drivers/block/elevator.c

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