[BLOCK] reimplement handling of barrier request
[linux-2.6.git] / block / elevator.c
1 /*
2  *  Block device elevator/IO-scheduler.
3  *
4  *  Copyright (C) 2000 Andrea Arcangeli <andrea@suse.de> SuSE
5  *
6  * 30042000 Jens Axboe <axboe@suse.de> :
7  *
8  * Split the elevator a bit so that it is possible to choose a different
9  * one or even write a new "plug in". There are three pieces:
10  * - elevator_fn, inserts a new request in the queue list
11  * - elevator_merge_fn, decides whether a new buffer can be merged with
12  *   an existing request
13  * - elevator_dequeue_fn, called when a request is taken off the active list
14  *
15  * 20082000 Dave Jones <davej@suse.de> :
16  * Removed tests for max-bomb-segments, which was breaking elvtune
17  *  when run without -bN
18  *
19  * Jens:
20  * - Rework again to work with bio instead of buffer_heads
21  * - loose bi_dev comparisons, partition handling is right now
22  * - completely modularize elevator setup and teardown
23  *
24  */
25 #include <linux/kernel.h>
26 #include <linux/fs.h>
27 #include <linux/blkdev.h>
28 #include <linux/elevator.h>
29 #include <linux/bio.h>
30 #include <linux/config.h>
31 #include <linux/module.h>
32 #include <linux/slab.h>
33 #include <linux/init.h>
34 #include <linux/compiler.h>
35 #include <linux/delay.h>
36
37 #include <asm/uaccess.h>
38
39 static DEFINE_SPINLOCK(elv_list_lock);
40 static LIST_HEAD(elv_list);
41
42 /*
43  * can we safely merge with this request?
44  */
45 inline int elv_rq_merge_ok(struct request *rq, struct bio *bio)
46 {
47         if (!rq_mergeable(rq))
48                 return 0;
49
50         /*
51          * different data direction or already started, don't merge
52          */
53         if (bio_data_dir(bio) != rq_data_dir(rq))
54                 return 0;
55
56         /*
57          * same device and no special stuff set, merge is ok
58          */
59         if (rq->rq_disk == bio->bi_bdev->bd_disk &&
60             !rq->waiting && !rq->special)
61                 return 1;
62
63         return 0;
64 }
65 EXPORT_SYMBOL(elv_rq_merge_ok);
66
67 inline int elv_try_merge(struct request *__rq, struct bio *bio)
68 {
69         int ret = ELEVATOR_NO_MERGE;
70
71         /*
72          * we can merge and sequence is ok, check if it's possible
73          */
74         if (elv_rq_merge_ok(__rq, bio)) {
75                 if (__rq->sector + __rq->nr_sectors == bio->bi_sector)
76                         ret = ELEVATOR_BACK_MERGE;
77                 else if (__rq->sector - bio_sectors(bio) == bio->bi_sector)
78                         ret = ELEVATOR_FRONT_MERGE;
79         }
80
81         return ret;
82 }
83 EXPORT_SYMBOL(elv_try_merge);
84
85 static struct elevator_type *elevator_find(const char *name)
86 {
87         struct elevator_type *e = NULL;
88         struct list_head *entry;
89
90         list_for_each(entry, &elv_list) {
91                 struct elevator_type *__e;
92
93                 __e = list_entry(entry, struct elevator_type, list);
94
95                 if (!strcmp(__e->elevator_name, name)) {
96                         e = __e;
97                         break;
98                 }
99         }
100
101         return e;
102 }
103
104 static void elevator_put(struct elevator_type *e)
105 {
106         module_put(e->elevator_owner);
107 }
108
109 static struct elevator_type *elevator_get(const char *name)
110 {
111         struct elevator_type *e;
112
113         spin_lock_irq(&elv_list_lock);
114
115         e = elevator_find(name);
116         if (e && !try_module_get(e->elevator_owner))
117                 e = NULL;
118
119         spin_unlock_irq(&elv_list_lock);
120
121         return e;
122 }
123
124 static int elevator_attach(request_queue_t *q, struct elevator_type *e,
125                            struct elevator_queue *eq)
126 {
127         int ret = 0;
128
129         memset(eq, 0, sizeof(*eq));
130         eq->ops = &e->ops;
131         eq->elevator_type = e;
132
133         q->elevator = eq;
134
135         if (eq->ops->elevator_init_fn)
136                 ret = eq->ops->elevator_init_fn(q, eq);
137
138         return ret;
139 }
140
141 static char chosen_elevator[16];
142
143 static void elevator_setup_default(void)
144 {
145         struct elevator_type *e;
146
147         /*
148          * If default has not been set, use the compiled-in selection.
149          */
150         if (!chosen_elevator[0])
151                 strcpy(chosen_elevator, CONFIG_DEFAULT_IOSCHED);
152
153         /*
154          * If the given scheduler is not available, fall back to no-op.
155          */
156         if ((e = elevator_find(chosen_elevator)))
157                 elevator_put(e);
158         else
159                 strcpy(chosen_elevator, "noop");
160 }
161
162 static int __init elevator_setup(char *str)
163 {
164         strncpy(chosen_elevator, str, sizeof(chosen_elevator) - 1);
165         return 0;
166 }
167
168 __setup("elevator=", elevator_setup);
169
170 int elevator_init(request_queue_t *q, char *name)
171 {
172         struct elevator_type *e = NULL;
173         struct elevator_queue *eq;
174         int ret = 0;
175
176         INIT_LIST_HEAD(&q->queue_head);
177         q->last_merge = NULL;
178         q->end_sector = 0;
179         q->boundary_rq = NULL;
180
181         elevator_setup_default();
182
183         if (!name)
184                 name = chosen_elevator;
185
186         e = elevator_get(name);
187         if (!e)
188                 return -EINVAL;
189
190         eq = kmalloc(sizeof(struct elevator_queue), GFP_KERNEL);
191         if (!eq) {
192                 elevator_put(e);
193                 return -ENOMEM;
194         }
195
196         ret = elevator_attach(q, e, eq);
197         if (ret) {
198                 kfree(eq);
199                 elevator_put(e);
200         }
201
202         return ret;
203 }
204
205 void elevator_exit(elevator_t *e)
206 {
207         if (e->ops->elevator_exit_fn)
208                 e->ops->elevator_exit_fn(e);
209
210         elevator_put(e->elevator_type);
211         e->elevator_type = NULL;
212         kfree(e);
213 }
214
215 /*
216  * Insert rq into dispatch queue of q.  Queue lock must be held on
217  * entry.  If sort != 0, rq is sort-inserted; otherwise, rq will be
218  * appended to the dispatch queue.  To be used by specific elevators.
219  */
220 void elv_dispatch_sort(request_queue_t *q, struct request *rq)
221 {
222         sector_t boundary;
223         struct list_head *entry;
224
225         if (q->last_merge == rq)
226                 q->last_merge = NULL;
227         q->nr_sorted--;
228
229         boundary = q->end_sector;
230
231         list_for_each_prev(entry, &q->queue_head) {
232                 struct request *pos = list_entry_rq(entry);
233
234                 if (pos->flags & (REQ_SOFTBARRIER|REQ_HARDBARRIER|REQ_STARTED))
235                         break;
236                 if (rq->sector >= boundary) {
237                         if (pos->sector < boundary)
238                                 continue;
239                 } else {
240                         if (pos->sector >= boundary)
241                                 break;
242                 }
243                 if (rq->sector >= pos->sector)
244                         break;
245         }
246
247         list_add(&rq->queuelist, entry);
248 }
249
250 int elv_merge(request_queue_t *q, struct request **req, struct bio *bio)
251 {
252         elevator_t *e = q->elevator;
253         int ret;
254
255         if (q->last_merge) {
256                 ret = elv_try_merge(q->last_merge, bio);
257                 if (ret != ELEVATOR_NO_MERGE) {
258                         *req = q->last_merge;
259                         return ret;
260                 }
261         }
262
263         if (e->ops->elevator_merge_fn)
264                 return e->ops->elevator_merge_fn(q, req, bio);
265
266         return ELEVATOR_NO_MERGE;
267 }
268
269 void elv_merged_request(request_queue_t *q, struct request *rq)
270 {
271         elevator_t *e = q->elevator;
272
273         if (e->ops->elevator_merged_fn)
274                 e->ops->elevator_merged_fn(q, rq);
275
276         q->last_merge = rq;
277 }
278
279 void elv_merge_requests(request_queue_t *q, struct request *rq,
280                              struct request *next)
281 {
282         elevator_t *e = q->elevator;
283
284         if (e->ops->elevator_merge_req_fn)
285                 e->ops->elevator_merge_req_fn(q, rq, next);
286         q->nr_sorted--;
287
288         q->last_merge = rq;
289 }
290
291 void elv_requeue_request(request_queue_t *q, struct request *rq)
292 {
293         elevator_t *e = q->elevator;
294
295         /*
296          * it already went through dequeue, we need to decrement the
297          * in_flight count again
298          */
299         if (blk_account_rq(rq)) {
300                 q->in_flight--;
301                 if (blk_sorted_rq(rq) && e->ops->elevator_deactivate_req_fn)
302                         e->ops->elevator_deactivate_req_fn(q, rq);
303         }
304
305         rq->flags &= ~REQ_STARTED;
306
307         __elv_add_request(q, rq, ELEVATOR_INSERT_REQUEUE, 0);
308 }
309
310 static void elv_drain_elevator(request_queue_t *q)
311 {
312         static int printed;
313         while (q->elevator->ops->elevator_dispatch_fn(q, 1))
314                 ;
315         if (q->nr_sorted == 0)
316                 return;
317         if (printed++ < 10) {
318                 printk(KERN_ERR "%s: forced dispatching is broken "
319                        "(nr_sorted=%u), please report this\n",
320                        q->elevator->elevator_type->elevator_name, q->nr_sorted);
321         }
322 }
323
324 void __elv_add_request(request_queue_t *q, struct request *rq, int where,
325                        int plug)
326 {
327         struct list_head *pos;
328         unsigned ordseq;
329
330         if (q->ordcolor)
331                 rq->flags |= REQ_ORDERED_COLOR;
332
333         if (rq->flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER)) {
334                 /*
335                  * toggle ordered color
336                  */
337                 q->ordcolor ^= 1;
338
339                 /*
340                  * barriers implicitly indicate back insertion
341                  */
342                 if (where == ELEVATOR_INSERT_SORT)
343                         where = ELEVATOR_INSERT_BACK;
344
345                 /*
346                  * this request is scheduling boundary, update end_sector
347                  */
348                 if (blk_fs_request(rq)) {
349                         q->end_sector = rq_end_sector(rq);
350                         q->boundary_rq = rq;
351                 }
352         } else if (!(rq->flags & REQ_ELVPRIV) && where == ELEVATOR_INSERT_SORT)
353                 where = ELEVATOR_INSERT_BACK;
354
355         if (plug)
356                 blk_plug_device(q);
357
358         rq->q = q;
359
360         switch (where) {
361         case ELEVATOR_INSERT_FRONT:
362                 rq->flags |= REQ_SOFTBARRIER;
363
364                 list_add(&rq->queuelist, &q->queue_head);
365                 break;
366
367         case ELEVATOR_INSERT_BACK:
368                 rq->flags |= REQ_SOFTBARRIER;
369                 elv_drain_elevator(q);
370                 list_add_tail(&rq->queuelist, &q->queue_head);
371                 /*
372                  * We kick the queue here for the following reasons.
373                  * - The elevator might have returned NULL previously
374                  *   to delay requests and returned them now.  As the
375                  *   queue wasn't empty before this request, ll_rw_blk
376                  *   won't run the queue on return, resulting in hang.
377                  * - Usually, back inserted requests won't be merged
378                  *   with anything.  There's no point in delaying queue
379                  *   processing.
380                  */
381                 blk_remove_plug(q);
382                 q->request_fn(q);
383                 break;
384
385         case ELEVATOR_INSERT_SORT:
386                 BUG_ON(!blk_fs_request(rq));
387                 rq->flags |= REQ_SORTED;
388                 q->nr_sorted++;
389                 if (q->last_merge == NULL && rq_mergeable(rq))
390                         q->last_merge = rq;
391                 /*
392                  * Some ioscheds (cfq) run q->request_fn directly, so
393                  * rq cannot be accessed after calling
394                  * elevator_add_req_fn.
395                  */
396                 q->elevator->ops->elevator_add_req_fn(q, rq);
397                 break;
398
399         case ELEVATOR_INSERT_REQUEUE:
400                 /*
401                  * If ordered flush isn't in progress, we do front
402                  * insertion; otherwise, requests should be requeued
403                  * in ordseq order.
404                  */
405                 rq->flags |= REQ_SOFTBARRIER;
406
407                 if (q->ordseq == 0) {
408                         list_add(&rq->queuelist, &q->queue_head);
409                         break;
410                 }
411
412                 ordseq = blk_ordered_req_seq(rq);
413
414                 list_for_each(pos, &q->queue_head) {
415                         struct request *pos_rq = list_entry_rq(pos);
416                         if (ordseq <= blk_ordered_req_seq(pos_rq))
417                                 break;
418                 }
419
420                 list_add_tail(&rq->queuelist, pos);
421                 break;
422
423         default:
424                 printk(KERN_ERR "%s: bad insertion point %d\n",
425                        __FUNCTION__, where);
426                 BUG();
427         }
428
429         if (blk_queue_plugged(q)) {
430                 int nrq = q->rq.count[READ] + q->rq.count[WRITE]
431                         - q->in_flight;
432
433                 if (nrq >= q->unplug_thresh)
434                         __generic_unplug_device(q);
435         }
436 }
437
438 void elv_add_request(request_queue_t *q, struct request *rq, int where,
439                      int plug)
440 {
441         unsigned long flags;
442
443         spin_lock_irqsave(q->queue_lock, flags);
444         __elv_add_request(q, rq, where, plug);
445         spin_unlock_irqrestore(q->queue_lock, flags);
446 }
447
448 static inline struct request *__elv_next_request(request_queue_t *q)
449 {
450         struct request *rq;
451
452         while (1) {
453                 while (!list_empty(&q->queue_head)) {
454                         rq = list_entry_rq(q->queue_head.next);
455                         if (blk_do_ordered(q, &rq))
456                                 return rq;
457                 }
458
459                 if (!q->elevator->ops->elevator_dispatch_fn(q, 0))
460                         return NULL;
461         }
462 }
463
464 struct request *elv_next_request(request_queue_t *q)
465 {
466         struct request *rq;
467         int ret;
468
469         while ((rq = __elv_next_request(q)) != NULL) {
470                 if (!(rq->flags & REQ_STARTED)) {
471                         elevator_t *e = q->elevator;
472
473                         /*
474                          * This is the first time the device driver
475                          * sees this request (possibly after
476                          * requeueing).  Notify IO scheduler.
477                          */
478                         if (blk_sorted_rq(rq) &&
479                             e->ops->elevator_activate_req_fn)
480                                 e->ops->elevator_activate_req_fn(q, rq);
481
482                         /*
483                          * just mark as started even if we don't start
484                          * it, a request that has been delayed should
485                          * not be passed by new incoming requests
486                          */
487                         rq->flags |= REQ_STARTED;
488                 }
489
490                 if (!q->boundary_rq || q->boundary_rq == rq) {
491                         q->end_sector = rq_end_sector(rq);
492                         q->boundary_rq = NULL;
493                 }
494
495                 if ((rq->flags & REQ_DONTPREP) || !q->prep_rq_fn)
496                         break;
497
498                 ret = q->prep_rq_fn(q, rq);
499                 if (ret == BLKPREP_OK) {
500                         break;
501                 } else if (ret == BLKPREP_DEFER) {
502                         /*
503                          * the request may have been (partially) prepped.
504                          * we need to keep this request in the front to
505                          * avoid resource deadlock.  REQ_STARTED will
506                          * prevent other fs requests from passing this one.
507                          */
508                         rq = NULL;
509                         break;
510                 } else if (ret == BLKPREP_KILL) {
511                         int nr_bytes = rq->hard_nr_sectors << 9;
512
513                         if (!nr_bytes)
514                                 nr_bytes = rq->data_len;
515
516                         blkdev_dequeue_request(rq);
517                         rq->flags |= REQ_QUIET;
518                         end_that_request_chunk(rq, 0, nr_bytes);
519                         end_that_request_last(rq, 0);
520                 } else {
521                         printk(KERN_ERR "%s: bad return=%d\n", __FUNCTION__,
522                                                                 ret);
523                         break;
524                 }
525         }
526
527         return rq;
528 }
529
530 void elv_dequeue_request(request_queue_t *q, struct request *rq)
531 {
532         BUG_ON(list_empty(&rq->queuelist));
533
534         list_del_init(&rq->queuelist);
535
536         /*
537          * the time frame between a request being removed from the lists
538          * and to it is freed is accounted as io that is in progress at
539          * the driver side.
540          */
541         if (blk_account_rq(rq))
542                 q->in_flight++;
543 }
544
545 int elv_queue_empty(request_queue_t *q)
546 {
547         elevator_t *e = q->elevator;
548
549         if (!list_empty(&q->queue_head))
550                 return 0;
551
552         if (e->ops->elevator_queue_empty_fn)
553                 return e->ops->elevator_queue_empty_fn(q);
554
555         return 1;
556 }
557
558 struct request *elv_latter_request(request_queue_t *q, struct request *rq)
559 {
560         elevator_t *e = q->elevator;
561
562         if (e->ops->elevator_latter_req_fn)
563                 return e->ops->elevator_latter_req_fn(q, rq);
564         return NULL;
565 }
566
567 struct request *elv_former_request(request_queue_t *q, struct request *rq)
568 {
569         elevator_t *e = q->elevator;
570
571         if (e->ops->elevator_former_req_fn)
572                 return e->ops->elevator_former_req_fn(q, rq);
573         return NULL;
574 }
575
576 int elv_set_request(request_queue_t *q, struct request *rq, struct bio *bio,
577                     gfp_t gfp_mask)
578 {
579         elevator_t *e = q->elevator;
580
581         if (e->ops->elevator_set_req_fn)
582                 return e->ops->elevator_set_req_fn(q, rq, bio, gfp_mask);
583
584         rq->elevator_private = NULL;
585         return 0;
586 }
587
588 void elv_put_request(request_queue_t *q, struct request *rq)
589 {
590         elevator_t *e = q->elevator;
591
592         if (e->ops->elevator_put_req_fn)
593                 e->ops->elevator_put_req_fn(q, rq);
594 }
595
596 int elv_may_queue(request_queue_t *q, int rw, struct bio *bio)
597 {
598         elevator_t *e = q->elevator;
599
600         if (e->ops->elevator_may_queue_fn)
601                 return e->ops->elevator_may_queue_fn(q, rw, bio);
602
603         return ELV_MQUEUE_MAY;
604 }
605
606 void elv_completed_request(request_queue_t *q, struct request *rq)
607 {
608         elevator_t *e = q->elevator;
609
610         /*
611          * request is released from the driver, io must be done
612          */
613         if (blk_account_rq(rq)) {
614                 struct request *first_rq = list_entry_rq(q->queue_head.next);
615
616                 q->in_flight--;
617
618                 /*
619                  * Check if the queue is waiting for fs requests to be
620                  * drained for flush sequence.
621                  */
622                 if (q->ordseq && q->in_flight == 0 &&
623                     blk_ordered_cur_seq(q) == QUEUE_ORDSEQ_DRAIN &&
624                     blk_ordered_req_seq(first_rq) > QUEUE_ORDSEQ_DRAIN) {
625                         blk_ordered_complete_seq(q, QUEUE_ORDSEQ_DRAIN, 0);
626                         q->request_fn(q);
627                 }
628
629                 if (blk_sorted_rq(rq) && e->ops->elevator_completed_req_fn)
630                         e->ops->elevator_completed_req_fn(q, rq);
631         }
632 }
633
634 int elv_register_queue(struct request_queue *q)
635 {
636         elevator_t *e = q->elevator;
637
638         e->kobj.parent = kobject_get(&q->kobj);
639         if (!e->kobj.parent)
640                 return -EBUSY;
641
642         snprintf(e->kobj.name, KOBJ_NAME_LEN, "%s", "iosched");
643         e->kobj.ktype = e->elevator_type->elevator_ktype;
644
645         return kobject_register(&e->kobj);
646 }
647
648 void elv_unregister_queue(struct request_queue *q)
649 {
650         if (q) {
651                 elevator_t *e = q->elevator;
652                 kobject_unregister(&e->kobj);
653                 kobject_put(&q->kobj);
654         }
655 }
656
657 int elv_register(struct elevator_type *e)
658 {
659         spin_lock_irq(&elv_list_lock);
660         if (elevator_find(e->elevator_name))
661                 BUG();
662         list_add_tail(&e->list, &elv_list);
663         spin_unlock_irq(&elv_list_lock);
664
665         printk(KERN_INFO "io scheduler %s registered", e->elevator_name);
666         if (!strcmp(e->elevator_name, chosen_elevator))
667                 printk(" (default)");
668         printk("\n");
669         return 0;
670 }
671 EXPORT_SYMBOL_GPL(elv_register);
672
673 void elv_unregister(struct elevator_type *e)
674 {
675         struct task_struct *g, *p;
676
677         /*
678          * Iterate every thread in the process to remove the io contexts.
679          */
680         read_lock(&tasklist_lock);
681         do_each_thread(g, p) {
682                 struct io_context *ioc = p->io_context;
683                 if (ioc && ioc->cic) {
684                         ioc->cic->exit(ioc->cic);
685                         ioc->cic->dtor(ioc->cic);
686                         ioc->cic = NULL;
687                 }
688                 if (ioc && ioc->aic) {
689                         ioc->aic->exit(ioc->aic);
690                         ioc->aic->dtor(ioc->aic);
691                         ioc->aic = NULL;
692                 }
693         } while_each_thread(g, p);
694         read_unlock(&tasklist_lock);
695
696         spin_lock_irq(&elv_list_lock);
697         list_del_init(&e->list);
698         spin_unlock_irq(&elv_list_lock);
699 }
700 EXPORT_SYMBOL_GPL(elv_unregister);
701
702 /*
703  * switch to new_e io scheduler. be careful not to introduce deadlocks -
704  * we don't free the old io scheduler, before we have allocated what we
705  * need for the new one. this way we have a chance of going back to the old
706  * one, if the new one fails init for some reason.
707  */
708 static void elevator_switch(request_queue_t *q, struct elevator_type *new_e)
709 {
710         elevator_t *old_elevator, *e;
711
712         /*
713          * Allocate new elevator
714          */
715         e = kmalloc(sizeof(elevator_t), GFP_KERNEL);
716         if (!e)
717                 goto error;
718
719         /*
720          * Turn on BYPASS and drain all requests w/ elevator private data
721          */
722         spin_lock_irq(q->queue_lock);
723
724         set_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
725
726         elv_drain_elevator(q);
727
728         while (q->rq.elvpriv) {
729                 blk_remove_plug(q);
730                 q->request_fn(q);
731                 spin_unlock_irq(q->queue_lock);
732                 msleep(10);
733                 spin_lock_irq(q->queue_lock);
734                 elv_drain_elevator(q);
735         }
736
737         spin_unlock_irq(q->queue_lock);
738
739         /*
740          * unregister old elevator data
741          */
742         elv_unregister_queue(q);
743         old_elevator = q->elevator;
744
745         /*
746          * attach and start new elevator
747          */
748         if (elevator_attach(q, new_e, e))
749                 goto fail;
750
751         if (elv_register_queue(q))
752                 goto fail_register;
753
754         /*
755          * finally exit old elevator and turn off BYPASS.
756          */
757         elevator_exit(old_elevator);
758         clear_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
759         return;
760
761 fail_register:
762         /*
763          * switch failed, exit the new io scheduler and reattach the old
764          * one again (along with re-adding the sysfs dir)
765          */
766         elevator_exit(e);
767         e = NULL;
768 fail:
769         q->elevator = old_elevator;
770         elv_register_queue(q);
771         clear_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
772         kfree(e);
773 error:
774         elevator_put(new_e);
775         printk(KERN_ERR "elevator: switch to %s failed\n",new_e->elevator_name);
776 }
777
778 ssize_t elv_iosched_store(request_queue_t *q, const char *name, size_t count)
779 {
780         char elevator_name[ELV_NAME_MAX];
781         size_t len;
782         struct elevator_type *e;
783
784         elevator_name[sizeof(elevator_name) - 1] = '\0';
785         strncpy(elevator_name, name, sizeof(elevator_name) - 1);
786         len = strlen(elevator_name);
787
788         if (len && elevator_name[len - 1] == '\n')
789                 elevator_name[len - 1] = '\0';
790
791         e = elevator_get(elevator_name);
792         if (!e) {
793                 printk(KERN_ERR "elevator: type %s not found\n", elevator_name);
794                 return -EINVAL;
795         }
796
797         if (!strcmp(elevator_name, q->elevator->elevator_type->elevator_name)) {
798                 elevator_put(e);
799                 return count;
800         }
801
802         elevator_switch(q, e);
803         return count;
804 }
805
806 ssize_t elv_iosched_show(request_queue_t *q, char *name)
807 {
808         elevator_t *e = q->elevator;
809         struct elevator_type *elv = e->elevator_type;
810         struct list_head *entry;
811         int len = 0;
812
813         spin_lock_irq(q->queue_lock);
814         list_for_each(entry, &elv_list) {
815                 struct elevator_type *__e;
816
817                 __e = list_entry(entry, struct elevator_type, list);
818                 if (!strcmp(elv->elevator_name, __e->elevator_name))
819                         len += sprintf(name+len, "[%s] ", elv->elevator_name);
820                 else
821                         len += sprintf(name+len, "%s ", __e->elevator_name);
822         }
823         spin_unlock_irq(q->queue_lock);
824
825         len += sprintf(len+name, "\n");
826         return len;
827 }
828
829 EXPORT_SYMBOL(elv_dispatch_sort);
830 EXPORT_SYMBOL(elv_add_request);
831 EXPORT_SYMBOL(__elv_add_request);
832 EXPORT_SYMBOL(elv_requeue_request);
833 EXPORT_SYMBOL(elv_next_request);
834 EXPORT_SYMBOL(elv_dequeue_request);
835 EXPORT_SYMBOL(elv_queue_empty);
836 EXPORT_SYMBOL(elv_completed_request);
837 EXPORT_SYMBOL(elevator_exit);
838 EXPORT_SYMBOL(elevator_init);