ALSA: hda - Fix no sound after Windows boot with ALC269
[linux-2.6.git] / block / elevator.c
index 2e0fb21..0cdb4e7 100644 (file)
 #include <linux/compiler.h>
 #include <linux/delay.h>
 #include <linux/blktrace_api.h>
-#include <trace/block.h>
 #include <linux/hash.h>
 #include <linux/uaccess.h>
 
+#include <trace/events/block.h>
+
 #include "blk.h"
 
 static DEFINE_SPINLOCK(elv_list_lock);
 static LIST_HEAD(elv_list);
 
-DEFINE_TRACE(block_rq_abort);
-
 /*
  * Merge hash stuff.
  */
@@ -52,11 +51,7 @@ static const int elv_hash_shift = 6;
 #define ELV_HASH_FN(sec)       \
                (hash_long(ELV_HASH_BLOCK((sec)), elv_hash_shift))
 #define ELV_HASH_ENTRIES       (1 << elv_hash_shift)
-#define rq_hash_key(rq)                ((rq)->sector + (rq)->nr_sectors)
-#define ELV_ON_HASH(rq)                (!hlist_unhashed(&(rq)->hash))
-
-DEFINE_TRACE(block_rq_insert);
-DEFINE_TRACE(block_rq_issue);
+#define rq_hash_key(rq)                (blk_rq_pos(rq) + blk_rq_sectors(rq))
 
 /*
  * Query io scheduler to see if the current process issuing bio may be
@@ -84,7 +79,13 @@ int elv_rq_merge_ok(struct request *rq, struct bio *bio)
        /*
         * Don't merge file system requests and discard requests
         */
-       if (bio_discard(bio) != bio_discard(rq->bio))
+       if ((bio->bi_rw & REQ_DISCARD) != (rq->bio->bi_rw & REQ_DISCARD))
+               return 0;
+
+       /*
+        * Don't merge discard requests and secure discard requests
+        */
+       if ((bio->bi_rw & REQ_SECURE) != (rq->bio->bi_rw & REQ_SECURE))
                return 0;
 
        /*
@@ -112,7 +113,7 @@ int elv_rq_merge_ok(struct request *rq, struct bio *bio)
 }
 EXPORT_SYMBOL(elv_rq_merge_ok);
 
-static inline int elv_try_merge(struct request *__rq, struct bio *bio)
+int elv_try_merge(struct request *__rq, struct bio *bio)
 {
        int ret = ELEVATOR_NO_MERGE;
 
@@ -120,9 +121,9 @@ static inline int elv_try_merge(struct request *__rq, struct bio *bio)
         * we can merge and sequence is ok, check if it's possible
         */
        if (elv_rq_merge_ok(__rq, bio)) {
-               if (__rq->sector + __rq->nr_sectors == bio->bi_sector)
+               if (blk_rq_pos(__rq) + blk_rq_sectors(__rq) == bio->bi_sector)
                        ret = ELEVATOR_BACK_MERGE;
-               else if (__rq->sector - bio_sectors(bio) == bio->bi_sector)
+               else if (blk_rq_pos(__rq) - bio_sectors(bio) == bio->bi_sector)
                        ret = ELEVATOR_FRONT_MERGE;
        }
 
@@ -158,10 +159,7 @@ static struct elevator_type *elevator_get(const char *name)
 
                spin_unlock(&elv_list_lock);
 
-               if (!strcmp(name, "anticipatory"))
-                       sprintf(elv, "as-iosched");
-               else
-                       sprintf(elv, "%s-iosched", name);
+               snprintf(elv, sizeof(elv), "%s-iosched", name);
 
                request_module("%s", elv);
                spin_lock(&elv_list_lock);
@@ -197,10 +195,7 @@ static int __init elevator_setup(char *str)
         * Be backwards-compatible with previous kernels, so users
         * won't get the wrong elevator.
         */
-       if (!strcmp(str, "as"))
-               strcpy(chosen_elevator, "anticipatory");
-       else
-               strncpy(chosen_elevator, str, sizeof(chosen_elevator) - 1);
+       strncpy(chosen_elevator, str, sizeof(chosen_elevator) - 1);
        return 1;
 }
 
@@ -252,9 +247,11 @@ int elevator_init(struct request_queue *q, char *name)
 {
        struct elevator_type *e = NULL;
        struct elevator_queue *eq;
-       int ret = 0;
        void *data;
 
+       if (unlikely(q->elevator))
+               return 0;
+
        INIT_LIST_HEAD(&q->queue_head);
        q->last_merge = NULL;
        q->end_sector = 0;
@@ -294,7 +291,7 @@ int elevator_init(struct request_queue *q, char *name)
        }
 
        elevator_attach(q, eq, data);
-       return ret;
+       return 0;
 }
 EXPORT_SYMBOL(elevator_init);
 
@@ -310,22 +307,6 @@ void elevator_exit(struct elevator_queue *e)
 }
 EXPORT_SYMBOL(elevator_exit);
 
-static void elv_activate_rq(struct request_queue *q, struct request *rq)
-{
-       struct elevator_queue *e = q->elevator;
-
-       if (e->ops->elevator_activate_req_fn)
-               e->ops->elevator_activate_req_fn(q, rq);
-}
-
-static void elv_deactivate_rq(struct request_queue *q, struct request *rq)
-{
-       struct elevator_queue *e = q->elevator;
-
-       if (e->ops->elevator_deactivate_req_fn)
-               e->ops->elevator_deactivate_req_fn(q, rq);
-}
-
 static inline void __elv_rqhash_del(struct request *rq)
 {
        hlist_del_init(&rq->hash);
@@ -387,9 +368,9 @@ struct request *elv_rb_add(struct rb_root *root, struct request *rq)
                parent = *p;
                __rq = rb_entry(parent, struct request, rb_node);
 
-               if (rq->sector < __rq->sector)
+               if (blk_rq_pos(rq) < blk_rq_pos(__rq))
                        p = &(*p)->rb_left;
-               else if (rq->sector > __rq->sector)
+               else if (blk_rq_pos(rq) > blk_rq_pos(__rq))
                        p = &(*p)->rb_right;
                else
                        return __rq;
@@ -417,9 +398,9 @@ struct request *elv_rb_find(struct rb_root *root, sector_t sector)
        while (n) {
                rq = rb_entry(n, struct request, rb_node);
 
-               if (sector < rq->sector)
+               if (sector < blk_rq_pos(rq))
                        n = n->rb_left;
-               else if (sector > rq->sector)
+               else if (sector > blk_rq_pos(rq))
                        n = n->rb_right;
                else
                        return rq;
@@ -440,6 +421,8 @@ void elv_dispatch_sort(struct request_queue *q, struct request *rq)
        struct list_head *entry;
        int stop_flags;
 
+       BUG_ON(rq->cmd_flags & REQ_ON_PLUG);
+
        if (q->last_merge == rq)
                q->last_merge = NULL;
 
@@ -448,24 +431,25 @@ void elv_dispatch_sort(struct request_queue *q, struct request *rq)
        q->nr_sorted--;
 
        boundary = q->end_sector;
-       stop_flags = REQ_SOFTBARRIER | REQ_HARDBARRIER | REQ_STARTED;
+       stop_flags = REQ_SOFTBARRIER | REQ_STARTED;
        list_for_each_prev(entry, &q->queue_head) {
                struct request *pos = list_entry_rq(entry);
 
-               if (blk_discard_rq(rq) != blk_discard_rq(pos))
+               if ((rq->cmd_flags & REQ_DISCARD) !=
+                   (pos->cmd_flags & REQ_DISCARD))
                        break;
                if (rq_data_dir(rq) != rq_data_dir(pos))
                        break;
                if (pos->cmd_flags & stop_flags)
                        break;
-               if (rq->sector >= boundary) {
-                       if (pos->sector < boundary)
+               if (blk_rq_pos(rq) >= boundary) {
+                       if (blk_rq_pos(pos) < boundary)
                                continue;
                } else {
-                       if (pos->sector >= boundary)
+                       if (blk_rq_pos(pos) >= boundary)
                                break;
                }
-               if (rq->sector >= pos->sector)
+               if (blk_rq_pos(rq) >= blk_rq_pos(pos))
                        break;
        }
 
@@ -500,6 +484,15 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
        int ret;
 
        /*
+        * Levels of merges:
+        *      nomerges:  No merges at all attempted
+        *      noxmerges: Only simple one-hit cache try
+        *      merges:    All merge tries attempted
+        */
+       if (blk_queue_nomerges(q))
+               return ELEVATOR_NO_MERGE;
+
+       /*
         * First try one-hit cache.
         */
        if (q->last_merge) {
@@ -510,7 +503,7 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
                }
        }
 
-       if (blk_queue_nomerges(q))
+       if (blk_queue_noxmerges(q))
                return ELEVATOR_NO_MERGE;
 
        /*
@@ -528,6 +521,40 @@ int elv_merge(struct request_queue *q, struct request **req, struct bio *bio)
        return ELEVATOR_NO_MERGE;
 }
 
+/*
+ * Attempt to do an insertion back merge. Only check for the case where
+ * we can append 'rq' to an existing request, so we can throw 'rq' away
+ * afterwards.
+ *
+ * Returns true if we merged, false otherwise
+ */
+static bool elv_attempt_insert_merge(struct request_queue *q,
+                                    struct request *rq)
+{
+       struct request *__rq;
+
+       if (blk_queue_nomerges(q))
+               return false;
+
+       /*
+        * First try one-hit cache.
+        */
+       if (q->last_merge && blk_attempt_req_merge(q, q->last_merge, rq))
+               return true;
+
+       if (blk_queue_noxmerges(q))
+               return false;
+
+       /*
+        * See if our hash lookup can find a potential backmerge.
+        */
+       __rq = elv_rqhash_find(q, blk_rq_pos(rq));
+       if (__rq && blk_attempt_req_merge(q, __rq, rq))
+               return true;
+
+       return false;
+}
+
 void elv_merged_request(struct request_queue *q, struct request *rq, int type)
 {
        struct elevator_queue *e = q->elevator;
@@ -545,17 +572,30 @@ void elv_merge_requests(struct request_queue *q, struct request *rq,
                             struct request *next)
 {
        struct elevator_queue *e = q->elevator;
+       const int next_sorted = next->cmd_flags & REQ_SORTED;
 
-       if (e->ops->elevator_merge_req_fn)
+       if (next_sorted && e->ops->elevator_merge_req_fn)
                e->ops->elevator_merge_req_fn(q, rq, next);
 
        elv_rqhash_reposition(q, rq);
-       elv_rqhash_del(q, next);
 
-       q->nr_sorted--;
+       if (next_sorted) {
+               elv_rqhash_del(q, next);
+               q->nr_sorted--;
+       }
+
        q->last_merge = rq;
 }
 
+void elv_bio_merged(struct request_queue *q, struct request *rq,
+                       struct bio *bio)
+{
+       struct elevator_queue *e = q->elevator;
+
+       if (e->ops->elevator_bio_merged_fn)
+               e->ops->elevator_bio_merged_fn(q, rq, bio);
+}
+
 void elv_requeue_request(struct request_queue *q, struct request *rq)
 {
        /*
@@ -563,14 +603,14 @@ void elv_requeue_request(struct request_queue *q, struct request *rq)
         * in_flight count again
         */
        if (blk_account_rq(rq)) {
-               q->in_flight--;
-               if (blk_sorted_rq(rq))
+               q->in_flight[rq_is_sync(rq)]--;
+               if (rq->cmd_flags & REQ_SORTED)
                        elv_deactivate_rq(q, rq);
        }
 
        rq->cmd_flags &= ~REQ_STARTED;
 
-       elv_insert(q, rq, ELEVATOR_INSERT_REQUEUE);
+       __elv_add_request(q, rq, ELEVATOR_INSERT_REQUEUE);
 }
 
 void elv_drain_elevator(struct request_queue *q)
@@ -592,6 +632,9 @@ void elv_drain_elevator(struct request_queue *q)
  */
 void elv_quiesce_start(struct request_queue *q)
 {
+       if (!q->elevator)
+               return;
+
        queue_flag_set(QUEUE_FLAG_ELVSWITCH, q);
 
        /*
@@ -599,7 +642,7 @@ void elv_quiesce_start(struct request_queue *q)
         */
        elv_drain_elevator(q);
        while (q->rq.elvpriv) {
-               __blk_run_queue(q);
+               __blk_run_queue(q, false);
                spin_unlock_irq(q->queue_lock);
                msleep(10);
                spin_lock_irq(q->queue_lock);
@@ -612,20 +655,29 @@ void elv_quiesce_end(struct request_queue *q)
        queue_flag_clear(QUEUE_FLAG_ELVSWITCH, q);
 }
 
-void elv_insert(struct request_queue *q, struct request *rq, int where)
+void __elv_add_request(struct request_queue *q, struct request *rq, int where)
 {
-       struct list_head *pos;
-       unsigned ordseq;
-       int unplug_it = 1;
-
        trace_block_rq_insert(q, rq);
 
        rq->q = q;
 
+       BUG_ON(rq->cmd_flags & REQ_ON_PLUG);
+
+       if (rq->cmd_flags & REQ_SOFTBARRIER) {
+               /* barriers are scheduling boundary, update end_sector */
+               if (rq->cmd_type == REQ_TYPE_FS ||
+                   (rq->cmd_flags & REQ_DISCARD)) {
+                       q->end_sector = rq_end_sector(rq);
+                       q->boundary_rq = rq;
+               }
+       } else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
+                   where == ELEVATOR_INSERT_SORT)
+               where = ELEVATOR_INSERT_BACK;
+
        switch (where) {
+       case ELEVATOR_INSERT_REQUEUE:
        case ELEVATOR_INSERT_FRONT:
                rq->cmd_flags |= REQ_SOFTBARRIER;
-
                list_add(&rq->queuelist, &q->queue_head);
                break;
 
@@ -643,11 +695,20 @@ void elv_insert(struct request_queue *q, struct request *rq, int where)
                 *   with anything.  There's no point in delaying queue
                 *   processing.
                 */
-               __blk_run_queue(q);
+               __blk_run_queue(q, false);
                break;
 
+       case ELEVATOR_INSERT_SORT_MERGE:
+               /*
+                * If we succeed in merging this request with one in the
+                * queue already, we are done - rq has now been freed,
+                * so no need to do anything further.
+                */
+               if (elv_attempt_insert_merge(q, rq))
+                       break;
        case ELEVATOR_INSERT_SORT:
-               BUG_ON(!blk_fs_request(rq) && !blk_discard_rq(rq));
+               BUG_ON(rq->cmd_type != REQ_TYPE_FS &&
+                      !(rq->cmd_flags & REQ_DISCARD));
                rq->cmd_flags |= REQ_SORTED;
                q->nr_sorted++;
                if (rq_mergeable(rq)) {
@@ -664,225 +725,28 @@ void elv_insert(struct request_queue *q, struct request *rq, int where)
                q->elevator->ops->elevator_add_req_fn(q, rq);
                break;
 
-       case ELEVATOR_INSERT_REQUEUE:
-               /*
-                * If ordered flush isn't in progress, we do front
-                * insertion; otherwise, requests should be requeued
-                * in ordseq order.
-                */
+       case ELEVATOR_INSERT_FLUSH:
                rq->cmd_flags |= REQ_SOFTBARRIER;
-
-               /*
-                * Most requeues happen because of a busy condition,
-                * don't force unplug of the queue for that case.
-                */
-               unplug_it = 0;
-
-               if (q->ordseq == 0) {
-                       list_add(&rq->queuelist, &q->queue_head);
-                       break;
-               }
-
-               ordseq = blk_ordered_req_seq(rq);
-
-               list_for_each(pos, &q->queue_head) {
-                       struct request *pos_rq = list_entry_rq(pos);
-                       if (ordseq <= blk_ordered_req_seq(pos_rq))
-                               break;
-               }
-
-               list_add_tail(&rq->queuelist, pos);
+               blk_insert_flush(rq);
                break;
-
        default:
                printk(KERN_ERR "%s: bad insertion point %d\n",
                       __func__, where);
                BUG();
        }
-
-       if (unplug_it && blk_queue_plugged(q)) {
-               int nrq = q->rq.count[BLK_RW_SYNC] + q->rq.count[BLK_RW_ASYNC]
-                       - q->in_flight;
-
-               if (nrq >= q->unplug_thresh)
-                       __generic_unplug_device(q);
-       }
-}
-
-void __elv_add_request(struct request_queue *q, struct request *rq, int where,
-                      int plug)
-{
-       if (q->ordcolor)
-               rq->cmd_flags |= REQ_ORDERED_COLOR;
-
-       if (rq->cmd_flags & (REQ_SOFTBARRIER | REQ_HARDBARRIER)) {
-               /*
-                * toggle ordered color
-                */
-               if (blk_barrier_rq(rq))
-                       q->ordcolor ^= 1;
-
-               /*
-                * barriers implicitly indicate back insertion
-                */
-               if (where == ELEVATOR_INSERT_SORT)
-                       where = ELEVATOR_INSERT_BACK;
-
-               /*
-                * this request is scheduling boundary, update
-                * end_sector
-                */
-               if (blk_fs_request(rq) || blk_discard_rq(rq)) {
-                       q->end_sector = rq_end_sector(rq);
-                       q->boundary_rq = rq;
-               }
-       } else if (!(rq->cmd_flags & REQ_ELVPRIV) &&
-                   where == ELEVATOR_INSERT_SORT)
-               where = ELEVATOR_INSERT_BACK;
-
-       if (plug)
-               blk_plug_device(q);
-
-       elv_insert(q, rq, where);
 }
 EXPORT_SYMBOL(__elv_add_request);
 
-void elv_add_request(struct request_queue *q, struct request *rq, int where,
-                    int plug)
+void elv_add_request(struct request_queue *q, struct request *rq, int where)
 {
        unsigned long flags;
 
        spin_lock_irqsave(q->queue_lock, flags);
-       __elv_add_request(q, rq, where, plug);
+       __elv_add_request(q, rq, where);
        spin_unlock_irqrestore(q->queue_lock, flags);
 }
 EXPORT_SYMBOL(elv_add_request);
 
-static inline struct request *__elv_next_request(struct request_queue *q)
-{
-       struct request *rq;
-
-       while (1) {
-               while (!list_empty(&q->queue_head)) {
-                       rq = list_entry_rq(q->queue_head.next);
-                       if (blk_do_ordered(q, &rq))
-                               return rq;
-               }
-
-               if (!q->elevator->ops->elevator_dispatch_fn(q, 0))
-                       return NULL;
-       }
-}
-
-struct request *elv_next_request(struct request_queue *q)
-{
-       struct request *rq;
-       int ret;
-
-       while ((rq = __elv_next_request(q)) != NULL) {
-               if (!(rq->cmd_flags & REQ_STARTED)) {
-                       /*
-                        * This is the first time the device driver
-                        * sees this request (possibly after
-                        * requeueing).  Notify IO scheduler.
-                        */
-                       if (blk_sorted_rq(rq))
-                               elv_activate_rq(q, rq);
-
-                       /*
-                        * just mark as started even if we don't start
-                        * it, a request that has been delayed should
-                        * not be passed by new incoming requests
-                        */
-                       rq->cmd_flags |= REQ_STARTED;
-                       trace_block_rq_issue(q, rq);
-               }
-
-               if (!q->boundary_rq || q->boundary_rq == rq) {
-                       q->end_sector = rq_end_sector(rq);
-                       q->boundary_rq = NULL;
-               }
-
-               if (rq->cmd_flags & REQ_DONTPREP)
-                       break;
-
-               if (q->dma_drain_size && rq->data_len) {
-                       /*
-                        * make sure space for the drain appears we
-                        * know we can do this because max_hw_segments
-                        * has been adjusted to be one fewer than the
-                        * device can handle
-                        */
-                       rq->nr_phys_segments++;
-               }
-
-               if (!q->prep_rq_fn)
-                       break;
-
-               ret = q->prep_rq_fn(q, rq);
-               if (ret == BLKPREP_OK) {
-                       break;
-               } else if (ret == BLKPREP_DEFER) {
-                       /*
-                        * the request may have been (partially) prepped.
-                        * we need to keep this request in the front to
-                        * avoid resource deadlock.  REQ_STARTED will
-                        * prevent other fs requests from passing this one.
-                        */
-                       if (q->dma_drain_size && rq->data_len &&
-                           !(rq->cmd_flags & REQ_DONTPREP)) {
-                               /*
-                                * remove the space for the drain we added
-                                * so that we don't add it again
-                                */
-                               --rq->nr_phys_segments;
-                       }
-
-                       rq = NULL;
-                       break;
-               } else if (ret == BLKPREP_KILL) {
-                       rq->cmd_flags |= REQ_QUIET;
-                       __blk_end_request(rq, -EIO, blk_rq_bytes(rq));
-               } else {
-                       printk(KERN_ERR "%s: bad return=%d\n", __func__, ret);
-                       break;
-               }
-       }
-
-       return rq;
-}
-EXPORT_SYMBOL(elv_next_request);
-
-void elv_dequeue_request(struct request_queue *q, struct request *rq)
-{
-       BUG_ON(list_empty(&rq->queuelist));
-       BUG_ON(ELV_ON_HASH(rq));
-
-       list_del_init(&rq->queuelist);
-
-       /*
-        * the time frame between a request being removed from the lists
-        * and to it is freed is accounted as io that is in progress at
-        * the driver side.
-        */
-       if (blk_account_rq(rq))
-               q->in_flight++;
-}
-
-int elv_queue_empty(struct request_queue *q)
-{
-       struct elevator_queue *e = q->elevator;
-
-       if (!list_empty(&q->queue_head))
-               return 0;
-
-       if (e->ops->elevator_queue_empty_fn)
-               return e->ops->elevator_queue_empty_fn(q);
-
-       return 1;
-}
-EXPORT_SYMBOL(elv_queue_empty);
-
 struct request *elv_latter_request(struct request_queue *q, struct request *rq)
 {
        struct elevator_queue *e = q->elevator;
@@ -908,7 +772,7 @@ int elv_set_request(struct request_queue *q, struct request *rq, gfp_t gfp_mask)
        if (e->ops->elevator_set_req_fn)
                return e->ops->elevator_set_req_fn(q, rq, gfp_mask);
 
-       rq->elevator_private = NULL;
+       rq->elevator_private[0] = NULL;
        return 0;
 }
 
@@ -934,11 +798,18 @@ void elv_abort_queue(struct request_queue *q)
 {
        struct request *rq;
 
+       blk_abort_flushes(q);
+
        while (!list_empty(&q->queue_head)) {
                rq = list_entry_rq(q->queue_head.next);
                rq->cmd_flags |= REQ_QUIET;
                trace_block_rq_abort(q, rq);
-               __blk_end_request(rq, -EIO, blk_rq_bytes(rq));
+               /*
+                * Mark this request as started so we don't trigger
+                * any debug logic in the end I/O path.
+                */
+               blk_start_request(rq);
+               __blk_end_request_all(rq, -EIO);
        }
 }
 EXPORT_SYMBOL(elv_abort_queue);
@@ -951,28 +822,11 @@ void elv_completed_request(struct request_queue *q, struct request *rq)
         * request is released from the driver, io must be done
         */
        if (blk_account_rq(rq)) {
-               q->in_flight--;
-               if (blk_sorted_rq(rq) && e->ops->elevator_completed_req_fn)
+               q->in_flight[rq_is_sync(rq)]--;
+               if ((rq->cmd_flags & REQ_SORTED) &&
+                   e->ops->elevator_completed_req_fn)
                        e->ops->elevator_completed_req_fn(q, rq);
        }
-
-       /*
-        * Check if the queue is waiting for fs requests to be
-        * drained for flush sequence.
-        */
-       if (unlikely(q->ordseq)) {
-               struct request *next = NULL;
-
-               if (!list_empty(&q->queue_head))
-                       next = list_entry_rq(q->queue_head.next);
-
-               if (!q->in_flight &&
-                   blk_ordered_cur_seq(q) == QUEUE_ORDSEQ_DRAIN &&
-                   (!next || blk_ordered_req_seq(next) > QUEUE_ORDSEQ_DRAIN)) {
-                       blk_ordered_complete_seq(q, QUEUE_ORDSEQ_DRAIN, 0);
-                       __blk_run_queue(q);
-               }
-       }
 }
 
 #define to_elv(atr) container_of((atr), struct elv_fs_entry, attr)
@@ -1012,7 +866,7 @@ elv_attr_store(struct kobject *kobj, struct attribute *attr,
        return error;
 }
 
-static struct sysfs_ops elv_sysfs_ops = {
+static const struct sysfs_ops elv_sysfs_ops = {
        .show   = elv_attr_show,
        .store  = elv_attr_store,
 };
@@ -1038,14 +892,17 @@ int elv_register_queue(struct request_queue *q)
                        }
                }
                kobject_uevent(&e->kobj, KOBJ_ADD);
+               e->registered = 1;
        }
        return error;
 }
+EXPORT_SYMBOL(elv_register_queue);
 
 static void __elv_unregister_queue(struct elevator_queue *e)
 {
        kobject_uevent(&e->kobj, KOBJ_REMOVE);
        kobject_del(&e->kobj);
+       e->registered = 0;
 }
 
 void elv_unregister_queue(struct request_queue *q)
@@ -1053,6 +910,7 @@ void elv_unregister_queue(struct request_queue *q)
        if (q)
                __elv_unregister_queue(q->elevator);
 }
+EXPORT_SYMBOL(elv_unregister_queue);
 
 void elv_register(struct elevator_type *e)
 {
@@ -1107,18 +965,19 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 {
        struct elevator_queue *old_elevator, *e;
        void *data;
+       int err;
 
        /*
         * Allocate new elevator
         */
        e = elevator_alloc(q, new_e);
        if (!e)
-               return 0;
+               return -ENOMEM;
 
        data = elevator_init_queue(q, e);
        if (!data) {
                kobject_put(&e->kobj);
-               return 0;
+               return -ENOMEM;
        }
 
        /*
@@ -1139,10 +998,13 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 
        spin_unlock_irq(q->queue_lock);
 
-       __elv_unregister_queue(old_elevator);
+       if (old_elevator->registered) {
+               __elv_unregister_queue(old_elevator);
 
-       if (elv_register_queue(q))
-               goto fail_register;
+               err = elv_register_queue(q);
+               if (err)
+                       goto fail_register;
+       }
 
        /*
         * finally exit old elevator and turn off BYPASS.
@@ -1154,7 +1016,7 @@ static int elevator_switch(struct request_queue *q, struct elevator_type *new_e)
 
        blk_add_trace_msg(q, "elv switch: %s", e->elevator_type->elevator_name);
 
-       return 1;
+       return 0;
 
 fail_register:
        /*
@@ -1169,19 +1031,22 @@ fail_register:
        queue_flag_clear(QUEUE_FLAG_ELVSWITCH, q);
        spin_unlock_irq(q->queue_lock);
 
-       return 0;
+       return err;
 }
 
-ssize_t elv_iosched_store(struct request_queue *q, const char *name,
-                         size_t count)
+/*
+ * Switch this queue to the given IO scheduler.
+ */
+int elevator_change(struct request_queue *q, const char *name)
 {
        char elevator_name[ELV_NAME_MAX];
        struct elevator_type *e;
 
-       strlcpy(elevator_name, name, sizeof(elevator_name));
-       strstrip(elevator_name);
+       if (!q->elevator)
+               return -ENXIO;
 
-       e = elevator_get(elevator_name);
+       strlcpy(elevator_name, name, sizeof(elevator_name));
+       e = elevator_get(strstrip(elevator_name));
        if (!e) {
                printk(KERN_ERR "elevator: type %s not found\n", elevator_name);
                return -EINVAL;
@@ -1189,22 +1054,41 @@ ssize_t elv_iosched_store(struct request_queue *q, const char *name,
 
        if (!strcmp(elevator_name, q->elevator->elevator_type->elevator_name)) {
                elevator_put(e);
-               return count;
+               return 0;
        }
 
-       if (!elevator_switch(q, e))
-               printk(KERN_ERR "elevator: switch to %s failed\n",
-                                                       elevator_name);
-       return count;
+       return elevator_switch(q, e);
+}
+EXPORT_SYMBOL(elevator_change);
+
+ssize_t elv_iosched_store(struct request_queue *q, const char *name,
+                         size_t count)
+{
+       int ret;
+
+       if (!q->elevator)
+               return count;
+
+       ret = elevator_change(q, name);
+       if (!ret)
+               return count;
+
+       printk(KERN_ERR "elevator: switch to %s failed\n", name);
+       return ret;
 }
 
 ssize_t elv_iosched_show(struct request_queue *q, char *name)
 {
        struct elevator_queue *e = q->elevator;
-       struct elevator_type *elv = e->elevator_type;
+       struct elevator_type *elv;
        struct elevator_type *__e;
        int len = 0;
 
+       if (!q->elevator || !blk_queue_stackable(q))
+               return sprintf(name, "none\n");
+
+       elv = e->elevator_type;
+
        spin_lock(&elv_list_lock);
        list_for_each_entry(__e, &elv_list, list) {
                if (!strcmp(elv->elevator_name, __e->elevator_name))