioprio.h>
#include <linux/kdev_t.h>
#include <linux/module.h>
#include <linux/err.h>
#include <linux/blkdev.h>
#include <linux/slab.h>
#include <linux/genhd.h>
#include <linux/delay.h>
#include <linux/atomic.h>
#include "blk-cgroup.h"
#include "blk.h"

#define MAX_KEY_LEN 100

static DEFINE_MUTEX(blkcg_pol_mutex);

struct blkcg blkcg_root = { .cfq_weight = 2 * CFQ_WEIGHT_DEFAULT };
EXPORT_SYMBOL_GPL(blkcg_root);

static struct blkcg_policy *blkcg_policy[BLKCG_MAX_POLS];

static bool blkcg_policy_enabled(struct request_queue *q,
                 const struct blkcg_policy *pol)
{
    return pol && test_bit(pol->plid, q->blkcg_pols);
}

static void blkg_free(struct blkcg_gq *blkg)
{
    int i;

    if (!blkg)
        return;

    for (i = 0; i < BLKCG_MAX_POLS; i++) {
        struct blkcg_policy *pol = blkcg_policy[i];
        struct blkg_policy_data *pd = blkg->pd[i];

        if (!pd)
            continue;

        if (pol && pol->pd_exit_fn)
            pol->pd_exit_fn(blkg);

        kfree(pd);
    }

    blk_exit_rl(&blkg->rl);
    kfree(blkg);
}

static struct blkcg_gq *blkg_alloc(struct blkcg *blkcg, struct request_queue *q,
                   gfp_t gfp_mask)
{
    struct blkcg_gq *blkg;
    int i;

    /* alloc and init base part */
    blkg = kzalloc_node(sizeof(*blkg), gfp_mask, q->node);
    if (!blkg)
        return NULL;

    blkg->q = q;
    INIT_LIST_HEAD(&blkg->q_node);
    blkg->blkcg = blkcg;
    blkg->refcnt = 1;

    /* root blkg uses @q->root_rl, init rl only for !root blkgs */
    if (blkcg != &blkcg_root) {
        if (blk_init_rl(&blkg->rl, q, gfp_mask))
            goto err_free;
        blkg->rl.blkg = blkg;
    }

    for (i = 0; i < BLKCG_MAX_POLS; i++) {
        struct blkcg_policy *pol = blkcg_policy[i];
        struct blkg_policy_data *pd;

        if (!blkcg_policy_enabled(q, pol))
            continue;

        /* alloc per-policy data and attach it to blkg */
        pd = kzalloc_node(pol->pd_size, gfp_mask, q->node);
        if (!pd)
            goto err_free;

        blkg->pd[i] = pd;
        pd->blkg = blkg;

        /* invoke per-policy init */
        if (blkcg_policy_enabled(blkg->q, pol))
            pol->pd_init_fn(blkg);
    }

    return blkg;

err_free:
    blkg_free(blkg);
    return NULL;
}

static struct blkcg_gq *__blkg_lookup(struct blkcg *blkcg,
                      struct request_queue *q)
{
    struct blkcg_gq *blkg;

    blkg = rcu_dereference(blkcg->blkg_hint);
    if (blkg && blkg->q == q)
        return blkg;

    /*
     * Hint didn't match.  Look up from the radix tree.  Note that we
     * may not be holding queue_lock and thus are not sure whether
     * @blkg from blkg_tree has already been removed or not, so we
     * can't update hint to the lookup result.  Leave it to the caller.
     */
    blkg = radix_tree_lookup(&blkcg->blkg_tree, q->id);
    if (blkg && blkg->q == q)
        return blkg;

    return NULL;
}

struct blkcg_gq *blkg_lookup(struct blkcg *blkcg, struct request_queue *q)
{
    WARN_ON_ONCE(!rcu_read_lock_held());

    if (unlikely(blk_queue_bypass(q)))
        return NULL;
    return __blkg_lookup(blkcg, q);
}
EXPORT_SYMBOL_GPL(blkg_lookup);

/*
 * If @new_blkg is %NULL, this function tries to allocate a new one as
 * necessary using %GFP_ATOMIC.  @new_blkg is always consumed on return.
 */
static struct blkcg_gq *__blkg_lookup_create(struct blkcg *blkcg,
                         struct request_queue *q,
                         struct blkcg_gq *new_blkg)
{
    struct blkcg_gq *blkg;
    int ret;

    WARN_ON_ONCE(!rcu_read_lock_held());
    lockdep_assert_held(q->queue_lock);

    /* lookup and update hint on success, see __blkg_lookup() for details */
    blkg = __blkg_lookup(blkcg, q);
    if (blkg) {
        rcu_assign_pointer(blkcg->blkg_hint, blkg);
        goto out_free;
    }

    /* blkg holds a reference to blkcg */
    if (!css_tryget(&blkcg->css)) {
        blkg = ERR_PTR(-EINVAL);
        goto out_free;
    }

    /* allocate */
    if (!new_blkg) {
        new_blkg = blkg_alloc(blkcg, q, GFP_ATOMIC);
        if (unlikely(!new_blkg)) {
            blkg = ERR_PTR(-ENOMEM);
            goto out_put;
        }
    }
    blkg = new_blkg;

    /* insert */
    spin_lock(&blkcg->lock);
    ret = radix_tree_insert(&blkcg->blkg_tree, q->id, blkg);
    if (likely(!ret)) {
        hlist_add_head_rcu(&blkg->blkcg_node, &blkcg->blkg_list);
        list_add(&blkg->q_node, &q->blkg_list);
    }
    spin_unlock(&blkcg->lock);

    if (!ret)
        return blkg;

    blkg = ERR_PTR(ret);
out_put:
    css_put(&blkcg->css);
out_free:
    blkg_free(new_blkg);
    return blkg;
}

struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
                    struct request_queue *q)
{
    /*
     * This could be the first entry point of blkcg implementation and
     * we shouldn't allow anything to go through for a bypassing queue.
     */
    if (unlikely(blk_queue_bypass(q)))
        return ERR_PTR(blk_queue_dead(q) ? -EINVAL : -EBUSY);
    return __blkg_lookup_create(blkcg, q, NULL);
}
EXPORT_SYMBOL_GPL(blkg_lookup_create);

static void blkg_destroy(struct blkcg_gq *blkg)
{
    struct blkcg *blkcg = blkg->blkcg;

    lockdep_assert_held(blkg->q->queue_lock);
    lockdep_assert_held(&blkcg->lock);

    /* Something wrong if we are trying to remove same group twice */
    WARN_ON_ONCE(list_empty(&blkg->q_node));
    WARN_ON_ONCE(hlist_unhashed(&blkg->blkcg_node));

    radix_tree_delete(&blkcg->blkg_tree, blkg->q->id);
    list_del_init(&blkg->q_node);
    hlist_del_init_rcu(&blkg->blkcg_node);

    /*
     * Both setting lookup hint to and clearing it from @blkg are done
     * under queue_lock.  If it's not pointing to @blkg now, it never
     * will.  Hint assignment itself can race safely.
     */
    if (rcu_dereference_raw(blkcg->blkg_hint) == blkg)
        rcu_assign_pointer(blkcg->blkg_hint, NULL);

    /*
     * Put the reference taken at the time of creation so that when all
     * queues are gone, group can be destroyed.
     */
    blkg_put(blkg);
}

static void blkg_destroy_all(struct request_queue *q)
{
    struct blkcg_gq *blkg, *n;

    lockdep_assert_held(q->queue_lock);

    list_for_each_entry_safe(blkg, n, &q->blkg_list, q_node) {
        struct blkcg *blkcg = blkg->blkcg;

        spin_lock(&blkcg->lock);
        blkg_destroy(blkg);
        spin_unlock(&blkcg->lock);
    }

    /*
     * root blkg is destroyed.  Just clear the pointer since
     * root_rl does not take reference on root blkg.
     */
    q->root_blkg = NULL;
    q->root_rl.blkg = NULL;
}

static void blkg_rcu_free(struct rcu_head *rcu_head)
{
    blkg_free(container_of(rcu_head, struct blkcg_gq, rcu_head));
}

void __blkg_release(struct blkcg_gq *blkg)
{
    /* release the extra blkcg reference this blkg has been holding */
    css_put(&blkg->blkcg->css);

    /*
     * A group is freed in rcu manner. But having an rcu lock does not
     * mean that one can access all the fields of blkg and assume these
     * are valid. For example, don't try to follow throtl_data and
     * request queue links.
     *
     * Having a reference to blkg under an rcu allows acess to only
     * values local to groups like group stats and group rate limits
     */
    call_rcu(&blkg->rcu_head, blkg_rcu_free);
}
EXPORT_SYMBOL_GPL(__blkg_release);

/*
 * The next function used by blk_queue_for_each_rl().  It's a bit tricky
 * because the root blkg uses @q->root_rl instead of its own rl.
 */
struct request_list *__blk_queue_next_rl(struct request_list *rl,
                     struct request_queue *q)
{
    struct list_head *ent;
    struct blkcg_gq *blkg;

    /*
     * Determine the current blkg list_head.  The first entry is
     * root_rl which is off @q->blkg_list and mapped to the head.
     */
    if (rl == &q->root_rl) {
        ent = &q->blkg_list;
        /* There are no more block groups, hence no request lists */
        if (list_empty(ent))
            return NULL;
    } else {
        blkg = container_of(rl, struct blkcg_gq, rl);
        ent = &blkg->q_node;
    }

    /* walk to the next list_head, skip root blkcg */
    ent = ent->next;
    if (ent == &q->root_blkg->q_node)
        ent = ent->next;
    if (ent == &q->blkg_list)
        return NULL;

    blkg = container_of(ent, struct blkcg_gq, q_node);
    return &blkg->rl;
}

static int blkcg_reset_stats(struct cgroup *cgroup, struct cftype *cftype,
                 u64 val)
{
    struct blkcg *blkcg = cgroup_to_blkcg(cgroup);
    struct blkcg_gq *blkg;
    struct hlist_node *n;
    int i;

    mutex_lock(&blkcg_pol_mutex);
    spin_lock_irq(&blkcg->lock);

    /*
     * Note that stat reset is racy - it doesn't synchronize against
     * stat updates.  This is a debug feature which shouldn't exist
     * anyway.  If you get hit by a race, retry.
     */
    hlist_for_each_entry(blkg, n, &blkcg->blkg_list, blkcg_node) {
        for (i = 0; i < BLKCG_MAX_POLS; i++) {
            struct blkcg_policy *pol = blkcg_policy[i];

            if (blkcg_policy_enabled(blkg->q, pol) &&
                pol->pd_reset_stats_fn)
                pol->pd_reset_stats_fn(blkg);
        }
    }

    spin_unlock_irq(&blkcg->lock);
    mutex_unlock(&blkcg_pol_mutex);
    return 0;
}

static const char *blkg_dev_name(struct blkcg_gq *blkg)
{
    /* some drivers (floppy) instantiate a queue w/o disk registered */
    if (blkg->q->backing_dev_info.dev)
        return dev_name(blkg->q->backing_dev_info.dev);
    return NULL;
}

void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
               u64 (*prfill)(struct seq_file *,
                     struct blkg_policy_data *, int),
               const struct blkcg_policy *pol, int data,
               bool show_total)
{
    struct blkcg_gq *blkg;
    struct hlist_node *n;
    u64 total = 0;

    spin_lock_irq(&blkcg->lock);
    hlist_for_each_entry(blkg, n, &blkcg->blkg_list, blkcg_node)
        if (blkcg_policy_enabled(blkg->q, pol))
            total += prfill(sf, blkg->pd[pol->plid], data);
    spin_unlock_irq(&blkcg->lock);

    if (show_total)
        seq_printf(sf, "Total %llu\n", (unsigned long long)total);
}
EXPORT_SYMBOL_GPL(blkcg_print_blkgs);

u64 __blkg_prfill_u64(struct seq_file *sf, struct blkg_policy_data *pd, u64 v)
{
    const char *dname = blkg_dev_name(pd->blkg);

    if (!dname)
        return 0;

    seq_printf(sf, "%s %llu\n", dname, (unsigned long long)v);
    return v;
}
EXPORT_SYMBOL_GPL(__blkg_prfill_u64);

u64 __blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
             const struct blkg_rwstat *rwstat)
{
    static const char *rwstr[] = {
        [BLKG_RWSTAT_READ]  = "Read",
        [BLKG_RWSTAT_WRITE] = "Write",
        [BLKG_RWSTAT_SYNC]  = "Sync",
        [BLKG_RWSTAT_ASYNC] = "Async",
    };
    const char *dname = blkg_dev_name(pd->blkg);
    u64 v;
    int i;

    if (!dname)
        return 0;

    for (i = 0; i < BLKG_RWSTAT_NR; i++)
        seq_printf(sf, "%s %s %llu\n", dname, rwstr[i],
               (unsigned long long)rwstat->cnt[i]);

    v = rwstat->cnt[BLKG_RWSTAT_READ] + rwstat->cnt[BLKG_RWSTAT_WRITE];
    seq_printf(sf, "%s Total %llu\n", dname, (unsigned long long)v);
    return v;
}

u64 blkg_prfill_stat(struct seq_file *sf, struct blkg_policy_data *pd, int off)
{
    return __blkg_prfill_u64(sf, pd, blkg_stat_read((void *)pd + off));
}
EXPORT_SYMBOL_GPL(blkg_prfill_stat);

u64 blkg_prfill_rwstat(struct seq_file *sf, struct blkg_policy_data *pd,
               int off)
{
    struct blkg_rwstat rwstat = blkg_rwstat_read((void *)pd + off);

    return __blkg_prfill_rwstat(sf, pd, &rwstat);
}
EXPORT_SYMBOL_GPL(blkg_prfill_rwstat);

int blkg_conf_prep(struct blkcg *blkcg, const struct blkcg_policy *pol,
           const char *input, struct blkg_conf_ctx *ctx)
    __acquires(rcu) __acquires(disk->queue->queue_lock)
{
    struct gendisk *disk;
    struct blkcg_gq *blkg;
    unsigned int major, minor;
    unsigned long long v;
    int part, ret;

    if (sscanf(input, "%u:%u %llu", &major, &minor, &v) != 3)
        return -EINVAL;

    disk = get_gendisk(MKDEV(major, minor), &part);
    if (!disk || part)
        return -EINVAL;

    rcu_read_lock();
    spin_lock_irq(disk->queue->queue_lock);

    if (blkcg_policy_enabled(disk->queue, pol))
        blkg = blkg_lookup_create(blkcg, disk->queue);
    else
        blkg = ERR_PTR(-EINVAL);

    if (IS_ERR(blkg)) {
        ret = PTR_ERR(blkg);
        rcu_read_unlock();
        spin_unlock_irq(disk->queue->queue_lock);
        put_disk(disk);
        /*
         * If queue was bypassing, we should retry.  Do so after a
         * short msleep().  It isn't strictly necessary but queue
         * can be bypassing for some time and it's always nice to
         * avoid busy looping.
         */
        if (ret == -EBUSY) {
            msleep(10);
            ret = restart_syscall();
        }
        return ret;
    }

    ctx->disk = disk;
    ctx->blkg = blkg;
    ctx->v = v;
    return 0;
}
EXPORT_SYMBOL_GPL(blkg_conf_prep);

void blkg_conf_finish(struct blkg_conf_ctx *ctx)
    __releases(ctx->disk->queue->queue_lock) __releases(rcu)
{
    spin_unlock_irq(ctx->disk->queue->queue_lock);
    rcu_read_unlock();
    put_disk(ctx->disk);
}
EXPORT_SYMBOL_GPL(blkg_conf_finish);

struct cftype blkcg_files[] = {
    {
        .name = "reset_stats",
        .write_u64 = blkcg_reset_stats,
    },
    { } /* terminate */
};

static int blkcg_pre_destroy(struct cgroup *cgroup)
{
    struct blkcg *blkcg = cgroup_to_blkcg(cgroup);

    spin_lock_irq(&blkcg->lock);

    while (!hlist_empty(&blkcg->blkg_list)) {
        struct blkcg_gq *blkg = hlist_entry(blkcg->blkg_list.first,
                        struct blkcg_gq, blkcg_node);
        struct request_queue *q = blkg->q;

        if (spin_trylock(q->queue_lock)) {
            blkg_destroy(blkg);
            spin_unlock(q->queue_lock);
        } else {
            spin_unlock_irq(&blkcg->lock);
            cpu_relax();
            spin_lock_irq(&blkcg->lock);
        }
    }

    spin_unlock_irq(&blkcg->lock);
    return 0;
}

static void blkcg_destroy(struct cgroup *cgroup)
{
    struct blkcg *blkcg = cgroup_to_blkcg(cgroup);

    if (blkcg != &blkcg_root)
        kfree(blkcg);
}

static struct cgroup_subsys_state *blkcg_create(struct cgroup *cgroup)
{
    static atomic64_t id_seq = ATOMIC64_INIT(0);
    struct blkcg *blkcg;
    struct cgroup *parent = cgroup->parent;

    if (!parent) {
        blkcg = &blkcg_root;
        goto done;
    }

    blkcg = kzalloc(sizeof(*blkcg), GFP_KERNEL);
    if (!blkcg)
        return ERR_PTR(-ENOMEM);

    blkcg->cfq_weight = CFQ_WEIGHT_DEFAULT;
    blkcg->id = atomic64_inc_return(&id_seq); /* root is 0, start from 1 */
done:
    spin_lock_init(&blkcg->lock);
    INIT_RADIX_TREE(&blkcg->blkg_tree, GFP_ATOMIC);
    INIT_HLIST_HEAD(&blkcg->blkg_list);

    return &blkcg->css;
}

int blkcg_init_queue(struct request_queue *q)
{
    might_sleep();

    return blk_throtl_init(q);
}

void blkcg_drain_queue(struct request_queue *q)
{
    lockdep_assert_held(q->queue_lock);

    blk_throtl_drain(q);
}

void blkcg_exit_queue(struct request_queue *q)
{
    spin_lock_irq(q->queue_lock);
    blkg_destroy_all(q);
    spin_unlock_irq(q->queue_lock);

    blk_throtl_exit(q);
}

/*
 * We cannot support shared io contexts, as we have no mean to support
 * two tasks with the same ioc in two different groups without major rework
 * of the main cic data structures.  For now we allow a task to change
 * its cgroup only if it's the only owner of its ioc.
 */
static int blkcg_can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
{
    struct task_struct *task;
    struct io_context *ioc;
    int ret = 0;

    /* task_lock() is needed to avoid races with exit_io_context() */
    cgroup_taskset_for_each(task, cgrp, tset) {
        task_lock(task);
        ioc = task->io_context;
        if (ioc && atomic_read(&ioc->nr_tasks) > 1)
            ret = -EINVAL;
        task_unlock(task);
        if (ret)
            break;
    }
    return ret;
}

struct cgroup_subsys blkio_subsys = {
    .name = "blkio",
    .create = blkcg_create,
    .can_attach = blkcg_can_attach,
    .pre_destroy = blkcg_pre_destroy,
    .destroy = blkcg_destroy,
    .subsys_id = blkio_subsys_id,
    .base_cftypes = blkcg_files,
    .module = THIS_MODULE,

    /*
     * blkio subsystem is utterly broken in terms of hierarchy support.
     * It treats all cgroups equally regardless of where they're
     * located in the hierarchy - all cgroups are treated as if they're
     * right below the root.  Fix it and remove the following.
     */
    .broken_hierarchy = true,
};
EXPORT_SYMBOL_GPL(blkio_subsys);

int blkcg_activate_policy(struct request_queue *q,
              const struct blkcg_policy *pol)
{
    LIST_HEAD(pds);
    struct blkcg_gq *blkg;
    struct blkg_policy_data *pd, *n;
    int cnt = 0, ret;
    bool preloaded;

    if (blkcg_policy_enabled(q, pol))
        return 0;

    /* preallocations for root blkg */
    blkg = blkg_alloc(&blkcg_root, q, GFP_KERNEL);
    if (!blkg)
        return -ENOMEM;

    preloaded = !radix_tree_preload(GFP_KERNEL);

    blk_queue_bypass_start(q);

    /* make sure the root blkg exists and count the existing blkgs */
    spin_lock_irq(q->queue_lock);

    rcu_read_lock();
    blkg = __blkg_lookup_create(&blkcg_root, q, blkg);
    rcu_read_unlock();

    if (preloaded)
        radix_tree_preload_end();

    if (IS_ERR(blkg)) {
        ret = PTR_ERR(blkg);
        goto out_unlock;
    }
    q->root_blkg = blkg;
    q->root_rl.blkg = blkg;

    list_for_each_entry(blkg, &q->blkg_list, q_node)
        cnt++;

    spin_unlock_irq(q->queue_lock);

    /* allocate policy_data for all existing blkgs */
    while (cnt--) {
        pd = kzalloc_node(pol->pd_size, GFP_KERNEL, q->node);
        if (!pd) {
            ret = -ENOMEM;
            goto out_free;
        }
        list_add_tail(&pd->alloc_node, &pds);
    }

    /*
     * Install the allocated pds.  With @q bypassing, no new blkg
     * should have been created while the queue lock was dropped.
     */
    spin_lock_irq(q->queue_lock);

    list_for_each_entry(blkg, &q->blkg_list, q_node) {
        if (WARN_ON(list_empty(&pds))) {
            /* umm... this shouldn't happen, just abort */
            ret = -ENOMEM;
            goto out_unlock;
        }
        pd = list_first_entry(&pds, struct blkg_policy_data, alloc_node);
        list_del_init(&pd->alloc_node);

        /* grab blkcg lock too while installing @pd on @blkg */
        spin_lock(&blkg->blkcg->lock);

        blkg->pd[pol->plid] = pd;
        pd->blkg = blkg;
        pol->pd_init_fn(blkg);

        spin_unlock(&blkg->blkcg->lock);
    }

    __set_bit(pol->plid, q->blkcg_pols);
    ret = 0;
out_unlock:
    spin_unlock_irq(q->queue_lock);
out_free:
    blk_queue_bypass_end(q);
    list_for_each_entry_safe(pd, n, &pds, alloc_node)
        kfree(pd);
    return ret;
}
EXPORT_SYMBOL_GPL(blkcg_activate_policy);

void blkcg_deactivate_policy(struct request_queue *q,
                 const struct blkcg_policy *pol)
{
    struct blkcg_gq *blkg;

    if (!blkcg_policy_enabled(q, pol))
        return;

    blk_queue_bypass_start(q);
    spin_lock_irq(q->queue_lock);

    __clear_bit(pol->plid, q->blkcg_pols);

    /* if no policy is left, no need for blkgs - shoot them down */
    if (bitmap_empty(q->blkcg_pols, BLKCG_MAX_POLS))
        blkg_destroy_all(q);

    list_for_each_entry(blkg, &q->blkg_list, q_node) {
        /* grab blkcg lock too while removing @pd from @blkg */
        spin_lock(&blkg->blkcg->lock);

        if (pol->pd_exit_fn)
            pol->pd_exit_fn(blkg);

        kfree(blkg->pd[pol->plid]);
        blkg->pd[pol->plid] = NULL;

        spin_unlock(&blkg->blkcg->lock);
    }

    spin_unlock_irq(q->queue_lock);
    blk_queue_bypass_end(q);
}
EXPORT_SYMBOL_GPL(blkcg_deactivate_policy);

int blkcg_policy_register(struct blkcg_policy *pol)
{
    int i, ret;

    if (WARN_ON(pol->pd_size < sizeof(struct blkg_policy_data)))
        return -EINVAL;

    mutex_lock(&blkcg_pol_mutex);

    /* find an empty slot */
    ret = -ENOSPC;
    for (i = 0; i < BLKCG_MAX_POLS; i++)
        if (!blkcg_policy[i])
            break;
    if (i >= BLKCG_MAX_POLS)
        goto out_unlock;

    /* register and update blkgs */
    pol->plid = i;
    blkcg_policy[i] = pol;

    /* everything is in place, add intf files for the new policy */
    if (pol->cftypes)
        WARN_ON(cgroup_add_cftypes(&blkio_subsys, pol->cftypes));
    ret = 0;
out_unlock:
    mutex_unlock(&blkcg_pol_mutex);
    return ret;
}
EXPORT_SYMBOL_GPL(blkcg_policy_register);

void blkcg_policy_unregister(struct blkcg_policy *pol)
{
    mutex_lock(&blkcg_pol_mutex);

    if (WARN_ON(blkcg_policy[pol->plid] != pol))
        goto out_unlock;

    /* kill the intf files first */
    if (pol->cftypes)
        cgroup_rm_cftypes(&blkio_subsys, pol->cftypes);

    /* unregister and update blkgs */
    blkcg_policy[pol->plid] = NULL;
out_unlock:
    mutex_unlock(&blkcg_pol_mutex);
}
EXPORT_SYMBOL_GPL(blkcg_policy_unregister);