block: do not merge requests without consulting with io scheduler

[mirror_ubuntu-artful-kernel.git] / block / elevator.c

/*
 *  Block device elevator/IO-scheduler.
 *
 *  Copyright (C) 2000 Andrea Arcangeli <andrea@suse.de> SuSE
 *
 * 30042000 Jens Axboe <axboe@kernel.dk> :
 *
 * 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 <davej@suse.de> :
 * 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 <linux/pm_runtime.h>
#include <linux/blk-cgroup.h>

#include <trace/events/block.h>

#include "blk.h"

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

/*
 * Merge hash stuff.
 */
#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_bio_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_bio_merge_fn)
                return e->type->ops.elevator_allow_bio_merge_fn(q, rq, bio);

        return 1;
}

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

        if (!elv_iosched_allow_bio_merge(rq, bio))
                return false;

        return true;
}
EXPORT_SYMBOL(elv_bio_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, bool try_loading)
{
        struct elevator_type *e;

        spin_lock(&elv_list_lock);

        e = elevator_find(name);
        if (!e && try_loading) {
                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);

/* called during boot to load the elevator chosen by the elevator param */
void __init load_default_elevator_module(void)
{
        struct elevator_type *e;

        if (!chosen_elevator[0])
                return;

        spin_lock(&elv_list_lock);
        e = elevator_find(chosen_elevator);
        spin_unlock(&elv_list_lock);

        if (!e)
                request_module("%s-iosched", chosen_elevator);
}

static struct kobj_type elv_ktype;

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

        eq = kzalloc_node(sizeof(*eq), GFP_KERNEL, q->node);
        if (unlikely(!eq))
                return NULL;

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

        return eq;
}
EXPORT_SYMBOL(elevator_alloc);

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);
}

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

        /*
         * q->sysfs_lock must be held to provide mutual exclusion between
         * elevator_switch() and here.
         */
        lockdep_assert_held(&q->sysfs_lock);

        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, true);
                if (!e)
                        return -EINVAL;
        }

        /*
         * Use the default elevator specified by config boot param or
         * config option.  Don't try to load modules as we could be running
         * off async and request_module() isn't allowed from async.
         */
        if (!e && *chosen_elevator) {
                e = elevator_get(chosen_elevator, false);
                if (!e)
                        printk(KERN_ERR "I/O scheduler %s not found\n",
                                                        chosen_elevator);
        }

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

        err = e->ops.elevator_init_fn(q, e);
        if (err)
                elevator_put(e);
        return err;
}
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)
{
        hash_del(&rq->hash);
        rq->cmd_flags &= ~REQ_HASHED;
}

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));
        hash_add(e->hash, &rq->hash, rq_hash_key(rq));
        rq->cmd_flags |= REQ_HASHED;
}

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_node *next;
        struct request *rq;

        hash_for_each_possible_safe(e->hash, rq, next, hash, offset) {
                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 ((req_op(rq) == REQ_OP_DISCARD) != (req_op(pos) == REQ_OP_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) || !bio_mergeable(bio))
                return ELEVATOR_NO_MERGE;

        /*
         * First try one-hit cache.
         */
        if (q->last_merge && elv_bio_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_iter.bi_sector);
        if (__rq && elv_bio_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;
        bool ret;

        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;

        ret = false;
        /*
         * See if our hash lookup can find a potential backmerge.
         */
        while (1) {
                __rq = elv_rqhash_find(q, blk_rq_pos(rq));
                if (!__rq || !blk_attempt_req_merge(q, __rq, rq))
                        break;

                /* The merged request could be merged with others, try again */
                ret = true;
                rq = __rq;
        }

        return ret;
}

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);
}

#ifdef CONFIG_PM
static void blk_pm_requeue_request(struct request *rq)
{
        if (rq->q->dev && !(rq->cmd_flags & REQ_PM))
                rq->q->nr_pending--;
}

static void blk_pm_add_request(struct request_queue *q, struct request *rq)
{
        if (q->dev && !(rq->cmd_flags & REQ_PM) && q->nr_pending++ == 0 &&
            (q->rpm_status == RPM_SUSPENDED || q->rpm_status == RPM_SUSPENDING))
                pm_request_resume(q->dev);
}
#else
static inline void blk_pm_requeue_request(struct request *rq) {}
static inline void blk_pm_add_request(struct request_queue *q,
                                      struct request *rq)
{
}
#endif

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;

        blk_pm_requeue_request(rq);

        __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);

        blk_pm_add_request(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 op, int op_flags)
{
        struct elevator_queue *e = q->elevator;

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

        return ELV_MQUEUE_MAY;
}

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;
                if (e->type->ops.elevator_registered_fn)
                        e->type->ops.elevator_registered_fn(q);
        }
        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 = new_e->ops.elevator_init_fn(q, new_e);
        if (err)
                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.
 */
static 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), true);
        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);
}

int elevator_change(struct request_queue *q, const char *name)
{
        int ret;

        /* Protect q->elevator from elevator_init() */
        mutex_lock(&q->sysfs_lock);
        ret = __elevator_change(q, name);
        mutex_unlock(&q->sysfs_lock);

        return ret;
}
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);