SAFE public projects git trees. - safe/jmp/linux-2.6/blob - 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);