blkcg: make blkcg_gq's hierarchical

[deliverable/linux.git] / block / blk-cgroup.c

/*
 * Common Block IO controller cgroup interface
 *
 * Based on ideas and code from CFQ, CFS and BFQ:
 * Copyright (C) 2003 Jens Axboe <axboe@kernel.dk>
 *
 * Copyright (C) 2008 Fabio Checconi <fabio@gandalf.sssup.it>
 *                    Paolo Valente <paolo.valente@unimore.it>
 *
 * Copyright (C) 2009 Vivek Goyal <vgoyal@redhat.com>
 *                    Nauman Rafique <nauman@google.com>
 */
#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);
}

/**
 * blkg_free - free a blkg
 * @blkg: blkg to free
 *
 * Free @blkg which may be partially allocated.
 */
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);
}

/**
 * blkg_alloc - allocate a blkg
 * @blkcg: block cgroup the new blkg is associated with
 * @q: request_queue the new blkg is associated with
 * @gfp_mask: allocation mask to use
 *
 * Allocate a new blkg assocating @blkcg and @q.
 */
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 (pol->pd_init_fn)
                        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, bool update_hint)
{
        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 the
         * hint can only be updated under queue_lock as otherwise @blkg
         * could have already been removed from blkg_tree.  The caller is
         * responsible for grabbing queue_lock if @update_hint.
         */
        blkg = radix_tree_lookup(&blkcg->blkg_tree, q->id);
        if (blkg && blkg->q == q) {
                if (update_hint) {
                        lockdep_assert_held(q->queue_lock);
                        rcu_assign_pointer(blkcg->blkg_hint, blkg);
                }
                return blkg;
        }

        return NULL;
}

/**
 * blkg_lookup - lookup blkg for the specified blkcg - q pair
 * @blkcg: blkcg of interest
 * @q: request_queue of interest
 *
 * Lookup blkg for the @blkcg - @q pair.  This function should be called
 * under RCU read lock and is guaranteed to return %NULL if @q is bypassing
 * - see blk_queue_bypass_start() for details.
 */
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, false);
}
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_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);

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

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

        /* link parent and insert */
        if (blkcg_parent(blkcg)) {
                blkg->parent = __blkg_lookup(blkcg_parent(blkcg), q, false);
                if (WARN_ON_ONCE(!blkg->parent)) {
                        blkg = ERR_PTR(-EINVAL);
                        goto err_put_css;
                }
                blkg_get(blkg->parent);
        }

        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 failed fully initialized, use the usual release path */
        blkg_put(blkg);
        return ERR_PTR(ret);

err_put_css:
        css_put(&blkcg->css);
err_free_blkg:
        blkg_free(new_blkg);
        return ERR_PTR(ret);
}

/**
 * blkg_lookup_create - lookup blkg, try to create one if not there
 * @blkcg: blkcg of interest
 * @q: request_queue of interest
 *
 * Lookup blkg for the @blkcg - @q pair.  If it doesn't exist, try to
 * create one.  blkg creation is performed recursively from blkcg_root such
 * that all non-root blkg's have access to the parent blkg.  This function
 * should be called under RCU read lock and @q->queue_lock.
 *
 * Returns pointer to the looked up or created blkg on success, ERR_PTR()
 * value on error.  If @q is dead, returns ERR_PTR(-EINVAL).  If @q is not
 * dead and bypassing, returns ERR_PTR(-EBUSY).
 */
struct blkcg_gq *blkg_lookup_create(struct blkcg *blkcg,
                                    struct request_queue *q)
{
        struct blkcg_gq *blkg;

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

        /*
         * 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_dying(q) ? -EINVAL : -EBUSY);

        blkg = __blkg_lookup(blkcg, q, true);
        if (blkg)
                return blkg;

        /*
         * Create blkgs walking down from blkcg_root to @blkcg, so that all
         * non-root blkgs have access to their parents.
         */
        while (true) {
                struct blkcg *pos = blkcg;
                struct blkcg *parent = blkcg_parent(blkcg);

                while (parent && !__blkg_lookup(parent, q, false)) {
                        pos = parent;
                        parent = blkcg_parent(parent);
                }

                blkg = blkg_create(pos, q, NULL);
                if (pos == blkcg || IS_ERR(blkg))
                        return blkg;
        }
}
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);
}

/**
 * blkg_destroy_all - destroy all blkgs associated with a request_queue
 * @q: request_queue of interest
 *
 * Destroy all blkgs associated with @q.
 */
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 blkcg and parent blkg refs this blkg has been holding */
        css_put(&blkg->blkcg->css);
        if (blkg->parent)
                blkg_put(blkg->parent);

        /*
         * 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;
}

/**
 * blkcg_print_blkgs - helper for printing per-blkg data
 * @sf: seq_file to print to
 * @blkcg: blkcg of interest
 * @prfill: fill function to print out a blkg
 * @pol: policy in question
 * @data: data to be passed to @prfill
 * @show_total: to print out sum of prfill return values or not
 *
 * This function invokes @prfill on each blkg of @blkcg if pd for the
 * policy specified by @pol exists.  @prfill is invoked with @sf, the
 * policy data and @data.  If @show_total is %true, the sum of the return
 * values from @prfill is printed with "Total" label at the end.
 *
 * This is to be used to construct print functions for
 * cftype->read_seq_string method.
 */
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);

/**
 * __blkg_prfill_u64 - prfill helper for a single u64 value
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @v: value to print
 *
 * Print @v to @sf for the device assocaited with @pd.
 */
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);

/**
 * __blkg_prfill_rwstat - prfill helper for a blkg_rwstat
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @rwstat: rwstat to print
 *
 * Print @rwstat to @sf for the device assocaited with @pd.
 */
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;
}

/**
 * blkg_prfill_stat - prfill callback for blkg_stat
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @off: offset to the blkg_stat in @pd
 *
 * prfill callback for printing a blkg_stat.
 */
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);

/**
 * blkg_prfill_rwstat - prfill callback for blkg_rwstat
 * @sf: seq_file to print to
 * @pd: policy private data of interest
 * @off: offset to the blkg_rwstat in @pd
 *
 * prfill callback for printing a blkg_rwstat.
 */
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);

/**
 * blkg_conf_prep - parse and prepare for per-blkg config update
 * @blkcg: target block cgroup
 * @pol: target policy
 * @input: input string
 * @ctx: blkg_conf_ctx to be filled
 *
 * Parse per-blkg config update from @input and initialize @ctx with the
 * result.  @ctx->blkg points to the blkg to be updated and @ctx->v the new
 * value.  This function returns with RCU read lock and queue lock held and
 * must be paired with blkg_conf_finish().
 */
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);

/**
 * blkg_conf_finish - finish up per-blkg config update
 * @ctx: blkg_conf_ctx intiailized by blkg_conf_prep()
 *
 * Finish up after per-blkg config update.  This function must be paired
 * with 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 */
};

/**
 * blkcg_css_offline - cgroup css_offline callback
 * @cgroup: cgroup of interest
 *
 * This function is called when @cgroup is about to go away and responsible
 * for shooting down all blkgs associated with @cgroup.  blkgs should be
 * removed while holding both q and blkcg locks.  As blkcg lock is nested
 * inside q lock, this function performs reverse double lock dancing.
 *
 * This is the blkcg counterpart of ioc_release_fn().
 */
static void blkcg_css_offline(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);
}

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

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

static struct cgroup_subsys_state *blkcg_css_alloc(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;
}

/**
 * blkcg_init_queue - initialize blkcg part of request queue
 * @q: request_queue to initialize
 *
 * Called from blk_alloc_queue_node(). Responsible for initializing blkcg
 * part of new request_queue @q.
 *
 * RETURNS:
 * 0 on success, -errno on failure.
 */
int blkcg_init_queue(struct request_queue *q)
{
        might_sleep();

        return blk_throtl_init(q);
}

/**
 * blkcg_drain_queue - drain blkcg part of request_queue
 * @q: request_queue to drain
 *
 * Called from blk_drain_queue().  Responsible for draining blkcg part.
 */
void blkcg_drain_queue(struct request_queue *q)
{
        lockdep_assert_held(q->queue_lock);

        blk_throtl_drain(q);
}

/**
 * blkcg_exit_queue - exit and release blkcg part of request_queue
 * @q: request_queue being released
 *
 * Called from blk_release_queue().  Responsible for exiting blkcg part.
 */
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",
        .css_alloc = blkcg_css_alloc,
        .css_offline = blkcg_css_offline,
        .css_free = blkcg_css_free,
        .can_attach = blkcg_can_attach,
        .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);

/**
 * blkcg_activate_policy - activate a blkcg policy on a request_queue
 * @q: request_queue of interest
 * @pol: blkcg policy to activate
 *
 * Activate @pol on @q.  Requires %GFP_KERNEL context.  @q goes through
 * bypass mode to populate its blkgs with policy_data for @pol.
 *
 * Activation happens with @q bypassed, so nobody would be accessing blkgs
 * from IO path.  Update of each blkg is protected by both queue and blkcg
 * locks so that holding either lock and testing blkcg_policy_enabled() is
 * always enough for dereferencing policy data.
 *
 * The caller is responsible for synchronizing [de]activations and policy
 * [un]registerations.  Returns 0 on success, -errno on failure.
 */
int blkcg_activate_policy(struct request_queue *q,
                          const struct blkcg_policy *pol)
{
        LIST_HEAD(pds);
        struct blkcg_gq *blkg, *new_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 */
        new_blkg = blkg_alloc(&blkcg_root, q, GFP_KERNEL);
        if (!new_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.  As
         * @q is bypassing at this point, blkg_lookup_create() can't be
         * used.  Open code it.
         */
        spin_lock_irq(q->queue_lock);

        rcu_read_lock();
        blkg = __blkg_lookup(&blkcg_root, q, false);
        if (blkg)
                blkg_free(new_blkg);
        else
                blkg = blkg_create(&blkcg_root, q, new_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);

/**
 * blkcg_deactivate_policy - deactivate a blkcg policy on a request_queue
 * @q: request_queue of interest
 * @pol: blkcg policy to deactivate
 *
 * Deactivate @pol on @q.  Follows the same synchronization rules as
 * 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);

/**
 * blkcg_policy_register - register a blkcg policy
 * @pol: blkcg policy to register
 *
 * Register @pol with blkcg core.  Might sleep and @pol may be modified on
 * successful registration.  Returns 0 on success and -errno on failure.
 */
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);

/**
 * blkcg_policy_unregister - unregister a blkcg policy
 * @pol: blkcg policy to unregister
 *
 * Undo blkcg_policy_register(@pol).  Might sleep.
 */
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);