text: // SPDX-License-Identifier: GPL-2.0
  /*
   * blk-mq scheduling framework
   *
   * Copyright (C) 2016 Jens Axboe
   */
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/blk-mq.h>
  #include <linux/list_sort.h>
  
  #include <trace/events/block.h>
  
  #include "blk.h"
  #include "blk-mq.h"
  #include "blk-mq-debugfs.h"
  #include "blk-mq-sched.h"
  #include "blk-mq-tag.h"
  #include "blk-wbt.h"
  
  void blk_mq_sched_assign_ioc(struct request *rq)
  {
  	struct request_queue *q = rq->q;
  	struct io_context *ioc;
  	struct io_cq *icq;
  
  	/*
  	 * May not have an IO context if it's a passthrough request
  	 */
  	ioc = current->io_context;
  	if (!ioc)
  		return;
  
  	spin_lock_irq(&q->queue_lock);
  	icq = ioc_lookup_icq(ioc, q);
  	spin_unlock_irq(&q->queue_lock);
  
  	if (!icq) {
  		icq = ioc_create_icq(ioc, q, GFP_ATOMIC);
  		if (!icq)
  			return;
  	}
  	get_io_context(icq->ioc);
  	rq->elv.icq = icq;
  }
  
  /*
   * Mark a hardware queue as needing a restart. For shared queues, maintain
   * a count of how many hardware queues are marked for restart.
   */
  void blk_mq_sched_mark_restart_hctx(struct blk_mq_hw_ctx *hctx)
  {
  	if (test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
  		return;
  
  	set_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
  }
  EXPORT_SYMBOL_GPL(blk_mq_sched_mark_restart_hctx);
  
  void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
  {
  	if (!test_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state))
  		return;
  	clear_bit(BLK_MQ_S_SCHED_RESTART, &hctx->state);
  
  	/*
  	 * Order clearing SCHED_RESTART and list_empty_careful(&hctx->dispatch)
  	 * in blk_mq_run_hw_queue(). Its pair is the barrier in
  	 * blk_mq_dispatch_rq_list(). So dispatch code won't see SCHED_RESTART,
  	 * meantime new request added to hctx->dispatch is missed to check in
  	 * blk_mq_run_hw_queue().
  	 */
  	smp_mb();
  
  	blk_mq_run_hw_queue(hctx, true);
  }
  
  static int sched_rq_cmp(void *priv, const struct list_head *a,
  			const struct list_head *b)
  {
  	struct request *rqa = container_of(a, struct request, queuelist);
  	struct request *rqb = container_of(b, struct request, queuelist);
  
  	return rqa->mq_hctx > rqb->mq_hctx;
  }
  
  static bool blk_mq_dispatch_hctx_list(struct list_head *rq_list)
  {
  	struct blk_mq_hw_ctx *hctx =
  		list_first_entry(rq_list, struct request, queuelist)->mq_hctx;
  	struct request *rq;
  	LIST_HEAD(hctx_list);
  	unsigned int count = 0;
  
  	list_for_each_entry(rq, rq_list, queuelist) {
  		if (rq->mq_hctx != hctx) {
  			list_cut_before(&hctx_list, rq_list, &rq->queuelist);
  			goto dispatch;
  		}
  		count++;
  	}
  	list_splice_tail_init(rq_list, &hctx_list);
  
  dispatch:
  	return blk_mq_dispatch_rq_list(hctx, &hctx_list, count);
  }
  
  #define BLK_MQ_BUDGET_DELAY	3		/* ms units */
  
  /*
   * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
   * its queue by itself in its completion handler, so we don't need to
   * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
   *
   * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
   * be run again.  This is necessary to avoid starving flushes.
   */
  static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
  {
  	struct request_queue *q = hctx->queue;
  	struct elevator_queue *e = q->elevator;
  	bool multi_hctxs = false, run_queue = false;
  	bool dispatched = false, busy = false;
  	unsigned int max_dispatch;
  	LIST_HEAD(rq_list);
  	int count = 0;
  
  	if (hctx->dispatch_busy)
  		max_dispatch = 1;
  	else
  		max_dispatch = hctx->queue->nr_requests;
  
  	do {
  		struct request *rq;
  		int budget_token;
  
  		if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
  			break;
  
  		if (!list_empty_careful(&hctx->dispatch)) {
  			busy = true;
  			break;
  		}
  
  		budget_token = blk_mq_get_dispatch_budget(q);
  		if (budget_token < 0)
  			break;
  
  		rq = e->type->ops.dispatch_request(hctx);
  		if (!rq) {
  			blk_mq_put_dispatch_budget(q, budget_token);
  			/*
  			 * We're releasing without dispatching. Holding the
  			 * budget could have blocked any "hctx"s with the
  			 * same queue and if we didn't dispatch then there's
  			 * no guarantee anyone will kick the queue.  Kick it
  			 * ourselves.
  			 */
  			run_queue = true;
  			break;
  		}
  
  		blk_mq_set_rq_budget_token(rq, budget_token);
  
  		/*
  		 * Now this rq owns the budget which has to be released
  		 * if this rq won't be queued to driver via .queue_rq()
  		 * in blk_mq_dispatch_rq_list().
  		 */
  		list_add_tail(&rq->queuelist, &rq_list);
  		count++;
  		if (rq->mq_hctx != hctx)
  			multi_hctxs = true;
  
  		/*
  		 * If we cannot get tag for the request, stop dequeueing
  		 * requests from the IO scheduler. We are unlikely to be able
  		 * to submit them anyway and it creates false impression for
  		 * scheduling heuristics that the device can take more IO.
  		 */
  		if (!blk_mq_get_driver_tag(rq))
  			break;
  	} while (count < max_dispatch);
  
  	if (!count) {
  		if (run_queue)
  			blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
  	} else if (multi_hctxs) {
  		/*
  		 * Requests from different hctx may be dequeued from some
  		 * schedulers, such as bfq and deadline.
  		 *
  		 * Sort the requests in the list according to their hctx,
  		 * dispatch batching requests from same hctx at a time.
  		 */
  		list_sort(NULL, &rq_list, sched_rq_cmp);
  		do {
  			dispatched |= blk_mq_dispatch_hctx_list(&rq_list);
  		} while (!list_empty(&rq_list));
  	} else {
  		dispatched = blk_mq_dispatch_rq_list(hctx, &rq_list, count);
  	}
  
  	if (busy)
  		return -EAGAIN;
  	return !!dispatched;
  }
  
  static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
  {
  	int ret;
  
  	do {
  		ret = __blk_mq_do_dispatch_sched(hctx);
  	} while (ret == 1);
  
  	return ret;
  }
  
  static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
  					  struct blk_mq_ctx *ctx)
  {
  	unsigned short idx = ctx->index_hw[hctx->type];
  
  	if (++idx == hctx->nr_ctx)
  		idx = 0;
  
  	return hctx->ctxs[idx];
  }
  
  /*
   * Only SCSI implements .get_budget and .put_budget, and SCSI restarts
   * its queue by itself in its completion handler, so we don't need to
   * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
   *
   * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
   * be run again.  This is necessary to avoid starving flushes.
   */
  static int blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
  {
  	struct request_queue *q = hctx->queue;
  	LIST_HEAD(rq_list);
  	struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
  	int ret = 0;
  	struct request *rq;
  
  	do {
  		int budget_token;
  
  		if (!list_empty_careful(&hctx->dispatch)) {
  			ret = -EAGAIN;
  			break;
  		}
  
  		if (!sbitmap_any_bit_set(&hctx->ctx_map))
  			break;
  
  		budget_token = blk_mq_get_dispatch_budget(q);
  		if (budget_token < 0)
  			break;
  
  		rq = blk_mq_dequeue_from_ctx(hctx, ctx);
  		if (!rq) {
  			blk_mq_put_dispatch_budget(q, budget_token);
  			/*
  			 * We're releasing without dispatching. Holding the
  			 * budget could have blocked any "hctx"s with the
  			 * same queue and if we didn't dispatch then there's
  			 * no guarantee anyone will kick the queue.  Kick it
  			 * ourselves.
  			 */
  			blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
  			break;
  		}
  
  		blk_mq_set_rq_budget_token(rq, budget_token);
  
  		/*
  		 * Now this rq owns the budget which has to be released
  		 * if this rq won't be queued to driver via .queue_rq()
  		 * in blk_mq_dispatch_rq_list().
  		 */
  		list_add(&rq->queuelist, &rq_list);
  
  		/* round robin for fair dispatch */
  		ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
  
  	} while (blk_mq_dispatch_rq_list(rq->mq_hctx, &rq_list, 1));
  
  	WRITE_ONCE(hctx->dispatch_from, ctx);
  	return ret;
  }
  
  static int __blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
  {
  	struct request_queue *q = hctx->queue;
  	const bool has_sched = q->elevator;
  	int ret = 0;
  	LIST_HEAD(rq_list);
  
  	/*
  	 * If we have previous entries on our dispatch list, grab them first for
  	 * more fair dispatch.
  	 */
  	if (!list_empty_careful(&hctx->dispatch)) {
  		spin_lock(&hctx->lock);
  		if (!list_empty(&hctx->dispatch))
  			list_splice_init(&hctx->dispatch, &rq_list);
  		spin_unlock(&hctx->lock);
  	}
  
  	/*
  	 * Only ask the scheduler for requests, if we didn't have residual
  	 * requests from the dispatch list. This is to avoid the case where
  	 * we only ever dispatch a fraction of the requests available because
  	 * of low device queue depth. Once we pull requests out of the IO
  	 * scheduler, we can no longer merge or sort them. So it's best to
  	 * leave them there for as long as we can. Mark the hw queue as
  	 * needing a restart in that case.
  	 *
  	 * We want to dispatch from the scheduler if there was nothing
  	 * on the dispatch list or we were able to dispatch from the
  	 * dispatch list.
  	 */
  	if (!list_empty(&rq_list)) {
  		blk_mq_sched_mark_restart_hctx(hctx);
  		if (blk_mq_dispatch_rq_list(hctx, &rq_list, 0)) {
  			if (has_sched)
  				ret = blk_mq_do_dispatch_sched(hctx);
  			else
  				ret = blk_mq_do_dispatch_ctx(hctx);
  		}
  	} else if (has_sched) {
  		ret = blk_mq_do_dispatch_sched(hctx);
  	} else if (hctx->dispatch_busy) {
  		/* dequeue request one by one from sw queue if queue is busy */
  		ret = blk_mq_do_dispatch_ctx(hctx);
  	} else {
  		blk_mq_flush_busy_ctxs(hctx, &rq_list);
  		blk_mq_dispatch_rq_list(hctx, &rq_list, 0);
  	}
  
  	return ret;
  }
  
  void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
  {
  	struct request_queue *q = hctx->queue;
  
  	/* RCU or SRCU read lock is needed before checking quiesced flag */
  	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
  		return;
  
  	hctx->run++;
  
  	/*
  	 * A return of -EAGAIN is an indication that hctx->dispatch is not
  	 * empty and we must run again in order to avoid starving flushes.
  	 */
  	if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) {
  		if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN)
  			blk_mq_run_hw_queue(hctx, true);
  	}
  }
  
  bool __blk_mq_sched_bio_merge(struct request_queue *q, struct bio *bio,
  		unsigned int nr_segs)
  {
  	struct elevator_queue *e = q->elevator;
  	struct blk_mq_ctx *ctx;
  	struct blk_mq_hw_ctx *hctx;
  	bool ret = false;
  	enum hctx_type type;
  
  	if (e && e->type->ops.bio_merge)
  		return e->type->ops.bio_merge(q, bio, nr_segs);
  
  	ctx = blk_mq_get_ctx(q);
  	hctx = blk_mq_map_queue(q, bio->bi_opf, ctx);
  	type = hctx->type;
  	if (!(hctx->flags & BLK_MQ_F_SHOULD_MERGE) ||
  	    list_empty_careful(&ctx->rq_lists[type]))
  		return false;
  
  	/* default per sw-queue merge */
  	spin_lock(&ctx->lock);
  	/*
  	 * Reverse check our software queue for entries that we could
  	 * potentially merge with. Currently includes a hand-wavy stop
  	 * count of 8, to not spend too much time checking for merges.
  	 */
  	if (blk_bio_list_merge(q, &ctx->rq_lists[type], bio, nr_segs)) {
  		ctx->rq_merged++;
  		ret = true;
  	}
  
  	spin_unlock(&ctx->lock);
  
  	return ret;
  }
  
  bool blk_mq_sched_try_insert_merge(struct request_queue *q, struct request *rq,
  				   struct list_head *free)
  {
  	return rq_mergeable(rq) && elv_attempt_insert_merge(q, rq, free);
  }
  EXPORT_SYMBOL_GPL(blk_mq_sched_try_insert_merge);
  
  static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
  				       struct request *rq)
  {
  	/*
  	 * dispatch flush and passthrough rq directly
  	 *
  	 * passthrough request has to be added to hctx->dispatch directly.
  	 * For some reason, device may be in one situation which can't
  	 * handle FS request, so STS_RESOURCE is always returned and the
  	 * FS request will be added to hctx->dispatch. However passthrough
  	 * request may be required at that time for fixing the problem. If
  	 * passthrough request is added to scheduler queue, there isn't any
  	 * chance to dispatch it given we prioritize requests in hctx->dispatch.
  	 */
  	if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq))
  		return true;
  
  	return false;
  }
  
  void blk_mq_sched_insert_request(struct request *rq, bool at_head,
  				 bool run_queue, bool async)
  {
  	struct request_queue *q = rq->q;
  	struct elevator_queue *e = q->elevator;
  	struct blk_mq_ctx *ctx = rq->mq_ctx;
  	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
  
  	WARN_ON(e && (rq->tag != BLK_MQ_NO_TAG));
  
  	if (blk_mq_sched_bypass_insert(hctx, rq)) {
  		/*
  		 * Firstly normal IO request is inserted to scheduler queue or
  		 * sw queue, meantime we add flush request to dispatch queue(
  		 * hctx->dispatch) directly and there is at most one in-flight
  		 * flush request for each hw queue, so it doesn't matter to add
  		 * flush request to tail or front of the dispatch queue.
  		 *
  		 * Secondly in case of NCQ, flush request belongs to non-NCQ
  		 * command, and queueing it will fail when there is any
  		 * in-flight normal IO request(NCQ command). When adding flush
  		 * rq to the front of hctx->dispatch, it is easier to introduce
  		 * extra time to flush rq's latency because of S_SCHED_RESTART
  		 * compared with adding to the tail of dispatch queue, then
  		 * chance of flush merge is increased, and less flush requests
  		 * will be issued to controller. It is observed that ~10% time
  		 * is saved in blktests block/004 on disk attached to AHCI/NCQ
  		 * drive when adding flush rq to the front of hctx->dispatch.
  		 *
  		 * Simply queue flush rq to the front of hctx->dispatch so that
  		 * intensive flush workloads can benefit in case of NCQ HW.
  		 */
  		at_head = (rq->rq_flags & RQF_FLUSH_SEQ) ? true : at_head;
  		blk_mq_request_bypass_insert(rq, at_head, false);
  		goto run;
  	}
  
  	if (e) {
  		LIST_HEAD(list);
  
  		list_add(&rq->queuelist, &list);
  		e->type->ops.insert_requests(hctx, &list, at_head);
  	} else {
  		spin_lock(&ctx->lock);
  		__blk_mq_insert_request(hctx, rq, at_head);
  		spin_unlock(&ctx->lock);
  	}
  
  run:
  	if (run_queue)
  		blk_mq_run_hw_queue(hctx, async);
  }
  
  void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx,
  				  struct blk_mq_ctx *ctx,
  				  struct list_head *list, bool run_queue_async)
  {
  	struct elevator_queue *e;
  	struct request_queue *q = hctx->queue;
  
  	/*
  	 * blk_mq_sched_insert_requests() is called from flush plug
  	 * context only, and hold one usage counter to prevent queue
  	 * from being released.
  	 */
  	percpu_ref_get(&q->q_usage_counter);
  
  	e = hctx->queue->elevator;
  	if (e) {
  		e->type->ops.insert_requests(hctx, list, false);
  	} else {
  		/*
  		 * try to issue requests directly if the hw queue isn't
  		 * busy in case of 'none' scheduler, and this way may save
  		 * us one extra enqueue & dequeue to sw queue.
  		 */
  		if (!hctx->dispatch_busy && !e && !run_queue_async) {
  			blk_mq_try_issue_list_directly(hctx, list);
  			if (list_empty(list))
  				goto out;
  		}
  		blk_mq_insert_requests(hctx, ctx, list);
  	}
  
  	blk_mq_run_hw_queue(hctx, run_queue_async);
   out:
  	percpu_ref_put(&q->q_usage_counter);
  }
  
  static int blk_mq_sched_alloc_tags(struct request_queue *q,
  				   struct blk_mq_hw_ctx *hctx,
  				   unsigned int hctx_idx)
  {
  	struct blk_mq_tag_set *set = q->tag_set;
  	int ret;
  
  	hctx->sched_tags = blk_mq_alloc_rq_map(set, hctx_idx, q->nr_requests,
  					       set->reserved_tags, set->flags);
  	if (!hctx->sched_tags)
  		return -ENOMEM;
  
  	ret = blk_mq_alloc_rqs(set, hctx->sched_tags, hctx_idx, q->nr_requests);
  	if (ret) {
  		blk_mq_free_rq_map(hctx->sched_tags, set->flags);
  		hctx->sched_tags = NULL;
  	}
  
  	return ret;
  }
  
  /* called in queue's release handler, tagset has gone away */
  static void blk_mq_sched_tags_teardown(struct request_queue *q)
  {
  	struct blk_mq_hw_ctx *hctx;
  	int i;
  
  	queue_for_each_hw_ctx(q, hctx, i) {
  		if (hctx->sched_tags) {
  			blk_mq_free_rq_map(hctx->sched_tags, hctx->flags);
  			hctx->sched_tags = NULL;
  		}
  	}
  }
  
  static int blk_mq_init_sched_shared_sbitmap(struct request_queue *queue)
  {
  	struct blk_mq_tag_set *set = queue->tag_set;
  	int alloc_policy = BLK_MQ_FLAG_TO_ALLOC_POLICY(set->flags);
  	struct blk_mq_hw_ctx *hctx;
  	int ret, i;
  
  	/*
  	 * Set initial depth at max so that we don't need to reallocate for
  	 * updating nr_requests.
  	 */
  	ret = blk_mq_init_bitmaps(&queue->sched_bitmap_tags,
  				  &queue->sched_breserved_tags,
  				  MAX_SCHED_RQ, set->reserved_tags,
  				  set->numa_node, alloc_policy);
  	if (ret)
  		return ret;
  
  	queue_for_each_hw_ctx(queue, hctx, i) {
  		hctx->sched_tags->bitmap_tags =
  					&queue->sched_bitmap_tags;
  		hctx->sched_tags->breserved_tags =
  					&queue->sched_breserved_tags;
  	}
  
  	sbitmap_queue_resize(&queue->sched_bitmap_tags,
  			     queue->nr_requests - set->reserved_tags);
  
  	return 0;
  }
  
  static void blk_mq_exit_sched_shared_sbitmap(struct request_queue *queue)
  {
  	sbitmap_queue_free(&queue->sched_bitmap_tags);
  	sbitmap_queue_free(&queue->sched_breserved_tags);
  }
  
  int blk_mq_init_sched(struct request_queue *q, struct elevator_type *e)
  {
  	struct blk_mq_hw_ctx *hctx;
  	struct elevator_queue *eq;
  	unsigned int i;
  	int ret;
  
  	if (!e) {
  		q->elevator = NULL;
  		q->nr_requests = q->tag_set->queue_depth;
  		return 0;
  	}
  
  	/*
  	 * Default to double of smaller one between hw queue_depth and 128,
  	 * since we don't split into sync/async like the old code did.
  	 * Additionally, this is a per-hw queue depth.
  	 */
  	q->nr_requests = 2 * min_t(unsigned int, q->tag_set->queue_depth,
  				   BLKDEV_MAX_RQ);
  
  	queue_for_each_hw_ctx(q, hctx, i) {
  		ret = blk_mq_sched_alloc_tags(q, hctx, i);
  		if (ret)
  			goto err_free_tags;
  	}
  
  	if (blk_mq_is_sbitmap_shared(q->tag_set->flags)) {
  		ret = blk_mq_init_sched_shared_sbitmap(q);
  		if (ret)
  			goto err_free_tags;
  	}
  
  	ret = e->ops.init_sched(q, e);
  	if (ret)
  		goto err_free_sbitmap;
  
  	blk_mq_debugfs_register_sched(q);
  
  	queue_for_each_hw_ctx(q, hctx, i) {
  		if (e->ops.init_hctx) {
  			ret = e->ops.init_hctx(hctx, i);
  			if (ret) {
  				eq = q->elevator;
  				blk_mq_sched_free_requests(q);
  				blk_mq_exit_sched(q, eq);
  				kobject_put(&eq->kobj);
  				return ret;
  			}
  		}
  		blk_mq_debugfs_register_sched_hctx(q, hctx);
  	}
  
  	return 0;
  
  err_free_sbitmap:
  	if (blk_mq_is_sbitmap_shared(q->tag_set->flags))
  		blk_mq_exit_sched_shared_sbitmap(q);
  err_free_tags:
  	blk_mq_sched_free_requests(q);
  	blk_mq_sched_tags_teardown(q);
  	q->elevator = NULL;
  	return ret;
  }
  
  /*
   * called in either blk_queue_cleanup or elevator_switch, tagset
   * is required for freeing requests
   */
  void blk_mq_sched_free_requests(struct request_queue *q)
  {
  	struct blk_mq_hw_ctx *hctx;
  	int i;
  
  	queue_for_each_hw_ctx(q, hctx, i) {
  		if (hctx->sched_tags)
  			blk_mq_free_rqs(q->tag_set, hctx->sched_tags, i);
  	}
  }
  
  void blk_mq_exit_sched(struct request_queue *q, struct elevator_queue *e)
  {
  	struct blk_mq_hw_ctx *hctx;
  	unsigned int i;
  	unsigned int flags = 0;
  
  	queue_for_each_hw_ctx(q, hctx, i) {
  		blk_mq_debugfs_unregister_sched_hctx(hctx);
  		if (e->type->ops.exit_hctx && hctx->sched_data) {
  			e->type->ops.exit_hctx(hctx, i);
  			hctx->sched_data = NULL;
  		}
  		flags = hctx->flags;
  	}
  	blk_mq_debugfs_unregister_sched(q);
  	if (e->type->ops.exit_sched)
  		e->type->ops.exit_sched(e);
  	blk_mq_sched_tags_teardown(q);
  	if (blk_mq_is_sbitmap_shared(flags))
  		blk_mq_exit_sched_shared_sbitmap(q);
  	q->elevator = NULL;
  }
  