text: /*
   *  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 <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.
   */
  #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, 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))
  		goto err;
  
  	eq->type = e;
  	kobject_init(&eq->kobj, &elv_ktype);
  	mutex_init(&eq->sysfs_lock);
  	hash_init(eq->hash);
  
  	return eq;
  err:
  	kfree(eq);
  	elevator_put(e);
  	return NULL;
  }
  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);
  	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)
  {
  	hash_del(&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));
  	hash_add(e->hash, &rq->hash, 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_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 ((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;
  	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_RUNTIME
  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 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 = 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);
  