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