elevator.c

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/*
 *  Block device elevator/IO-scheduler.
 *
 *  Copyright (C) 2000 Andrea Arcangeli <[email protected]> SuSE
 *
 * 30042000 Jens Axboe <[email protected]> :
 *
 * Split the elevator a bit so that it is possible to choose a different
 * one or even write a new "plug in". There are three pieces:
 * - elevator_fn, inserts a new request in the queue list
 * - elevator_merge_fn, decides whether a new buffer can be merged with
 *   an existing request
 * - elevator_dequeue_fn, called when a request is taken off the active list
 *
 * 20082000 Dave Jones <[email protected]> :
 * Removed tests for max-bomb-segments, which was breaking elvtune
 *  when run without -bN
 *
 * Jens:
 * - Rework again to work with bio instead of buffer_heads
 * - loose bi_dev comparisons, partition handling is right now
 * - completely modularize elevator setup and teardown
 *
 */
#include <linux/kernel.h>
#include <linux/fs.h>
#include <linux/blkdev.h>
#include <linux/elevator.h>
#include <linux/bio.h>
#include <linux/module.h>
#include <linux/slab.h>
#include <linux/init.h>
#include <linux/compiler.h>
#include <linux/blktrace_api.h>
#include <linux/hash.h>
#include <linux/uaccess.h>

#include <trace/events/block.h>

#include "blk.h"
#include "blk-cgroup.h"

static DEFINE_SPINLOCK(elv_list_lock);
static LIST_HEAD(elv_list);

/*
 * Merge hash stuff.
 */
static const int elv_hash_shift = 6;
#define ELV_HASH_BLOCK(sec) ((sec) >> 3)
#define ELV_HASH_FN(sec)    \
        (hash_long(ELV_HASH_BLOCK((sec)), elv_hash_shift))
#define ELV_HASH_ENTRIES    (1 << elv_hash_shift)
#define rq_hash_key(rq)     (blk_rq_pos(rq) + blk_rq_sectors(rq))

/*
 * Query io scheduler to see if the current process issuing bio may be
 * merged with rq.
 */
static int elv_iosched_allow_merge(struct request *rq, struct bio *bio)
{
    struct request_queue *q = rq->q;
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_allow_merge_fn)
        return e->type->ops.elevator_allow_merge_fn(q, rq, bio);

    return 1;
}

/*
 * can we safely merge with this request?
 */
bool elv_rq_merge_ok(struct request *rq, struct bio *bio)
{
    if (!blk_rq_merge_ok(rq, bio))
        return 0;

    if (!elv_iosched_allow_merge(rq, bio))
        return 0;

    return 1;
}
EXPORT_SYMBOL(elv_rq_merge_ok);

static struct elevator_type *elevator_find(const char *name)
{
    struct elevator_type *e;

    list_for_each_entry(e, &elv_list, list) {
        if (!strcmp(e->elevator_name, name))
            return e;
    }

    return NULL;
}

static void elevator_put(struct elevator_type *e)
{
    module_put(e->elevator_owner);
}

static struct elevator_type *elevator_get(const char *name)
{
    struct elevator_type *e;

    spin_lock(&elv_list_lock);

    e = elevator_find(name);
    if (!e) {
        spin_unlock(&elv_list_lock);
        request_module("%s-iosched", name);
        spin_lock(&elv_list_lock);
        e = elevator_find(name);
    }

    if (e && !try_module_get(e->elevator_owner))
        e = NULL;

    spin_unlock(&elv_list_lock);

    return e;
}

static char chosen_elevator[ELV_NAME_MAX];

static int __init elevator_setup(char *str)
{
    /*
     * Be backwards-compatible with previous kernels, so users
     * won't get the wrong elevator.
     */
    strncpy(chosen_elevator, str, sizeof(chosen_elevator) - 1);
    return 1;
}

__setup("elevator=", elevator_setup);

static struct kobj_type elv_ktype;

static struct elevator_queue *elevator_alloc(struct request_queue *q,
                  struct elevator_type *e)
{
    struct elevator_queue *eq;
    int i;

    eq = kmalloc_node(sizeof(*eq), GFP_KERNEL | __GFP_ZERO, q->node);
    if (unlikely(!eq))
        goto err;

    eq->type = e;
    kobject_init(&eq->kobj, &elv_ktype);
    mutex_init(&eq->sysfs_lock);

    eq->hash = kmalloc_node(sizeof(struct hlist_head) * ELV_HASH_ENTRIES,
                    GFP_KERNEL, q->node);
    if (!eq->hash)
        goto err;

    for (i = 0; i < ELV_HASH_ENTRIES; i++)
        INIT_HLIST_HEAD(&eq->hash[i]);

    return eq;
err:
    kfree(eq);
    elevator_put(e);
    return NULL;
}

static void elevator_release(struct kobject *kobj)
{
    struct elevator_queue *e;

    e = container_of(kobj, struct elevator_queue, kobj);
    elevator_put(e->type);
    kfree(e->hash);
    kfree(e);
}

int elevator_init(struct request_queue *q, char *name)
{
    struct elevator_type *e = NULL;
    int err;

    if (unlikely(q->elevator))
        return 0;

    INIT_LIST_HEAD(&q->queue_head);
    q->last_merge = NULL;
    q->end_sector = 0;
    q->boundary_rq = NULL;

    if (name) {
        e = elevator_get(name);
        if (!e)
            return -EINVAL;
    }

    if (!e && *chosen_elevator) {
        e = elevator_get(chosen_elevator);
        if (!e)
            printk(KERN_ERR "I/O scheduler %s not found\n",
                            chosen_elevator);
    }

    if (!e) {
        e = elevator_get(CONFIG_DEFAULT_IOSCHED);
        if (!e) {
            printk(KERN_ERR
                "Default I/O scheduler not found. " \
                "Using noop.\n");
            e = elevator_get("noop");
        }
    }

    q->elevator = elevator_alloc(q, e);
    if (!q->elevator)
        return -ENOMEM;

    err = e->ops.elevator_init_fn(q);
    if (err) {
        kobject_put(&q->elevator->kobj);
        return err;
    }

    return 0;
}
EXPORT_SYMBOL(elevator_init);

void elevator_exit(struct elevator_queue *e)
{
    mutex_lock(&e->sysfs_lock);
    if (e->type->ops.elevator_exit_fn)
        e->type->ops.elevator_exit_fn(e);
    mutex_unlock(&e->sysfs_lock);

    kobject_put(&e->kobj);
}
EXPORT_SYMBOL(elevator_exit);

static inline void __elv_rqhash_del(struct request *rq)
{
    hlist_del_init(&rq->hash);
}

static void elv_rqhash_del(struct request_queue *q, struct request *rq)
{
    if (ELV_ON_HASH(rq))
        __elv_rqhash_del(rq);
}

static void elv_rqhash_add(struct request_queue *q, struct request *rq)
{
    struct elevator_queue *e = q->elevator;

    BUG_ON(ELV_ON_HASH(rq));
    hlist_add_head(&rq->hash, &e->hash[ELV_HASH_FN(rq_hash_key(rq))]);
}

static void elv_rqhash_reposition(struct request_queue *q, struct request *rq)
{
    __elv_rqhash_del(rq);
    elv_rqhash_add(q, rq);
}

static struct request *elv_rqhash_find(struct request_queue *q, sector_t offset)
{
    struct elevator_queue *e = q->elevator;
    struct hlist_head *hash_list = &e->hash[ELV_HASH_FN(offset)];
    struct hlist_node *entry, *next;
    struct request *rq;

    hlist_for_each_entry_safe(rq, entry, next, hash_list, hash) {
        BUG_ON(!ELV_ON_HASH(rq));

        if (unlikely(!rq_mergeable(rq))) {
            __elv_rqhash_del(rq);
            continue;
        }

        if (rq_hash_key(rq) == offset)
            return rq;
    }

    return NULL;
}

/*
 * RB-tree support functions for inserting/lookup/removal of requests
 * in a sorted RB tree.
 */
void elv_rb_add(struct rb_root *root, struct request *rq)
{
    struct rb_node **p = &root->rb_node;
    struct rb_node *parent = NULL;
    struct request *__rq;

    while (*p) {
        parent = *p;
        __rq = rb_entry(parent, struct request, rb_node);

        if (blk_rq_pos(rq) < blk_rq_pos(__rq))
            p = &(*p)->rb_left;
        else if (blk_rq_pos(rq) >= blk_rq_pos(__rq))
            p = &(*p)->rb_right;
    }

    rb_link_node(&rq->rb_node, parent, p);
    rb_insert_color(&rq->rb_node, root);
}
EXPORT_SYMBOL(elv_rb_add);

void elv_rb_del(struct rb_root *root, struct request *rq)
{
    BUG_ON(RB_EMPTY_NODE(&rq->rb_node));
    rb_erase(&rq->rb_node, root);
    RB_CLEAR_NODE(&rq->rb_node);
}
EXPORT_SYMBOL(elv_rb_del);

struct request *elv_rb_find(struct rb_root *root, sector_t sector)
{
    struct rb_node *n = root->rb_node;
    struct request *rq;

    while (n) {
        rq = rb_entry(n, struct request, rb_node);

        if (sector < blk_rq_pos(rq))
            n = n->rb_left;
        else if (sector > blk_rq_pos(rq))
            n = n->rb_right;
        else
            return rq;
    }

    return NULL;
}
EXPORT_SYMBOL(elv_rb_find);

/*
 * Insert rq into dispatch queue of q.  Queue lock must be held on
 * entry.  rq is sort instead into the dispatch queue. To be used by
 * specific elevators.
 */
void elv_dispatch_sort(struct request_queue *q, struct request *rq)
{
    sector_t boundary;
    struct list_head *entry;
    int stop_flags;

    if (q->last_merge == rq)
        q->last_merge = NULL;

    elv_rqhash_del(q, rq);

    q->nr_sorted--;

    boundary = q->end_sector;
    stop_flags = REQ_SOFTBARRIER | REQ_STARTED;
    list_for_each_prev(entry, &q->queue_head) {
        struct request *pos = list_entry_rq(entry);

        if ((rq->cmd_flags & REQ_DISCARD) !=
            (pos->cmd_flags & REQ_DISCARD))
            break;
        if (rq_data_dir(rq) != rq_data_dir(pos))
            break;
        if (pos->cmd_flags & stop_flags)
            break;
        if (blk_rq_pos(rq) >= boundary) {
            if (blk_rq_pos(pos) < boundary)
                continue;
        } else {
            if (blk_rq_pos(pos) >= boundary)
                break;
        }
        if (blk_rq_pos(rq) >= blk_rq_pos(pos))
            break;
    }

    list_add(&rq->queuelist, entry);
}
EXPORT_SYMBOL(elv_dispatch_sort);

/*
 * Insert rq into dispatch queue of q.  Queue lock must be held on
 * entry.  rq is added to the back of the dispatch queue. To be used by
 * specific elevators.
 */
void elv_dispatch_add_tail(struct request_queue *q, struct request *rq)
{
    if (q->last_merge == rq)
        q->last_merge = NULL;

    elv_rqhash_del(q, rq);

    q->nr_sorted--;

    q->end_sector = rq_end_sector(rq);
    q->boundary_rq = rq;
    list_add_tail(&rq->queuelist, &q->queue_head);
}
EXPORT_SYMBOL(elv_dispatch_add_tail);

int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
{
    struct elevator_queue *e = q->elevator;
    struct request *__rq;
    int ret;

    /*
     * Levels of merges:
     *  nomerges:  No merges at all attempted
     *  noxmerges: Only simple one-hit cache try
     *  merges:    All merge tries attempted
     */
    if (blk_queue_nomerges(q))
        return ELEVATOR_NO_MERGE;

    /*
     * First try one-hit cache.
     */
    if (q->last_merge && elv_rq_merge_ok(q->last_merge, bio)) {
        ret = blk_try_merge(q->last_merge, bio);
        if (ret != ELEVATOR_NO_MERGE) {
            *req = q->last_merge;
            return ret;
        }
    }

    if (blk_queue_noxmerges(q))
        return ELEVATOR_NO_MERGE;

    /*
     * See if our hash lookup can find a potential backmerge.
     */
    __rq = elv_rqhash_find(q, bio->bi_sector);
    if (__rq && elv_rq_merge_ok(__rq, bio)) {
        *req = __rq;
        return ELEVATOR_BACK_MERGE;
    }

    if (e->type->ops.elevator_merge_fn)
        return e->type->ops.elevator_merge_fn(q, req, bio);

    return ELEVATOR_NO_MERGE;
}

/*
 * Attempt to do an insertion back merge. Only check for the case where
 * we can append 'rq' to an existing request, so we can throw 'rq' away
 * afterwards.
 *
 * Returns true if we merged, false otherwise
 */
static bool elv_attempt_insert_merge(struct request_queue *q,
                     struct request *rq)
{
    struct request *__rq;

    if (blk_queue_nomerges(q))
        return false;

    /*
     * First try one-hit cache.
     */
    if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))
        return true;

    if (blk_queue_noxmerges(q))
        return false;

    /*
     * See if our hash lookup can find a potential backmerge.
     */
    __rq = elv_rqhash_find(q, blk_rq_pos(rq));
    if (__rq && blk_attempt_req_merge(q, __rq, rq))
        return true;

    return false;
}

void elv_merged_request(struct request_queue *q, struct request *rq, int type)
{
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_merged_fn)
        e->type->ops.elevator_merged_fn(q, rq, type);

    if (type == ELEVATOR_BACK_MERGE)
        elv_rqhash_reposition(q, rq);

    q->last_merge = rq;
}

void elv_merge_requests(struct request_queue *q, struct request *rq,
                 struct request *next)
{
    struct elevator_queue *e = q->elevator;
    const int next_sorted = next->cmd_flags & REQ_SORTED;

    if (next_sorted && e->type->ops.elevator_merge_req_fn)
        e->type->ops.elevator_merge_req_fn(q, rq, next);

    elv_rqhash_reposition(q, rq);

    if (next_sorted) {
        elv_rqhash_del(q, next);
        q->nr_sorted--;
    }

    q->last_merge = rq;
}

void elv_bio_merged(struct request_queue *q, struct request *rq,
            struct bio *bio)
{
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_bio_merged_fn)
        e->type->ops.elevator_bio_merged_fn(q, rq, bio);
}

void elv_requeue_request(struct request_queue *q, struct request *rq)
{
    /*
     * it already went through dequeue, we need to decrement the
     * in_flight count again
     */
    if (blk_account_rq(rq)) {
        q->in_flight[rq_is_sync(rq)]--;
        if (rq->cmd_flags & REQ_SORTED)
            elv_deactivate_rq(q, rq);
    }

    rq->cmd_flags &= ~REQ_STARTED;

    __elv_add_request(q, rq, ELEVATOR_INSERT_REQUEUE);
}

void elv_drain_elevator(struct request_queue *q)
{
    static int printed;

    lockdep_assert_held(q->queue_lock);

    while (q->elevator->type->ops.elevator_dispatch_fn(q, 1))
        ;
    if (q->nr_sorted && printed++ < 10) {
        printk(KERN_ERR "%s: forced dispatching is broken "
               "(nr_sorted=%u), please report this\n",
               q->elevator->type->elevator_name, q->nr_sorted);
    }
}

void __elv_add_request(struct request_queue *q, struct request *rq, int where)
{
    trace_block_rq_insert(q, rq);

    rq->q = q;

    if (rq->cmd_flags & REQ_SOFTBARRIER) {
        /* barriers are scheduling boundary, update end_sector */
        if (rq->cmd_type == REQ_TYPE_FS) {
            q->end_sector = rq_end_sector(rq);
            q->boundary_rq = rq;
        }
    } else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
            (where == ELEVATOR_INSERT_SORT ||
             where == ELEVATOR_INSERT_SORT_MERGE))
        where = ELEVATOR_INSERT_BACK;

    switch (where) {
    case ELEVATOR_INSERT_REQUEUE:
    case ELEVATOR_INSERT_FRONT:
        rq->cmd_flags |= REQ_SOFTBARRIER;
        list_add(&rq->queuelist, &q->queue_head);
        break;

    case ELEVATOR_INSERT_BACK:
        rq->cmd_flags |= REQ_SOFTBARRIER;
        elv_drain_elevator(q);
        list_add_tail(&rq->queuelist, &q->queue_head);
        /*
         * We kick the queue here for the following reasons.
         * - The elevator might have returned NULL previously
         *   to delay requests and returned them now.  As the
         *   queue wasn't empty before this request, ll_rw_blk
         *   won't run the queue on return, resulting in hang.
         * - Usually, back inserted requests won't be merged
         *   with anything.  There's no point in delaying queue
         *   processing.
         */
        __blk_run_queue(q);
        break;

    case ELEVATOR_INSERT_SORT_MERGE:
        /*
         * If we succeed in merging this request with one in the
         * queue already, we are done - rq has now been freed,
         * so no need to do anything further.
         */
        if (elv_attempt_insert_merge(q, rq))
            break;
    case ELEVATOR_INSERT_SORT:
        BUG_ON(rq->cmd_type != REQ_TYPE_FS);
        rq->cmd_flags |= REQ_SORTED;
        q->nr_sorted++;
        if (rq_mergeable(rq)) {
            elv_rqhash_add(q, rq);
            if (!q->last_merge)
                q->last_merge = rq;
        }

        /*
         * Some ioscheds (cfq) run q->request_fn directly, so
         * rq cannot be accessed after calling
         * elevator_add_req_fn.
         */
        q->elevator->type->ops.elevator_add_req_fn(q, rq);
        break;

    case ELEVATOR_INSERT_FLUSH:
        rq->cmd_flags |= REQ_SOFTBARRIER;
        blk_insert_flush(rq);
        break;
    default:
        printk(KERN_ERR "%s: bad insertion point %d\n",
               __func__, where);
        BUG();
    }
}
EXPORT_SYMBOL(__elv_add_request);

void elv_add_request(struct request_queue *q, struct request *rq, int where)
{
    unsigned long flags;

    spin_lock_irqsave(q->queue_lock, flags);
    __elv_add_request(q, rq, where);
    spin_unlock_irqrestore(q->queue_lock, flags);
}
EXPORT_SYMBOL(elv_add_request);

struct request *elv_latter_request(struct request_queue *q, struct request *rq)
{
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_latter_req_fn)
        return e->type->ops.elevator_latter_req_fn(q, rq);
    return NULL;
}

struct request *elv_former_request(struct request_queue *q, struct request *rq)
{
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_former_req_fn)
        return e->type->ops.elevator_former_req_fn(q, rq);
    return NULL;
}

int elv_set_request(struct request_queue *q, struct request *rq,
            struct bio *bio, gfp_t gfp_mask)
{
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_set_req_fn)
        return e->type->ops.elevator_set_req_fn(q, rq, bio, gfp_mask);
    return 0;
}

void elv_put_request(struct request_queue *q, struct request *rq)
{
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_put_req_fn)
        e->type->ops.elevator_put_req_fn(rq);
}

int elv_may_queue(struct request_queue *q, int rw)
{
    struct elevator_queue *e = q->elevator;

    if (e->type->ops.elevator_may_queue_fn)
        return e->type->ops.elevator_may_queue_fn(q, rw);

    return ELV_MQUEUE_MAY;
}

void elv_abort_queue(struct request_queue *q)
{
    struct request *rq;

    blk_abort_flushes(q);

    while (!list_empty(&q->queue_head)) {
        rq = list_entry_rq(q->queue_head.next);
        rq->cmd_flags |= REQ_QUIET;
        trace_block_rq_abort(q, rq);
        /*
         * Mark this request as started so we don't trigger
         * any debug logic in the end I/O path.
         */
        blk_start_request(rq);
        __blk_end_request_all(rq, -EIO);
    }
}
EXPORT_SYMBOL(elv_abort_queue);

void elv_completed_request(struct request_queue *q, struct request *rq)
{
    struct elevator_queue *e = q->elevator;

    /*
     * request is released from the driver, io must be done
     */
    if (blk_account_rq(rq)) {
        q->in_flight[rq_is_sync(rq)]--;
        if ((rq->cmd_flags & REQ_SORTED) &&
            e->type->ops.elevator_completed_req_fn)
            e->type->ops.elevator_completed_req_fn(q, rq);
    }
}

#define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)

static ssize_t
elv_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
{
    struct elv_fs_entry *entry = to_elv(attr);
    struct elevator_queue *e;
    ssize_t error;

    if (!entry->show)
        return -EIO;

    e = container_of(kobj, struct elevator_queue, kobj);
    mutex_lock(&e->sysfs_lock);
    error = e->type ? entry->show(e, page) : -ENOENT;
    mutex_unlock(&e->sysfs_lock);
    return error;
}

static ssize_t
elv_attr_store(struct kobject *kobj, struct attribute *attr,
           const char *page, size_t length)
{
    struct elv_fs_entry *entry = to_elv(attr);
    struct elevator_queue *e;
    ssize_t error;

    if (!entry->store)
        return -EIO;

    e = container_of(kobj, struct elevator_queue, kobj);
    mutex_lock(&e->sysfs_lock);
    error = e->type ? entry->store(e, page, length) : -ENOENT;
    mutex_unlock(&e->sysfs_lock);
    return error;
}

static const struct sysfs_ops elv_sysfs_ops = {
    .show   = elv_attr_show,
    .store  = elv_attr_store,
};

static struct kobj_type elv_ktype = {
    .sysfs_ops  = &elv_sysfs_ops,
    .release    = elevator_release,
};

int elv_register_queue(struct request_queue *q)
{
    struct elevator_queue *e = q->elevator;
    int error;

    error = kobject_add(&e->kobj, &q->kobj, "%s", "iosched");
    if (!error) {
        struct elv_fs_entry *attr = e->type->elevator_attrs;
        if (attr) {
            while (attr->attr.name) {
                if (sysfs_create_file(&e->kobj, &attr->attr))
                    break;
                attr++;
            }
        }
        kobject_uevent(&e->kobj, KOBJ_ADD);
        e->registered = 1;
    }
    return error;
}
EXPORT_SYMBOL(elv_register_queue);

void elv_unregister_queue(struct request_queue *q)
{
    if (q) {
        struct elevator_queue *e = q->elevator;

        kobject_uevent(&e->kobj, KOBJ_REMOVE);
        kobject_del(&e->kobj);
        e->registered = 0;
    }
}
EXPORT_SYMBOL(elv_unregister_queue);

int elv_register(struct elevator_type *e)
{
    char *def = "";

    /* create icq_cache if requested */
    if (e->icq_size) {
        if (WARN_ON(e->icq_size < sizeof(struct io_cq)) ||
            WARN_ON(e->icq_align < __alignof__(struct io_cq)))
            return -EINVAL;

        snprintf(e->icq_cache_name, sizeof(e->icq_cache_name),
             "%s_io_cq", e->elevator_name);
        e->icq_cache = kmem_cache_create(e->icq_cache_name, e->icq_size,
                         e->icq_align, 0, NULL);
        if (!e->icq_cache)
            return -ENOMEM;
    }

    /* register, don't allow duplicate names */
    spin_lock(&elv_list_lock);
    if (elevator_find(e->elevator_name)) {
        spin_unlock(&elv_list_lock);
        if (e->icq_cache)
            kmem_cache_destroy(e->icq_cache);
        return -EBUSY;
    }
    list_add_tail(&e->list, &elv_list);
    spin_unlock(&elv_list_lock);

    /* print pretty message */
    if (!strcmp(e->elevator_name, chosen_elevator) ||
            (!*chosen_elevator &&
             !strcmp(e->elevator_name, CONFIG_DEFAULT_IOSCHED)))
                def = " (default)";

    printk(KERN_INFO "io scheduler %s registered%s\n", e->elevator_name,
                                def);
    return 0;
}
EXPORT_SYMBOL_GPL(elv_register);

void elv_unregister(struct elevator_type *e)
{
    /* unregister */
    spin_lock(&elv_list_lock);
    list_del_init(&e->list);
    spin_unlock(&elv_list_lock);

    /*
     * Destroy icq_cache if it exists.  icq's are RCU managed.  Make
     * sure all RCU operations are complete before proceeding.
     */
    if (e->icq_cache) {
        rcu_barrier();
        kmem_cache_destroy(e->icq_cache);
        e->icq_cache = NULL;
    }
}
EXPORT_SYMBOL_GPL(elv_unregister);

/*
 * switch to new_e io scheduler. be careful not to introduce deadlocks -
 * we don't free the old io scheduler, before we have allocated what we
 * need for the new one. this way we have a chance of going back to the old
 * one, if the new one fails init for some reason.
 */
static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
{
    struct elevator_queue *old = q->elevator;
    bool registered = old->registered;
    int err;

    /*
     * Turn on BYPASS and drain all requests w/ elevator private data.
     * Block layer doesn't call into a quiesced elevator - all requests
     * are directly put on the dispatch list without elevator data
     * using INSERT_BACK.  All requests have SOFTBARRIER set and no
     * merge happens either.
     */
    blk_queue_bypass_start(q);

    /* unregister and clear all auxiliary data of the old elevator */
    if (registered)
        elv_unregister_queue(q);

    spin_lock_irq(q->queue_lock);
    ioc_clear_queue(q);
    spin_unlock_irq(q->queue_lock);

    /* allocate, init and register new elevator */
    err = -ENOMEM;
    q->elevator = elevator_alloc(q, new_e);
    if (!q->elevator)
        goto fail_init;

    err = new_e->ops.elevator_init_fn(q);
    if (err) {
        kobject_put(&q->elevator->kobj);
        goto fail_init;
    }

    if (registered) {
        err = elv_register_queue(q);
        if (err)
            goto fail_register;
    }

    /* done, kill the old one and finish */
    elevator_exit(old);
    blk_queue_bypass_end(q);

    blk_add_trace_msg(q, "elv switch: %s", new_e->elevator_name);

    return 0;

fail_register:
    elevator_exit(q->elevator);
fail_init:
    /* switch failed, restore and re-register old elevator */
    q->elevator = old;
    elv_register_queue(q);
    blk_queue_bypass_end(q);

    return err;
}

/*
 * Switch this queue to the given IO scheduler.
 */
int elevator_change(struct request_queue *q, const char *name)
{
    char elevator_name[ELV_NAME_MAX];
    struct elevator_type *e;

    if (!q->elevator)
        return -ENXIO;

    strlcpy(elevator_name, name, sizeof(elevator_name));
    e = elevator_get(strstrip(elevator_name));
    if (!e) {
        printk(KERN_ERR "elevator: type %s not found\n", elevator_name);
        return -EINVAL;
    }

    if (!strcmp(elevator_name, q->elevator->type->elevator_name)) {
        elevator_put(e);
        return 0;
    }

    return elevator_switch(q, e);
}
EXPORT_SYMBOL(elevator_change);

ssize_t elv_iosched_store(struct request_queue *q, const char *name,
              size_t count)
{
    int ret;

    if (!q->elevator)
        return count;

    ret = elevator_change(q, name);
    if (!ret)
        return count;

    printk(KERN_ERR "elevator: switch to %s failed\n", name);
    return ret;
}

ssize_t elv_iosched_show(struct request_queue *q, char *name)
{
    struct elevator_queue *e = q->elevator;
    struct elevator_type *elv;
    struct elevator_type *__e;
    int len = 0;

    if (!q->elevator || !blk_queue_stackable(q))
        return sprintf(name, "none\n");

    elv = e->type;

    spin_lock(&elv_list_lock);
    list_for_each_entry(__e, &elv_list, list) {
        if (!strcmp(elv->elevator_name, __e->elevator_name))
            len += sprintf(name+len, "[%s] ", elv->elevator_name);
        else
            len += sprintf(name+len, "%s ", __e->elevator_name);
    }
    spin_unlock(&elv_list_lock);

    len += sprintf(len+name, "\n");
    return len;
}

struct request *elv_rb_former_request(struct request_queue *q,
                      struct request *rq)
{
    struct rb_node *rbprev = rb_prev(&rq->rb_node);

    if (rbprev)
        return rb_entry_rq(rbprev);

    return NULL;
}
EXPORT_SYMBOL(elv_rb_former_request);

struct request *elv_rb_latter_request(struct request_queue *q,
                      struct request *rq)
{
    struct rb_node *rbnext = rb_next(&rq->rb_node);

    if (rbnext)
        return rb_entry_rq(rbnext);

    return NULL;
}
EXPORT_SYMBOL(elv_rb_latter_request);