Cross-Referenced Linux and Device Driver Code

   /*
    *  linux/drivers/block/ll_rw_blk.c
    *
    * Copyright (C) 1991, 1992 Linus Torvalds
    * Copyright (C) 1994,      Karl Keyte: Added support for disk statistics
    * Elevator latency, (C) 2000  Andrea Arcangeli <andrea@suse.de> SuSE
    * Queue request tables / lock, selectable elevator, Jens Axboe <axboe@suse.de>
    * kernel-doc documentation started by NeilBrown <neilb@cse.unsw.edu.au> -  July2000
    * bio rewrite, highmem i/o, etc, Jens Axboe <axboe@suse.de> - may 2001
   */
  
  /*
   * This handles all read/write requests to block devices
   */
  #include <linux/config.h>
  #include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/backing-dev.h>
  #include <linux/bio.h>
  #include <linux/blkdev.h>
  #include <linux/highmem.h>
  #include <linux/mm.h>
  #include <linux/kernel_stat.h>
  #include <linux/string.h>
  #include <linux/init.h>
  #include <linux/bootmem.h>      /* for max_pfn/max_low_pfn */
  #include <linux/completion.h>
  #include <linux/slab.h>
  #include <linux/swap.h>
  #include <linux/writeback.h>
  
  /*
   * for max sense size
   */
  #include <scsi/scsi_cmnd.h>
  
  static void blk_unplug_work(void *data);
  static void blk_unplug_timeout(unsigned long data);
  
  /*
   * For the allocated request tables
   */
  static kmem_cache_t *request_cachep;
  
  /*
   * For queue allocation
   */
  static kmem_cache_t *requestq_cachep;
  
  /*
   * For io context allocations
   */
  static kmem_cache_t *iocontext_cachep;
  
  static wait_queue_head_t congestion_wqh[2] = {
                  __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[0]),
                  __WAIT_QUEUE_HEAD_INITIALIZER(congestion_wqh[1])
          };
  
  /*
   * Controlling structure to kblockd
   */
  static struct workqueue_struct *kblockd_workqueue; 
  
  unsigned long blk_max_low_pfn, blk_max_pfn;
  
  EXPORT_SYMBOL(blk_max_low_pfn);
  EXPORT_SYMBOL(blk_max_pfn);
  
  /* Amount of time in which a process may batch requests */
  #define BLK_BATCH_TIME  (HZ/50UL)
  
  /* Number of requests a "batching" process may submit */
  #define BLK_BATCH_REQ   32
  
  /*
   * Return the threshold (number of used requests) at which the queue is
   * considered to be congested.  It include a little hysteresis to keep the
   * context switch rate down.
   */
  static inline int queue_congestion_on_threshold(struct request_queue *q)
  {
          return q->nr_congestion_on;
  }
  
  /*
   * The threshold at which a queue is considered to be uncongested
   */
  static inline int queue_congestion_off_threshold(struct request_queue *q)
  {
          return q->nr_congestion_off;
  }
  
  static void blk_queue_congestion_threshold(struct request_queue *q)
  {
          int nr;
  
          nr = q->nr_requests - (q->nr_requests / 8) + 1;
          if (nr > q->nr_requests)
                 nr = q->nr_requests;
         q->nr_congestion_on = nr;
 
         nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests / 16) - 1;
         if (nr < 1)
                 nr = 1;
         q->nr_congestion_off = nr;
 }
 
 /*
  * A queue has just exitted congestion.  Note this in the global counter of
  * congested queues, and wake up anyone who was waiting for requests to be
  * put back.
  */
 static void clear_queue_congested(request_queue_t *q, int rw)
 {
         enum bdi_state bit;
         wait_queue_head_t *wqh = &congestion_wqh[rw];
 
         bit = (rw == WRITE) ? BDI_write_congested : BDI_read_congested;
         clear_bit(bit, &q->backing_dev_info.state);
         smp_mb__after_clear_bit();
         if (waitqueue_active(wqh))
                 wake_up(wqh);
 }
 
 /*
  * A queue has just entered congestion.  Flag that in the queue's VM-visible
  * state flags and increment the global gounter of congested queues.
  */
 static void set_queue_congested(request_queue_t *q, int rw)
 {
         enum bdi_state bit;
 
         bit = (rw == WRITE) ? BDI_write_congested : BDI_read_congested;
         set_bit(bit, &q->backing_dev_info.state);
 }
 
 /**
  * blk_get_backing_dev_info - get the address of a queue's backing_dev_info
  * @bdev:       device
  *
  * Locates the passed device's request queue and returns the address of its
  * backing_dev_info
  *
  * Will return NULL if the request queue cannot be located.
  */
 struct backing_dev_info *blk_get_backing_dev_info(struct block_device *bdev)
 {
         struct backing_dev_info *ret = NULL;
         request_queue_t *q = bdev_get_queue(bdev);
 
         if (q)
                 ret = &q->backing_dev_info;
         return ret;
 }
 
 EXPORT_SYMBOL(blk_get_backing_dev_info);
 
 void blk_queue_activity_fn(request_queue_t *q, activity_fn *fn, void *data)
 {
         q->activity_fn = fn;
         q->activity_data = data;
 }
 
 EXPORT_SYMBOL(blk_queue_activity_fn);
 
 /**
  * blk_queue_prep_rq - set a prepare_request function for queue
  * @q:          queue
  * @pfn:        prepare_request function
  *
  * It's possible for a queue to register a prepare_request callback which
  * is invoked before the request is handed to the request_fn. The goal of
  * the function is to prepare a request for I/O, it can be used to build a
  * cdb from the request data for instance.
  *
  */
 void blk_queue_prep_rq(request_queue_t *q, prep_rq_fn *pfn)
 {
         q->prep_rq_fn = pfn;
 }
 
 EXPORT_SYMBOL(blk_queue_prep_rq);
 
 /**
  * blk_queue_merge_bvec - set a merge_bvec function for queue
  * @q:          queue
  * @mbfn:       merge_bvec_fn
  *
  * Usually queues have static limitations on the max sectors or segments that
  * we can put in a request. Stacking drivers may have some settings that
  * are dynamic, and thus we have to query the queue whether it is ok to
  * add a new bio_vec to a bio at a given offset or not. If the block device
  * has such limitations, it needs to register a merge_bvec_fn to control
  * the size of bio's sent to it. Note that a block device *must* allow a
  * single page to be added to an empty bio. The block device driver may want
  * to use the bio_split() function to deal with these bio's. By default
  * no merge_bvec_fn is defined for a queue, and only the fixed limits are
  * honored.
  */
 void blk_queue_merge_bvec(request_queue_t *q, merge_bvec_fn *mbfn)
 {
         q->merge_bvec_fn = mbfn;
 }
 
 EXPORT_SYMBOL(blk_queue_merge_bvec);
 
 /**
  * blk_queue_make_request - define an alternate make_request function for a device
  * @q:  the request queue for the device to be affected
  * @mfn: the alternate make_request function
  *
  * Description:
  *    The normal way for &struct bios to be passed to a device
  *    driver is for them to be collected into requests on a request
  *    queue, and then to allow the device driver to select requests
  *    off that queue when it is ready.  This works well for many block
  *    devices. However some block devices (typically virtual devices
  *    such as md or lvm) do not benefit from the processing on the
  *    request queue, and are served best by having the requests passed
  *    directly to them.  This can be achieved by providing a function
  *    to blk_queue_make_request().
  *
  * Caveat:
  *    The driver that does this *must* be able to deal appropriately
  *    with buffers in "highmemory". This can be accomplished by either calling
  *    __bio_kmap_atomic() to get a temporary kernel mapping, or by calling
  *    blk_queue_bounce() to create a buffer in normal memory.
  **/
 void blk_queue_make_request(request_queue_t * q, make_request_fn * mfn)
 {
         /*
          * set defaults
          */
         q->nr_requests = BLKDEV_MAX_RQ;
         q->max_phys_segments = MAX_PHYS_SEGMENTS;
         q->max_hw_segments = MAX_HW_SEGMENTS;
         q->make_request_fn = mfn;
         q->backing_dev_info.ra_pages = (VM_MAX_READAHEAD * 1024) / PAGE_CACHE_SIZE;
         q->backing_dev_info.state = 0;
         q->backing_dev_info.memory_backed = 0;
         blk_queue_max_sectors(q, MAX_SECTORS);
         blk_queue_hardsect_size(q, 512);
         blk_queue_dma_alignment(q, 511);
         blk_queue_congestion_threshold(q);
         q->nr_batching = BLK_BATCH_REQ;
 
         q->unplug_thresh = 4;           /* hmm */
         q->unplug_delay = (3 * HZ) / 1000;      /* 3 milliseconds */
         if (q->unplug_delay == 0)
                 q->unplug_delay = 1;
 
         INIT_WORK(&q->unplug_work, blk_unplug_work, q);
 
         q->unplug_timer.function = blk_unplug_timeout;
         q->unplug_timer.data = (unsigned long)q;
 
         /*
          * by default assume old behaviour and bounce for any highmem page
          */
         blk_queue_bounce_limit(q, BLK_BOUNCE_HIGH);
 
         blk_queue_activity_fn(q, NULL, NULL);
 
         INIT_LIST_HEAD(&q->drain_list);
 }
 
 EXPORT_SYMBOL(blk_queue_make_request);
 
 /**
  * blk_queue_ordered - does this queue support ordered writes
  * @q:     the request queue
  * @flag:  see below
  *
  * Description:
  *   For journalled file systems, doing ordered writes on a commit
  *   block instead of explicitly doing wait_on_buffer (which is bad
  *   for performance) can be a big win. Block drivers supporting this
  *   feature should call this function and indicate so.
  *
  **/
 void blk_queue_ordered(request_queue_t *q, int flag)
 {
         if (flag)
                 set_bit(QUEUE_FLAG_ORDERED, &q->queue_flags);
         else
                 clear_bit(QUEUE_FLAG_ORDERED, &q->queue_flags);
 }
 
 EXPORT_SYMBOL(blk_queue_ordered);
 
 /**
  * blk_queue_issue_flush_fn - set function for issuing a flush
  * @q:     the request queue
  * @iff:   the function to be called issuing the flush
  *
  * Description:
  *   If a driver supports issuing a flush command, the support is notified
  *   to the block layer by defining it through this call.
  *
  **/
 void blk_queue_issue_flush_fn(request_queue_t *q, issue_flush_fn *iff)
 {
         q->issue_flush_fn = iff;
 }
 
 EXPORT_SYMBOL(blk_queue_issue_flush_fn);
 
 /**
  * blk_queue_bounce_limit - set bounce buffer limit for queue
  * @q:  the request queue for the device
  * @dma_addr:   bus address limit
  *
  * Description:
  *    Different hardware can have different requirements as to what pages
  *    it can do I/O directly to. A low level driver can call
  *    blk_queue_bounce_limit to have lower memory pages allocated as bounce
  *    buffers for doing I/O to pages residing above @page. By default
  *    the block layer sets this to the highest numbered "low" memory page.
  **/
 void blk_queue_bounce_limit(request_queue_t *q, u64 dma_addr)
 {
         unsigned long bounce_pfn = dma_addr >> PAGE_SHIFT;
 
         /*
          * set appropriate bounce gfp mask -- unfortunately we don't have a
          * full 4GB zone, so we have to resort to low memory for any bounces.
          * ISA has its own < 16MB zone.
          */
         if (bounce_pfn < blk_max_low_pfn) {
                 BUG_ON(dma_addr < BLK_BOUNCE_ISA);
                 init_emergency_isa_pool();
                 q->bounce_gfp = GFP_NOIO | GFP_DMA;
         } else
                 q->bounce_gfp = GFP_NOIO;
 
         q->bounce_pfn = bounce_pfn;
 }
 
 EXPORT_SYMBOL(blk_queue_bounce_limit);
 
 /**
  * blk_queue_max_sectors - set max sectors for a request for this queue
  * @q:  the request queue for the device
  * @max_sectors:  max sectors in the usual 512b unit
  *
  * Description:
  *    Enables a low level driver to set an upper limit on the size of
  *    received requests.
  **/
 void blk_queue_max_sectors(request_queue_t *q, unsigned short max_sectors)
 {
         if ((max_sectors << 9) < PAGE_CACHE_SIZE) {
                 max_sectors = 1 << (PAGE_CACHE_SHIFT - 9);
                 printk("%s: set to minimum %d\n", __FUNCTION__, max_sectors);
         }
 
         q->max_sectors = q->max_hw_sectors = max_sectors;
 }
 
 EXPORT_SYMBOL(blk_queue_max_sectors);
 
 /**
  * blk_queue_max_phys_segments - set max phys segments for a request for this queue
  * @q:  the request queue for the device
  * @max_segments:  max number of segments
  *
  * Description:
  *    Enables a low level driver to set an upper limit on the number of
  *    physical data segments in a request.  This would be the largest sized
  *    scatter list the driver could handle.
  **/
 void blk_queue_max_phys_segments(request_queue_t *q, unsigned short max_segments)
 {
         if (!max_segments) {
                 max_segments = 1;
                 printk("%s: set to minimum %d\n", __FUNCTION__, max_segments);
         }
 
         q->max_phys_segments = max_segments;
 }
 
 EXPORT_SYMBOL(blk_queue_max_phys_segments);
 
 /**
  * blk_queue_max_hw_segments - set max hw segments for a request for this queue
  * @q:  the request queue for the device
  * @max_segments:  max number of segments
  *
  * Description:
  *    Enables a low level driver to set an upper limit on the number of
  *    hw data segments in a request.  This would be the largest number of
  *    address/length pairs the host adapter can actually give as once
  *    to the device.
  **/
 void blk_queue_max_hw_segments(request_queue_t *q, unsigned short max_segments)
 {
         if (!max_segments) {
                 max_segments = 1;
                 printk("%s: set to minimum %d\n", __FUNCTION__, max_segments);
         }
 
         q->max_hw_segments = max_segments;
 }
 
 EXPORT_SYMBOL(blk_queue_max_hw_segments);
 
 /**
  * blk_queue_max_segment_size - set max segment size for blk_rq_map_sg
  * @q:  the request queue for the device
  * @max_size:  max size of segment in bytes
  *
  * Description:
  *    Enables a low level driver to set an upper limit on the size of a
  *    coalesced segment
  **/
 void blk_queue_max_segment_size(request_queue_t *q, unsigned int max_size)
 {
         if (max_size < PAGE_CACHE_SIZE) {
                 max_size = PAGE_CACHE_SIZE;
                 printk("%s: set to minimum %d\n", __FUNCTION__, max_size);
         }
 
         q->max_segment_size = max_size;
 }
 
 EXPORT_SYMBOL(blk_queue_max_segment_size);
 
 /**
  * blk_queue_hardsect_size - set hardware sector size for the queue
  * @q:  the request queue for the device
  * @size:  the hardware sector size, in bytes
  *
  * Description:
  *   This should typically be set to the lowest possible sector size
  *   that the hardware can operate on (possible without reverting to
  *   even internal read-modify-write operations). Usually the default
  *   of 512 covers most hardware.
  **/
 void blk_queue_hardsect_size(request_queue_t *q, unsigned short size)
 {
         q->hardsect_size = size;
 }
 
 EXPORT_SYMBOL(blk_queue_hardsect_size);
 
 /*
  * Returns the minimum that is _not_ zero, unless both are zero.
  */
 #define min_not_zero(l, r) (l == 0) ? r : ((r == 0) ? l : min(l, r))
 
 /**
  * blk_queue_stack_limits - inherit underlying queue limits for stacked drivers
  * @t:  the stacking driver (top)
  * @b:  the underlying device (bottom)
  **/
 void blk_queue_stack_limits(request_queue_t *t, request_queue_t *b)
 {
         /* zero is "infinity" */
         t->max_sectors = t->max_hw_sectors =
                 min_not_zero(t->max_sectors,b->max_sectors);
 
         t->max_phys_segments = min(t->max_phys_segments,b->max_phys_segments);
         t->max_hw_segments = min(t->max_hw_segments,b->max_hw_segments);
         t->max_segment_size = min(t->max_segment_size,b->max_segment_size);
         t->hardsect_size = max(t->hardsect_size,b->hardsect_size);
 }
 
 EXPORT_SYMBOL(blk_queue_stack_limits);
 
 /**
  * blk_queue_segment_boundary - set boundary rules for segment merging
  * @q:  the request queue for the device
  * @mask:  the memory boundary mask
  **/
 void blk_queue_segment_boundary(request_queue_t *q, unsigned long mask)
 {
         if (mask < PAGE_CACHE_SIZE - 1) {
                 mask = PAGE_CACHE_SIZE - 1;
                 printk("%s: set to minimum %lx\n", __FUNCTION__, mask);
         }
 
         q->seg_boundary_mask = mask;
 }
 
 EXPORT_SYMBOL(blk_queue_segment_boundary);
 
 /**
  * blk_queue_dma_alignment - set dma length and memory alignment
  * @q:     the request queue for the device
  * @mask:  alignment mask
  *
  * description:
  *    set required memory and length aligment for direct dma transactions.
  *    this is used when buiding direct io requests for the queue.
  *
  **/
 void blk_queue_dma_alignment(request_queue_t *q, int mask)
 {
         q->dma_alignment = mask;
 }
 
 EXPORT_SYMBOL(blk_queue_dma_alignment);
 
 /**
  * blk_queue_find_tag - find a request by its tag and queue
  *
  * @q:   The request queue for the device
  * @tag: The tag of the request
  *
  * Notes:
  *    Should be used when a device returns a tag and you want to match
  *    it with a request.
  *
  *    no locks need be held.
  **/
 struct request *blk_queue_find_tag(request_queue_t *q, int tag)
 {
         struct blk_queue_tag *bqt = q->queue_tags;
 
         if (unlikely(bqt == NULL || tag >= bqt->real_max_depth))
                 return NULL;
 
         return bqt->tag_index[tag];
 }
 
 EXPORT_SYMBOL(blk_queue_find_tag);
 
 /**
  * __blk_queue_free_tags - release tag maintenance info
  * @q:  the request queue for the device
  *
  *  Notes:
  *    blk_cleanup_queue() will take care of calling this function, if tagging
  *    has been used. So there's no need to call this directly.
  **/
 static void __blk_queue_free_tags(request_queue_t *q)
 {
         struct blk_queue_tag *bqt = q->queue_tags;
 
         if (!bqt)
                 return;
 
         if (atomic_dec_and_test(&bqt->refcnt)) {
                 BUG_ON(bqt->busy);
                 BUG_ON(!list_empty(&bqt->busy_list));
 
                 kfree(bqt->tag_index);
                 bqt->tag_index = NULL;
 
                 kfree(bqt->tag_map);
                 bqt->tag_map = NULL;
 
                 kfree(bqt);
         }
 
         q->queue_tags = NULL;
         q->queue_flags &= ~(1 << QUEUE_FLAG_QUEUED);
 }
 
 /**
  * blk_queue_free_tags - release tag maintenance info
  * @q:  the request queue for the device
  *
  *  Notes:
  *      This is used to disabled tagged queuing to a device, yet leave
  *      queue in function.
  **/
 void blk_queue_free_tags(request_queue_t *q)
 {
         clear_bit(QUEUE_FLAG_QUEUED, &q->queue_flags);
 }
 
 EXPORT_SYMBOL(blk_queue_free_tags);
 
 static int
 init_tag_map(request_queue_t *q, struct blk_queue_tag *tags, int depth)
 {
         int bits, i;
         struct request **tag_index;
         unsigned long *tag_map;
 
         if (depth > q->nr_requests * 2) {
                 depth = q->nr_requests * 2;
                 printk(KERN_ERR "%s: adjusted depth to %d\n",
                                 __FUNCTION__, depth);
         }
 
         tag_index = kmalloc(depth * sizeof(struct request *), GFP_ATOMIC);
         if (!tag_index)
                 goto fail;
 
         bits = (depth / BLK_TAGS_PER_LONG) + 1;
         tag_map = kmalloc(bits * sizeof(unsigned long), GFP_ATOMIC);
         if (!tag_map)
                 goto fail;
 
         memset(tag_index, 0, depth * sizeof(struct request *));
         memset(tag_map, 0, bits * sizeof(unsigned long));
         tags->max_depth = depth;
         tags->real_max_depth = bits * BITS_PER_LONG;
         tags->tag_index = tag_index;
         tags->tag_map = tag_map;
 
         /*
          * set the upper bits if the depth isn't a multiple of the word size
          */
         for (i = depth; i < bits * BLK_TAGS_PER_LONG; i++)
                 __set_bit(i, tag_map);
 
         return 0;
 fail:
         kfree(tag_index);
         return -ENOMEM;
 }
 
 /**
  * blk_queue_init_tags - initialize the queue tag info
  * @q:  the request queue for the device
  * @depth:  the maximum queue depth supported
  **/
 int blk_queue_init_tags(request_queue_t *q, int depth,
                         struct blk_queue_tag *tags)
 {
         int rc;
 
         BUG_ON(tags && q->queue_tags && tags != q->queue_tags);
 
         if (!tags && !q->queue_tags) {
                 tags = kmalloc(sizeof(struct blk_queue_tag), GFP_ATOMIC);
                 if (!tags)
                         goto fail;
 
                 if (init_tag_map(q, tags, depth))
                         goto fail;
 
                 INIT_LIST_HEAD(&tags->busy_list);
                 tags->busy = 0;
                 atomic_set(&tags->refcnt, 1);
         } else if (q->queue_tags) {
                 if ((rc = blk_queue_resize_tags(q, depth)))
                         return rc;
                 set_bit(QUEUE_FLAG_QUEUED, &q->queue_flags);
                 return 0;
         } else
                 atomic_inc(&tags->refcnt);
 
         /*
          * assign it, all done
          */
         q->queue_tags = tags;
         q->queue_flags |= (1 << QUEUE_FLAG_QUEUED);
         return 0;
 fail:
         kfree(tags);
         return -ENOMEM;
 }
 
 EXPORT_SYMBOL(blk_queue_init_tags);
 
 /**
  * blk_queue_resize_tags - change the queueing depth
  * @q:  the request queue for the device
  * @new_depth: the new max command queueing depth
  *
  *  Notes:
  *    Must be called with the queue lock held.
  **/
 int blk_queue_resize_tags(request_queue_t *q, int new_depth)
 {
         struct blk_queue_tag *bqt = q->queue_tags;
         struct request **tag_index;
         unsigned long *tag_map;
         int bits, max_depth;
 
         if (!bqt)
                 return -ENXIO;
 
         /*
          * don't bother sizing down
          */
         if (new_depth <= bqt->real_max_depth) {
                 bqt->max_depth = new_depth;
                 return 0;
         }
 
         /*
          * save the old state info, so we can copy it back
          */
         tag_index = bqt->tag_index;
         tag_map = bqt->tag_map;
         max_depth = bqt->real_max_depth;
 
         if (init_tag_map(q, bqt, new_depth))
                 return -ENOMEM;
 
         memcpy(bqt->tag_index, tag_index, max_depth * sizeof(struct request *));
         bits = max_depth / BLK_TAGS_PER_LONG;
         memcpy(bqt->tag_map, tag_map, bits * sizeof(unsigned long));
 
         kfree(tag_index);
         kfree(tag_map);
         return 0;
 }
 
 EXPORT_SYMBOL(blk_queue_resize_tags);
 
 /**
  * blk_queue_end_tag - end tag operations for a request
  * @q:  the request queue for the device
  * @rq: the request that has completed
  *
  *  Description:
  *    Typically called when end_that_request_first() returns 0, meaning
  *    all transfers have been done for a request. It's important to call
  *    this function before end_that_request_last(), as that will put the
  *    request back on the free list thus corrupting the internal tag list.
  *
  *  Notes:
  *   queue lock must be held.
  **/
 void blk_queue_end_tag(request_queue_t *q, struct request *rq)
 {
         struct blk_queue_tag *bqt = q->queue_tags;
         int tag = rq->tag;
 
         BUG_ON(tag == -1);
 
         if (unlikely(tag >= bqt->real_max_depth))
                 return;
 
         if (unlikely(!__test_and_clear_bit(tag, bqt->tag_map))) {
                 printk("attempt to clear non-busy tag (%d)\n", tag);
                 return;
         }
 
         list_del_init(&rq->queuelist);
         rq->flags &= ~REQ_QUEUED;
         rq->tag = -1;
 
         if (unlikely(bqt->tag_index[tag] == NULL))
                 printk("tag %d is missing\n", tag);
 
         bqt->tag_index[tag] = NULL;
         bqt->busy--;
 }
 
 EXPORT_SYMBOL(blk_queue_end_tag);
 
 /**
  * blk_queue_start_tag - find a free tag and assign it
  * @q:  the request queue for the device
  * @rq:  the block request that needs tagging
  *
  *  Description:
  *    This can either be used as a stand-alone helper, or possibly be
  *    assigned as the queue &prep_rq_fn (in which case &struct request
  *    automagically gets a tag assigned). Note that this function
  *    assumes that any type of request can be queued! if this is not
  *    true for your device, you must check the request type before
  *    calling this function.  The request will also be removed from
  *    the request queue, so it's the drivers responsibility to readd
  *    it if it should need to be restarted for some reason.
  *
  *  Notes:
  *   queue lock must be held.
  **/
 int blk_queue_start_tag(request_queue_t *q, struct request *rq)
 {
         struct blk_queue_tag *bqt = q->queue_tags;
         unsigned long *map = bqt->tag_map;
         int tag = 0;
 
         if (unlikely((rq->flags & REQ_QUEUED))) {
                 printk(KERN_ERR 
                        "request %p for device [%s] already tagged %d",
                        rq, rq->rq_disk ? rq->rq_disk->disk_name : "?", rq->tag);
                 BUG();
         }
 
         for (map = bqt->tag_map; *map == -1UL; map++) {
                 tag += BLK_TAGS_PER_LONG;
 
                 if (tag >= bqt->max_depth)
                         return 1;
         }
 
         tag += ffz(*map);
         __set_bit(tag, bqt->tag_map);
 
         rq->flags |= REQ_QUEUED;
         rq->tag = tag;
         bqt->tag_index[tag] = rq;
         blkdev_dequeue_request(rq);
         list_add(&rq->queuelist, &bqt->busy_list);
         bqt->busy++;
         return 0;
 }
 
 EXPORT_SYMBOL(blk_queue_start_tag);
 
 /**
  * blk_queue_invalidate_tags - invalidate all pending tags
  * @q:  the request queue for the device
  *
  *  Description:
  *   Hardware conditions may dictate a need to stop all pending requests.
  *   In this case, we will safely clear the block side of the tag queue and
  *   readd all requests to the request queue in the right order.
  *
  *  Notes:
  *   queue lock must be held.
  **/
 void blk_queue_invalidate_tags(request_queue_t *q)
 {
         struct blk_queue_tag *bqt = q->queue_tags;
         struct list_head *tmp, *n;
         struct request *rq;
 
         list_for_each_safe(tmp, n, &bqt->busy_list) {
                 rq = list_entry_rq(tmp);
 
                 if (rq->tag == -1) {
                         printk("bad tag found on list\n");
                         list_del_init(&rq->queuelist);
                         rq->flags &= ~REQ_QUEUED;
                 } else
                         blk_queue_end_tag(q, rq);
 
                 rq->flags &= ~REQ_STARTED;
                 __elv_add_request(q, rq, ELEVATOR_INSERT_BACK, 0);
         }
 }
 
 EXPORT_SYMBOL(blk_queue_invalidate_tags);
 
 static char *rq_flags[] = {
         "REQ_RW",
         "REQ_FAILFAST",
         "REQ_SOFTBARRIER",
         "REQ_HARDBARRIER",
         "REQ_CMD",
         "REQ_NOMERGE",
         "REQ_STARTED",
         "REQ_DONTPREP",
         "REQ_QUEUED",
         "REQ_PC",
         "REQ_BLOCK_PC",
         "REQ_SENSE",
         "REQ_FAILED",
         "REQ_QUIET",
         "REQ_SPECIAL",
         "REQ_DRIVE_CMD",
         "REQ_DRIVE_TASK",
         "REQ_DRIVE_TASKFILE",
         "REQ_PREEMPT",
         "REQ_PM_SUSPEND",
         "REQ_PM_RESUME",
         "REQ_PM_SHUTDOWN",
 };
 
 void blk_dump_rq_flags(struct request *rq, char *msg)
 {
         int bit;
 
         printk("%s: dev %s: flags = ", msg,
                 rq->rq_disk ? rq->rq_disk->disk_name : "?");
         bit = 0;
         do {
                 if (rq->flags & (1 << bit))
                         printk("%s ", rq_flags[bit]);
                 bit++;
         } while (bit < __REQ_NR_BITS);
 
         printk("\nsector %llu, nr/cnr %lu/%u\n", (unsigned long long)rq->sector,
                                                        rq->nr_sectors,
                                                        rq->current_nr_sectors);
         printk("bio %p, biotail %p, buffer %p, data %p, len %u\n", rq->bio, rq->biotail, rq->buffer, rq->data, rq->data_len);
 
         if (rq->flags & (REQ_BLOCK_PC | REQ_PC)) {
                 printk("cdb: ");
                 for (bit = 0; bit < sizeof(rq->cmd); bit++)
                         printk("%02x ", rq->cmd[bit]);
                 printk("\n");
         }
 }
 
 EXPORT_SYMBOL(blk_dump_rq_flags);
 
 void blk_recount_segments(request_queue_t *q, struct bio *bio)
 {
         struct bio_vec *bv, *bvprv = NULL;
         int i, nr_phys_segs, nr_hw_segs, seg_size, hw_seg_size, cluster;
         int high, highprv = 1;
 
         if (unlikely(!bio->bi_io_vec))
                 return;
 
         cluster = q->queue_flags & (1 << QUEUE_FLAG_CLUSTER);
         hw_seg_size = seg_size = nr_phys_segs = nr_hw_segs = 0;
         bio_for_each_segment(bv, bio, i) {
                 /*
                  * the trick here is making sure that a high page is never
                  * considered part of another segment, since that might
                  * change with the bounce page.
                  */
                 high = page_to_pfn(bv->bv_page) >= q->bounce_pfn;
                 if (high || highprv)
                         goto new_hw_segment;
                 if (cluster) {
                         if (seg_size + bv->bv_len > q->max_segment_size)
                                 goto new_segment;
                         if (!BIOVEC_PHYS_MERGEABLE(bvprv, bv))
                                 goto new_segment;
                         if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bv))
                                 goto new_segment;
                         if (BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len))
                                 goto new_hw_segment;
 
                         seg_size += bv->bv_len;
                         hw_seg_size += bv->bv_len;
                         bvprv = bv;
                         continue;
                 }
 new_segment:
                 if (BIOVEC_VIRT_MERGEABLE(bvprv, bv) &&
                     !BIOVEC_VIRT_OVERSIZE(hw_seg_size + bv->bv_len)) {
                         hw_seg_size += bv->bv_len;
                 } else {
 new_hw_segment:
                         if (hw_seg_size > bio->bi_hw_front_size)
                                 bio->bi_hw_front_size = hw_seg_size;
                         hw_seg_size = BIOVEC_VIRT_START_SIZE(bv) + bv->bv_len;
                         nr_hw_segs++;
                 }
 
                 nr_phys_segs++;
                 bvprv = bv;
                 seg_size = bv->bv_len;
                 highprv = high;
         }
         if (hw_seg_size > bio->bi_hw_back_size)
                 bio->bi_hw_back_size = hw_seg_size;
         if (nr_hw_segs == 1 && hw_seg_size > bio->bi_hw_front_size)
                 bio->bi_hw_front_size = hw_seg_size;
         bio->bi_phys_segments = nr_phys_segs;
         bio->bi_hw_segments = nr_hw_segs;
         bio->bi_flags |= (1 << BIO_SEG_VALID);
 }
 
 
 int blk_phys_contig_segment(request_queue_t *q, struct bio *bio,
                                    struct bio *nxt)
 {
         if (!(q->queue_flags & (1 << QUEUE_FLAG_CLUSTER)))
                 return 0;
 
         if (!BIOVEC_PHYS_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)))
                 return 0;
         if (bio->bi_size + nxt->bi_size > q->max_segment_size)
                 return 0;
 
         /*
          * bio and nxt are contigous in memory, check if the queue allows
          * these two to be merged into one
          */
         if (BIO_SEG_BOUNDARY(q, bio, nxt))
                 return 1;
 
         return 0;
 }
 
 EXPORT_SYMBOL(blk_phys_contig_segment);
 
 int blk_hw_contig_segment(request_queue_t *q, struct bio *bio,
                                  struct bio *nxt)
 {
         if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
                 blk_recount_segments(q, bio);
         if (unlikely(!bio_flagged(nxt, BIO_SEG_VALID)))
                 blk_recount_segments(q, nxt);
         if (!BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(nxt)) ||
             BIOVEC_VIRT_OVERSIZE(bio->bi_hw_front_size + bio->bi_hw_back_size))
                 return 0;
         if (bio->bi_size + nxt->bi_size > q->max_segment_size)
                 return 0;
 
         return 1;
 }
 
 EXPORT_SYMBOL(blk_hw_contig_segment);
 
 /*
  * map a request to scatterlist, return number of sg entries setup. Caller
  * must make sure sg can hold rq->nr_phys_segments entries
  */
 int blk_rq_map_sg(request_queue_t *q, struct request *rq, struct scatterlist *sg)
 {
         struct bio_vec *bvec, *bvprv;
         struct bio *bio;
         int nsegs, i, cluster;
 
         nsegs = 0;
         cluster = q->queue_flags & (1 << QUEUE_FLAG_CLUSTER);
 
         /*
          * for each bio in rq
          */
         bvprv = NULL;
         rq_for_each_bio(bio, rq) {
                 /*
                  * for each segment in bio
                  */
                 bio_for_each_segment(bvec, bio, i) {
                         int nbytes = bvec->bv_len;
 
                         if (bvprv && cluster) {
                                 if (sg[nsegs - 1].length + nbytes > q->max_segment_size)
                                         goto new_segment;
 
                                 if (!BIOVEC_PHYS_MERGEABLE(bvprv, bvec))
                                         goto new_segment;
                                 if (!BIOVEC_SEG_BOUNDARY(q, bvprv, bvec))
                                         goto new_segment;
 
                                 sg[nsegs - 1].length += nbytes;
                         } else {
 new_segment:
                                 memset(&sg[nsegs],0,sizeof(struct scatterlist));
                                 sg[nsegs].page = bvec->bv_page;
                                 sg[nsegs].length = nbytes;
                                 sg[nsegs].offset = bvec->bv_offset;
 
                                 nsegs++;
                         }
                         bvprv = bvec;
                 } /* segments in bio */
         } /* bios in rq */
 
         return nsegs;
 }
 
 EXPORT_SYMBOL(blk_rq_map_sg);
 
 /*
  * the standard queue merge functions, can be overridden with device
  * specific ones if so desired
  */
 
 static inline int ll_new_mergeable(request_queue_t *q,
                                    struct request *req,
                                    struct bio *bio)
 {
         int nr_phys_segs = bio_phys_segments(q, bio);
 
         if (req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
                 req->flags |= REQ_NOMERGE;
                 if (req == q->last_merge)
                         q->last_merge = NULL;
                 return 0;
         }
 
         /*
          * A hw segment is just getting larger, bump just the phys
          * counter.
          */
         req->nr_phys_segments += nr_phys_segs;
         return 1;
 }
 
 static inline int ll_new_hw_segment(request_queue_t *q,
                                     struct request *req,
                                     struct bio *bio)
 {
         int nr_hw_segs = bio_hw_segments(q, bio);
         int nr_phys_segs = bio_phys_segments(q, bio);
 
         if (req->nr_hw_segments + nr_hw_segs > q->max_hw_segments
             || req->nr_phys_segments + nr_phys_segs > q->max_phys_segments) {
                 req->flags |= REQ_NOMERGE;
                 if (req == q->last_merge)
                         q->last_merge = NULL;
                 return 0;
         }
 
         /*
          * This will form the start of a new hw segment.  Bump both
          * counters.
          */
         req->nr_hw_segments += nr_hw_segs;
         req->nr_phys_segments += nr_phys_segs;
         return 1;
 }
 
 static int ll_back_merge_fn(request_queue_t *q, struct request *req, 
                             struct bio *bio)
 {
         int len;
 
         if (req->nr_sectors + bio_sectors(bio) > q->max_sectors) {
                 req->flags |= REQ_NOMERGE;
                 if (req == q->last_merge)
                         q->last_merge = NULL;
                 return 0;
         }
         if (unlikely(!bio_flagged(req->biotail, BIO_SEG_VALID)))
                 blk_recount_segments(q, req->biotail);
         if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
                 blk_recount_segments(q, bio);
         len = req->biotail->bi_hw_back_size + bio->bi_hw_front_size;
         if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(req->biotail), __BVEC_START(bio)) &&
             !BIOVEC_VIRT_OVERSIZE(len)) {
                 int mergeable =  ll_new_mergeable(q, req, bio);
 
                 if (mergeable) {
                         if (req->nr_hw_segments == 1)
                                 req->bio->bi_hw_front_size = len;
                         if (bio->bi_hw_segments == 1)
                                 bio->bi_hw_back_size = len;
                 }
                 return mergeable;
         }
 
         return ll_new_hw_segment(q, req, bio);
 }
 
 static int ll_front_merge_fn(request_queue_t *q, struct request *req, 
                              struct bio *bio)
 {
         int len;
 
         if (req->nr_sectors + bio_sectors(bio) > q->max_sectors) {
                 req->flags |= REQ_NOMERGE;
                 if (req == q->last_merge)
                         q->last_merge = NULL;
                 return 0;
         }
         len = bio->bi_hw_back_size + req->bio->bi_hw_front_size;
         if (unlikely(!bio_flagged(bio, BIO_SEG_VALID)))
                 blk_recount_segments(q, bio);
         if (unlikely(!bio_flagged(req->bio, BIO_SEG_VALID)))
                 blk_recount_segments(q, req->bio);
         if (BIOVEC_VIRT_MERGEABLE(__BVEC_END(bio), __BVEC_START(req->bio)) &&
             !BIOVEC_VIRT_OVERSIZE(len)) {
                 int mergeable =  ll_new_mergeable(q, req, bio);
 
                 if (mergeable) {
                         if (bio->bi_hw_segments == 1)
                                 bio->bi_hw_front_size = len;
                         if (req->nr_hw_segments == 1)
                                 req->biotail->bi_hw_back_size = len;
                 }
                 return mergeable;
         }
 
         return ll_new_hw_segment(q, req, bio);
 }
 
 static int ll_merge_requests_fn(request_queue_t *q, struct request *req,
                                 struct request *next)
 {
         int total_phys_segments = req->nr_phys_segments +next->nr_phys_segments;
         int total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
 
         /*
          * First check if the either of the requests are re-queued
          * requests.  Can't merge them if they are.
          */
         if (req->special || next->special)
                 return 0;
 
         /*
          * Will it become to large?
          */
         if ((req->nr_sectors + next->nr_sectors) > q->max_sectors)
                 return 0;
 
         total_phys_segments = req->nr_phys_segments + next->nr_phys_segments;
         if (blk_phys_contig_segment(q, req->biotail, next->bio))
                 total_phys_segments--;
 
         if (total_phys_segments > q->max_phys_segments)
                 return 0;
 
         total_hw_segments = req->nr_hw_segments + next->nr_hw_segments;
         if (blk_hw_contig_segment(q, req->biotail, next->bio)) {
                 int len = req->biotail->bi_hw_back_size + next->bio->bi_hw_front_size;
                 /*
                  * propagate the combined length to the end of the requests
                  */
                 if (req->nr_hw_segments == 1)
                         req->bio->bi_hw_front_size = len;
                 if (next->nr_hw_segments == 1)
                         next->biotail->bi_hw_back_size = len;
                 total_hw_segments--;
         }
 
         if (total_hw_segments > q->max_hw_segments)
                 return 0;
 
         /* Merge is OK... */
         req->nr_phys_segments = total_phys_segments;
         req->nr_hw_segments = total_hw_segments;
         return 1;
 }
 
 /*
  * "plug" the device if there are no outstanding requests: this will
  * force the transfer to start only after we have put all the requests
  * on the list.
  *
  * This is called with interrupts off and no requests on the queue and
  * with the queue lock held.
  */
 void blk_plug_device(request_queue_t *q)
 {
         WARN_ON(!irqs_disabled());
 
         /*
          * don't plug a stopped queue, it must be paired with blk_start_queue()
          * which will restart the queueing
          */
         if (test_bit(QUEUE_FLAG_STOPPED, &q->queue_flags))
                 return;
 
         if (!test_and_set_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags))
                 mod_timer(&q->unplug_timer, jiffies + q->unplug_delay);
 }
 
 EXPORT_SYMBOL(blk_plug_device);
 
 /*
  * remove the queue from the plugged list, if present. called with
  * queue lock held and interrupts disabled.
  */
 int blk_remove_plug(request_queue_t *q)
 {
         WARN_ON(!irqs_disabled());
 
         if (!test_and_clear_bit(QUEUE_FLAG_PLUGGED, &q->queue_flags))
                 return 0;
 
         del_timer(&q->unplug_timer);
         return 1;
 }
 
 EXPORT_SYMBOL(blk_remove_plug);
 
 /*
  * remove the plug and let it rip..
  */
 void __generic_unplug_device(request_queue_t *q)
 {
         if (test_bit(QUEUE_FLAG_STOPPED, &q->queue_flags))
                 return;
 
         if (!blk_remove_plug(q))
                 return;
 
         /*
          * was plugged, fire request_fn if queue has stuff to do
          */
         if (elv_next_request(q))
                 q->request_fn(q);
 }
 EXPORT_SYMBOL(__generic_unplug_device);
 
 /**
  * generic_unplug_device - fire a request queue
  * @q:    The &request_queue_t in question
  *
  * Description:
  *   Linux uses plugging to build bigger requests queues before letting
  *   the device have at them. If a queue is plugged, the I/O scheduler
  *   is still adding and merging requests on the queue. Once the queue
  *   gets unplugged, the request_fn defined for the queue is invoked and
  *   transfers started.
  **/
 void generic_unplug_device(request_queue_t *q)
 {
         spin_lock_irq(q->queue_lock);
         __generic_unplug_device(q);
         spin_unlock_irq(q->queue_lock);
 }
 EXPORT_SYMBOL(generic_unplug_device);
 
 static void blk_backing_dev_unplug(struct backing_dev_info *bdi,
                                    struct page *page)
 {
         request_queue_t *q = bdi->unplug_io_data;
 
         /*
          * devices don't necessarily have an ->unplug_fn defined
          */
         if (q->unplug_fn)
                 q->unplug_fn(q);
 }
 
 static void blk_unplug_work(void *data)
 {
         request_queue_t *q = data;
 
         q->unplug_fn(q);
 }
 
 static void blk_unplug_timeout(unsigned long data)
 {
         request_queue_t *q = (request_queue_t *)data;
 
         kblockd_schedule_work(&q->unplug_work);
 }
 
 /**
  * blk_start_queue - restart a previously stopped queue
  * @q:    The &request_queue_t in question
  *
  * Description:
  *   blk_start_queue() will clear the stop flag on the queue, and call
  *   the request_fn for the queue if it was in a stopped state when
  *   entered. Also see blk_stop_queue(). Queue lock must be held.
  **/
 void blk_start_queue(request_queue_t *q)
 {
         clear_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
 
         /*
          * one level of recursion is ok and is much faster than kicking
          * the unplug handling
          */
         if (!test_and_set_bit(QUEUE_FLAG_REENTER, &q->queue_flags)) {
                 q->request_fn(q);
                 clear_bit(QUEUE_FLAG_REENTER, &q->queue_flags);
         } else {
                 blk_plug_device(q);
                 kblockd_schedule_work(&q->unplug_work);
         }
 }
 
 EXPORT_SYMBOL(blk_start_queue);
 
 /**
  * blk_stop_queue - stop a queue
  * @q:    The &request_queue_t in question
  *
  * Description:
  *   The Linux block layer assumes that a block driver will consume all
  *   entries on the request queue when the request_fn strategy is called.
  *   Often this will not happen, because of hardware limitations (queue
  *   depth settings). If a device driver gets a 'queue full' response,
  *   or if it simply chooses not to queue more I/O at one point, it can
  *   call this function to prevent the request_fn from being called until
  *   the driver has signalled it's ready to go again. This happens by calling
  *   blk_start_queue() to restart queue operations. Queue lock must be held.
  **/
 void blk_stop_queue(request_queue_t *q)
 {
         blk_remove_plug(q);
         set_bit(QUEUE_FLAG_STOPPED, &q->queue_flags);
 }
 EXPORT_SYMBOL(blk_stop_queue);
 
 /**
  * blk_sync_queue - cancel any pending callbacks on a queue
  * @q: the queue
  *
  * Description:
  *     The block layer may perform asynchronous callback activity
  *     on a queue, such as calling the unplug function after a timeout.
  *     A block device may call blk_sync_queue to ensure that any
  *     such activity is cancelled, thus allowing it to release resources
  *     the the callbacks might use. The caller must already have made sure
  *     that its ->make_request_fn will not re-add plugging prior to calling
  *     this function.
  *
  */
 void blk_sync_queue(struct request_queue *q)
 {
         del_timer_sync(&q->unplug_timer);
         kblockd_flush();
 }
 EXPORT_SYMBOL(blk_sync_queue);
 
 /**
  * blk_run_queue - run a single device queue
  * @q:  The queue to run
  */
 void blk_run_queue(struct request_queue *q)
 {
         unsigned long flags;
 
         spin_lock_irqsave(q->queue_lock, flags);
         blk_remove_plug(q);
         q->request_fn(q);
         spin_unlock_irqrestore(q->queue_lock, flags);
 }
 EXPORT_SYMBOL(blk_run_queue);
 
 /**
  * blk_cleanup_queue: - release a &request_queue_t when it is no longer needed
  * @q:    the request queue to be released
  *
  * Description:
  *     blk_cleanup_queue is the pair to blk_init_queue() or
  *     blk_queue_make_request().  It should be called when a request queue is
  *     being released; typically when a block device is being de-registered.
  *     Currently, its primary task it to free all the &struct request
  *     structures that were allocated to the queue and the queue itself.
  *
  * Caveat:
  *     Hopefully the low level driver will have finished any
  *     outstanding requests first...
  **/
 void blk_cleanup_queue(request_queue_t * q)
 {
         struct request_list *rl = &q->rq;
 
         if (!atomic_dec_and_test(&q->refcnt))
                 return;
 
         if (q->elevator)
                 elevator_exit(q->elevator);
 
         blk_sync_queue(q);
 
         if (rl->rq_pool)
                 mempool_destroy(rl->rq_pool);
 
         if (q->queue_tags)
                 __blk_queue_free_tags(q);
 
         kmem_cache_free(requestq_cachep, q);
 }
 
 EXPORT_SYMBOL(blk_cleanup_queue);
 
 static int blk_init_free_list(request_queue_t *q)
 {
         struct request_list *rl = &q->rq;
 
         rl->count[READ] = rl->count[WRITE] = 0;
         rl->starved[READ] = rl->starved[WRITE] = 0;
         init_waitqueue_head(&rl->wait[READ]);
         init_waitqueue_head(&rl->wait[WRITE]);
         init_waitqueue_head(&rl->drain);
 
         rl->rq_pool = mempool_create(BLKDEV_MIN_RQ, mempool_alloc_slab, mempool_free_slab, request_cachep);
 
         if (!rl->rq_pool)
                 return -ENOMEM;
 
         return 0;
 }
 
 static int __make_request(request_queue_t *, struct bio *);
 
 request_queue_t *blk_alloc_queue(int gfp_mask)
 {
         request_queue_t *q = kmem_cache_alloc(requestq_cachep, gfp_mask);
 
         if (!q)
                 return NULL;
 
         memset(q, 0, sizeof(*q));
         init_timer(&q->unplug_timer);
         atomic_set(&q->refcnt, 1);
 
         q->backing_dev_info.unplug_io_fn = blk_backing_dev_unplug;
         q->backing_dev_info.unplug_io_data = q;
 
         return q;
 }
 
 EXPORT_SYMBOL(blk_alloc_queue);
 
 /**
  * blk_init_queue  - prepare a request queue for use with a block device
  * @rfn:  The function to be called to process requests that have been
  *        placed on the queue.
  * @lock: Request queue spin lock
  *
  * Description:
  *    If a block device wishes to use the standard request handling procedures,
  *    which sorts requests and coalesces adjacent requests, then it must
  *    call blk_init_queue().  The function @rfn will be called when there
  *    are requests on the queue that need to be processed.  If the device
  *    supports plugging, then @rfn may not be called immediately when requests
  *    are available on the queue, but may be called at some time later instead.
  *    Plugged queues are generally unplugged when a buffer belonging to one
  *    of the requests on the queue is needed, or due to memory pressure.
  *
  *    @rfn is not required, or even expected, to remove all requests off the
  *    queue, but only as many as it can handle at a time.  If it does leave
  *    requests on the queue, it is responsible for arranging that the requests
  *    get dealt with eventually.
  *
  *    The queue spin lock must be held while manipulating the requests on the
  *    request queue.
  *
  *    Function returns a pointer to the initialized request queue, or NULL if
  *    it didn't succeed.
  *
  * Note:
  *    blk_init_queue() must be paired with a blk_cleanup_queue() call
  *    when the block device is deactivated (such as at module unload).
  **/
 request_queue_t *blk_init_queue(request_fn_proc *rfn, spinlock_t *lock)
 {
         request_queue_t *q = blk_alloc_queue(GFP_KERNEL);
 
         if (!q)
                 return NULL;
 
         if (blk_init_free_list(q))
                 goto out_init;
 
         q->request_fn           = rfn;
         q->back_merge_fn        = ll_back_merge_fn;
         q->front_merge_fn       = ll_front_merge_fn;
         q->merge_requests_fn    = ll_merge_requests_fn;
         q->prep_rq_fn           = NULL;
         q->unplug_fn            = generic_unplug_device;
         q->queue_flags          = (1 << QUEUE_FLAG_CLUSTER);
         q->queue_lock           = lock;
 
         blk_queue_segment_boundary(q, 0xffffffff);
 
         blk_queue_make_request(q, __make_request);
         blk_queue_max_segment_size(q, MAX_SEGMENT_SIZE);
 
         blk_queue_max_hw_segments(q, MAX_HW_SEGMENTS);
         blk_queue_max_phys_segments(q, MAX_PHYS_SEGMENTS);
 
         /*
          * all done
          */
         if (!elevator_init(q, NULL)) {
                 blk_queue_congestion_threshold(q);
                 return q;
         }
 
         blk_cleanup_queue(q);
 out_init:
         kmem_cache_free(requestq_cachep, q);
         return NULL;
 }
 
 EXPORT_SYMBOL(blk_init_queue);
 
 int blk_get_queue(request_queue_t *q)
 {
         if (!test_bit(QUEUE_FLAG_DEAD, &q->queue_flags)) {
                 atomic_inc(&q->refcnt);
                 return 0;
         }
 
         return 1;
 }
 
 EXPORT_SYMBOL(blk_get_queue);
 
 static inline void blk_free_request(request_queue_t *q, struct request *rq)
 {
         elv_put_request(q, rq);
         mempool_free(rq, q->rq.rq_pool);
 }
 
 static inline struct request *blk_alloc_request(request_queue_t *q, int rw,
                                                 int gfp_mask)
 {
         struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);
 
         if (!rq)
                 return NULL;
 
         /*
          * first three bits are identical in rq->flags and bio->bi_rw,
          * see bio.h and blkdev.h
          */
         rq->flags = rw;
 
         if (!elv_set_request(q, rq, gfp_mask))
                 return rq;
 
         mempool_free(rq, q->rq.rq_pool);
         return NULL;
 }
 
 /*
  * ioc_batching returns true if the ioc is a valid batching request and
  * should be given priority access to a request.
  */
 static inline int ioc_batching(request_queue_t *q, struct io_context *ioc)
 {
         if (!ioc)
                 return 0;
 
         /*
          * Make sure the process is able to allocate at least 1 request
          * even if the batch times out, otherwise we could theoretically
          * lose wakeups.
          */
         return ioc->nr_batch_requests == q->nr_batching ||
                 (ioc->nr_batch_requests > 0
                 && time_before(jiffies, ioc->last_waited + BLK_BATCH_TIME));
 }
 
 /*
  * ioc_set_batching sets ioc to be a new "batcher" if it is not one. This
  * will cause the process to be a "batcher" on all queues in the system. This
  * is the behaviour we want though - once it gets a wakeup it should be given
  * a nice run.
  */
 void ioc_set_batching(request_queue_t *q, struct io_context *ioc)
 {
         if (!ioc || ioc_batching(q, ioc))
                 return;
 
         ioc->nr_batch_requests = q->nr_batching;
         ioc->last_waited = jiffies;
 }
 
 static void __freed_request(request_queue_t *q, int rw)
 {
         struct request_list *rl = &q->rq;
 
         if (rl->count[rw] < queue_congestion_off_threshold(q))
                 clear_queue_congested(q, rw);
 
         if (rl->count[rw] + 1 <= q->nr_requests) {
                 smp_mb();
                 if (waitqueue_active(&rl->wait[rw]))
                         wake_up(&rl->wait[rw]);
 
                 blk_clear_queue_full(q, rw);
         }
 }
 
 /*
  * A request has just been released.  Account for it, update the full and
  * congestion status, wake up any waiters.   Called under q->queue_lock.
  */
 static void freed_request(request_queue_t *q, int rw)
 {
         struct request_list *rl = &q->rq;
 
         rl->count[rw]--;
 
         __freed_request(q, rw);
 
         if (unlikely(rl->starved[rw ^ 1]))
                 __freed_request(q, rw ^ 1);
 
         if (!rl->count[READ] && !rl->count[WRITE]) {
                 smp_mb();
                 if (unlikely(waitqueue_active(&rl->drain)))
                         wake_up(&rl->drain);
         }
 }
 
 #define blkdev_free_rq(list) list_entry((list)->next, struct request, queuelist)
 /*
  * Get a free request, queue_lock must not be held
  */
 static struct request *get_request(request_queue_t *q, int rw, int gfp_mask)
 {
         struct request *rq = NULL;
         struct request_list *rl = &q->rq;
         struct io_context *ioc = get_io_context(gfp_mask);
 
         if (unlikely(test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags)))
                 goto out;
 
         spin_lock_irq(q->queue_lock);
         if (rl->count[rw]+1 >= q->nr_requests) {
                 /*
                  * The queue will fill after this allocation, so set it as
                  * full, and mark this process as "batching". This process
                  * will be allowed to complete a batch of requests, others
                  * will be blocked.
                  */
                 if (!blk_queue_full(q, rw)) {
                         ioc_set_batching(q, ioc);
                         blk_set_queue_full(q, rw);
                 }
         }
 
         switch (elv_may_queue(q, rw)) {
                 case ELV_MQUEUE_NO:
                         goto rq_starved;
                 case ELV_MQUEUE_MAY:
                         break;
                 case ELV_MQUEUE_MUST:
                         goto get_rq;
         }
 
         if (blk_queue_full(q, rw) && !ioc_batching(q, ioc)) {
                 /*
                  * The queue is full and the allocating process is not a
                  * "batcher", and not exempted by the IO scheduler
                  */
                 spin_unlock_irq(q->queue_lock);
                 goto out;
         }
 
 get_rq:
         rl->count[rw]++;
         rl->starved[rw] = 0;
         if (rl->count[rw] >= queue_congestion_on_threshold(q))
                 set_queue_congested(q, rw);
         spin_unlock_irq(q->queue_lock);
 
         rq = blk_alloc_request(q, rw, gfp_mask);
         if (!rq) {
                 /*
                  * Allocation failed presumably due to memory. Undo anything
                  * we might have messed up.
                  *
                  * Allocating task should really be put onto the front of the
                  * wait queue, but this is pretty rare.
                  */
                 spin_lock_irq(q->queue_lock);
                 freed_request(q, rw);
 
                 /*
                  * in the very unlikely event that allocation failed and no
                  * requests for this direction was pending, mark us starved
                  * so that freeing of a request in the other direction will
                  * notice us. another possible fix would be to split the
                  * rq mempool into READ and WRITE
                  */
 rq_starved:
                 if (unlikely(rl->count[rw] == 0))
                         rl->starved[rw] = 1;
 
                 spin_unlock_irq(q->queue_lock);
                 goto out;
         }
 
         if (ioc_batching(q, ioc))
                 ioc->nr_batch_requests--;
         
         INIT_LIST_HEAD(&rq->queuelist);
 
         rq->errors = 0;
         rq->rq_status = RQ_ACTIVE;
         rq->bio = rq->biotail = NULL;
         rq->buffer = NULL;
         rq->ref_count = 1;
         rq->q = q;
         rq->rl = rl;
         rq->waiting = NULL;
         rq->special = NULL;
         rq->data_len = 0;
         rq->data = NULL;
         rq->sense = NULL;
 
 out:
         put_io_context(ioc);
         return rq;
 }
 
 /*
  * No available requests for this queue, unplug the device and wait for some
  * requests to become available.
  */
 static struct request *get_request_wait(request_queue_t *q, int rw)
 {
         DEFINE_WAIT(wait);
         struct request *rq;
 
         generic_unplug_device(q);
         do {
                 struct request_list *rl = &q->rq;
 
                 prepare_to_wait_exclusive(&rl->wait[rw], &wait,
                                 TASK_UNINTERRUPTIBLE);
 
                 rq = get_request(q, rw, GFP_NOIO);
 
                 if (!rq) {
                         struct io_context *ioc;
 
                         io_schedule();
 
                         /*
                          * After sleeping, we become a "batching" process and
                          * will be able to allocate at least one request, and
                          * up to a big batch of them for a small period time.
                          * See ioc_batching, ioc_set_batching
                          */
                         ioc = get_io_context(GFP_NOIO);
                         ioc_set_batching(q, ioc);
                         put_io_context(ioc);
                 }
                 finish_wait(&rl->wait[rw], &wait);
         } while (!rq);
 
         return rq;
 }
 
 struct request *blk_get_request(request_queue_t *q, int rw, int gfp_mask)
 {
         struct request *rq;
 
         BUG_ON(rw != READ && rw != WRITE);
 
         if (gfp_mask & __GFP_WAIT)
                 rq = get_request_wait(q, rw);
         else
                 rq = get_request(q, rw, gfp_mask);
 
         return rq;
 }
 
 EXPORT_SYMBOL(blk_get_request);
 
 /**
  * blk_requeue_request - put a request back on queue
  * @q:          request queue where request should be inserted
  * @rq:         request to be inserted
  *
  * Description:
  *    Drivers often keep queueing requests until the hardware cannot accept
  *    more, when that condition happens we need to put the request back
  *    on the queue. Must be called with queue lock held.
  */
 void blk_requeue_request(request_queue_t *q, struct request *rq)
 {
         if (blk_rq_tagged(rq))
                 blk_queue_end_tag(q, rq);
 
         elv_requeue_request(q, rq);
 }
 
 EXPORT_SYMBOL(blk_requeue_request);
 
 /**
  * blk_insert_request - insert a special request in to a request queue
  * @q:          request queue where request should be inserted
  * @rq:         request to be inserted
  * @at_head:    insert request at head or tail of queue
  * @data:       private data
  * @reinsert:   true if request it a reinsertion of previously processed one
  *
  * Description:
  *    Many block devices need to execute commands asynchronously, so they don't
  *    block the whole kernel from preemption during request execution.  This is
  *    accomplished normally by inserting aritficial requests tagged as
  *    REQ_SPECIAL in to the corresponding request queue, and letting them be
  *    scheduled for actual execution by the request queue.
  *
  *    We have the option of inserting the head or the tail of the queue.
  *    Typically we use the tail for new ioctls and so forth.  We use the head
  *    of the queue for things like a QUEUE_FULL message from a device, or a
  *    host that is unable to accept a particular command.
  */
 void blk_insert_request(request_queue_t *q, struct request *rq,
                         int at_head, void *data, int reinsert)
 {
         unsigned long flags;
 
         /*
          * tell I/O scheduler that this isn't a regular read/write (ie it
          * must not attempt merges on this) and that it acts as a soft
          * barrier
          */
         rq->flags |= REQ_SPECIAL | REQ_SOFTBARRIER;
 
         rq->special = data;
 
         spin_lock_irqsave(q->queue_lock, flags);
 
         /*
          * If command is tagged, release the tag
          */
         if (reinsert)
                 blk_requeue_request(q, rq);
         else {
                 int where = ELEVATOR_INSERT_BACK;
 
                 if (at_head)
                         where = ELEVATOR_INSERT_FRONT;
 
                 if (blk_rq_tagged(rq))
                         blk_queue_end_tag(q, rq);
 
                 drive_stat_acct(rq, rq->nr_sectors, 1);
                 __elv_add_request(q, rq, where, 0);
         }
         if (blk_queue_plugged(q))
                 __generic_unplug_device(q);
         else
                 q->request_fn(q);
         spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
 EXPORT_SYMBOL(blk_insert_request);
 
 /**
  * blk_rq_map_user - map user data to a request, for REQ_BLOCK_PC usage
  * @q:          request queue where request should be inserted
  * @rw:         READ or WRITE data
  * @ubuf:       the user buffer
  * @len:        length of user data
  *
  * Description:
  *    Data will be mapped directly for zero copy io, if possible. Otherwise
  *    a kernel bounce buffer is used.
  *
  *    A matching blk_rq_unmap_user() must be issued at the end of io, while
  *    still in process context.
  *
  *    Note: The mapped bio may need to be bounced through blk_queue_bounce()
  *    before being submitted to the device, as pages mapped may be out of
  *    reach. It's the callers responsibility to make sure this happens. The
  *    original bio must be passed back in to blk_rq_unmap_user() for proper
  *    unmapping.
  */
 struct request *blk_rq_map_user(request_queue_t *q, int rw, void __user *ubuf,
                                 unsigned int len)
 {
         unsigned long uaddr;
         struct request *rq;
         struct bio *bio;
 
         if (len > (q->max_sectors << 9))
                 return ERR_PTR(-EINVAL);
         if ((!len && ubuf) || (len && !ubuf))
                 return ERR_PTR(-EINVAL);
 
         rq = blk_get_request(q, rw, __GFP_WAIT);
         if (!rq)
                 return ERR_PTR(-ENOMEM);
 
         /*
          * if alignment requirement is satisfied, map in user pages for
          * direct dma. else, set up kernel bounce buffers
          */
         uaddr = (unsigned long) ubuf;
         if (!(uaddr & queue_dma_alignment(q)) && !(len & queue_dma_alignment(q)))
                 bio = bio_map_user(q, NULL, uaddr, len, rw == READ);
         else
                 bio = bio_copy_user(q, uaddr, len, rw == READ);
 
         if (!IS_ERR(bio)) {
                 rq->bio = rq->biotail = bio;
                 blk_rq_bio_prep(q, rq, bio);
 
                 rq->buffer = rq->data = NULL;
                 rq->data_len = len;
                 return rq;
         }
 
         /*
          * bio is the err-ptr
          */
         blk_put_request(rq);
         return (struct request *) bio;
 }
 
 EXPORT_SYMBOL(blk_rq_map_user);
 
 /**
  * blk_rq_unmap_user - unmap a request with user data
  * @rq:         request to be unmapped
  * @ubuf:       user buffer
  * @ulen:       length of user buffer
  *
  * Description:
  *    Unmap a request previously mapped by blk_rq_map_user().
  */
 int blk_rq_unmap_user(struct request *rq, struct bio *bio, unsigned int ulen)
 {
         int ret = 0;
 
         if (bio) {
                 if (bio_flagged(bio, BIO_USER_MAPPED))
                         bio_unmap_user(bio);
                 else
                         ret = bio_uncopy_user(bio);
         }
 
         blk_put_request(rq);
         return ret;
 }
 
 EXPORT_SYMBOL(blk_rq_unmap_user);
 
 /**
  * blk_execute_rq - insert a request into queue for execution
  * @q:          queue to insert the request in
  * @bd_disk:    matching gendisk
  * @rq:         request to insert
  *
  * Description:
  *    Insert a fully prepared request at the back of the io scheduler queue
  *    for execution.
  */
 int blk_execute_rq(request_queue_t *q, struct gendisk *bd_disk,
                    struct request *rq)
 {
         DECLARE_COMPLETION(wait);
         char sense[SCSI_SENSE_BUFFERSIZE];
         int err = 0;
 
         rq->rq_disk = bd_disk;
 
         /*
          * we need an extra reference to the request, so we can look at
          * it after io completion
          */
         rq->ref_count++;
 
         if (!rq->sense) {
                 memset(sense, 0, sizeof(sense));
                 rq->sense = sense;
                 rq->sense_len = 0;
         }
 
         rq->flags |= REQ_NOMERGE;
         if (!rq->waiting)
                 rq->waiting = &wait;
         elv_add_request(q, rq, ELEVATOR_INSERT_BACK, 1);
         generic_unplug_device(q);
         wait_for_completion(rq->waiting);
         rq->waiting = NULL;
 
         if (rq->errors)
                 err = -EIO;
 
         return err;
 }
 
 EXPORT_SYMBOL(blk_execute_rq);
 
 /**
  * blkdev_issue_flush - queue a flush
  * @bdev:       blockdev to issue flush for
  * @error_sector:       error sector
  *
  * Description:
  *    Issue a flush for the block device in question. Caller can supply
  *    room for storing the error offset in case of a flush error, if they
  *    wish to.  Caller must run wait_for_completion() on its own.
  */
 int blkdev_issue_flush(struct block_device *bdev, sector_t *error_sector)
 {
         request_queue_t *q;
 
         if (bdev->bd_disk == NULL)
                 return -ENXIO;
 
         q = bdev_get_queue(bdev);
         if (!q)
                 return -ENXIO;
         if (!q->issue_flush_fn)
                 return -EOPNOTSUPP;
 
         return q->issue_flush_fn(q, bdev->bd_disk, error_sector);
 }
 
 EXPORT_SYMBOL(blkdev_issue_flush);
 
 /**
  * blkdev_scsi_issue_flush_fn - issue flush for SCSI devices
  * @q:          device queue
  * @disk:       gendisk
  * @error_sector:       error offset
  *
  * Description:
  *    Devices understanding the SCSI command set, can use this function as
  *    a helper for issuing a cache flush. Note: driver is required to store
  *    the error offset (in case of error flushing) in ->sector of struct
  *    request.
  */
 int blkdev_scsi_issue_flush_fn(request_queue_t *q, struct gendisk *disk,
                                sector_t *error_sector)
 {
         struct request *rq = blk_get_request(q, WRITE, __GFP_WAIT);
         int ret;
 
         rq->flags |= REQ_BLOCK_PC | REQ_SOFTBARRIER;
         rq->sector = 0;
         memset(rq->cmd, 0, sizeof(rq->cmd));
         rq->cmd[0] = 0x35;
         rq->cmd_len = 12;
         rq->data = NULL;
         rq->data_len = 0;
         rq->timeout = 60 * HZ;
 
         ret = blk_execute_rq(q, disk, rq);
 
         if (ret && error_sector)
                 *error_sector = rq->sector;
 
         blk_put_request(rq);
         return ret;
 }
 
 EXPORT_SYMBOL(blkdev_scsi_issue_flush_fn);
 
 void drive_stat_acct(struct request *rq, int nr_sectors, int new_io)
 {
         int rw = rq_data_dir(rq);
 
         if (!blk_fs_request(rq) || !rq->rq_disk)
                 return;
 
         if (rw == READ) {
                 __disk_stat_add(rq->rq_disk, read_sectors, nr_sectors);
                 if (!new_io)
                         __disk_stat_inc(rq->rq_disk, read_merges);
         } else if (rw == WRITE) {
                 __disk_stat_add(rq->rq_disk, write_sectors, nr_sectors);
                 if (!new_io)
                         __disk_stat_inc(rq->rq_disk, write_merges);
         }
         if (new_io) {
                 disk_round_stats(rq->rq_disk);
                 rq->rq_disk->in_flight++;
         }
 }
 
 /*
  * add-request adds a request to the linked list.
  * queue lock is held and interrupts disabled, as we muck with the
  * request queue list.
  */
 static inline void add_request(request_queue_t * q, struct request * req)
 {
         drive_stat_acct(req, req->nr_sectors, 1);
 
         if (q->activity_fn)
                 q->activity_fn(q->activity_data, rq_data_dir(req));
 
         /*
          * elevator indicated where it wants this request to be
          * inserted at elevator_merge time
          */
         __elv_add_request(q, req, ELEVATOR_INSERT_SORT, 0);
 }
  
 /*
  * disk_round_stats()   - Round off the performance stats on a struct
  * disk_stats.
  *
  * The average IO queue length and utilisation statistics are maintained
  * by observing the current state of the queue length and the amount of
  * time it has been in this state for.
  *
  * Normally, that accounting is done on IO completion, but that can result
  * in more than a second's worth of IO being accounted for within any one
  * second, leading to >100% utilisation.  To deal with that, we call this
  * function to do a round-off before returning the results when reading
  * /proc/diskstats.  This accounts immediately for all queue usage up to
  * the current jiffies and restarts the counters again.
  */
 void disk_round_stats(struct gendisk *disk)
 {
         unsigned long now = jiffies;
 
         __disk_stat_add(disk, time_in_queue,
                         disk->in_flight * (now - disk->stamp));
         disk->stamp = now;
 
         if (disk->in_flight)
                 __disk_stat_add(disk, io_ticks, (now - disk->stamp_idle));
         disk->stamp_idle = now;
 }
 
 /*
  * queue lock must be held
  */
 void __blk_put_request(request_queue_t *q, struct request *req)
 {
         struct request_list *rl = req->rl;
 
         if (unlikely(!q))
                 return;
         if (unlikely(--req->ref_count))
                 return;
 
         req->rq_status = RQ_INACTIVE;
         req->q = NULL;
         req->rl = NULL;
 
         /*
          * Request may not have originated from ll_rw_blk. if not,
          * it didn't come out of our reserved rq pools
          */
         if (rl) {
                 int rw = rq_data_dir(req);
 
                 elv_completed_request(q, req);
 
                 BUG_ON(!list_empty(&req->queuelist));
 
                 blk_free_request(q, req);
                 freed_request(q, rw);
         }
 }
 
 void blk_put_request(struct request *req)
 {
         /*
          * if req->rl isn't set, this request didnt originate from the
          * block layer, so it's safe to just disregard it
          */
         if (req->rl) {
                 unsigned long flags;
                 request_queue_t *q = req->q;
 
                 spin_lock_irqsave(q->queue_lock, flags);
                 __blk_put_request(q, req);
                 spin_unlock_irqrestore(q->queue_lock, flags);
         }
 }
 
 EXPORT_SYMBOL(blk_put_request);
 
 /**
  * blk_congestion_wait - wait for a queue to become uncongested
  * @rw: READ or WRITE
  * @timeout: timeout in jiffies
  *
  * Waits for up to @timeout jiffies for a queue (any queue) to exit congestion.
  * If no queues are congested then just wait for the next request to be
  * returned.
  */
 long blk_congestion_wait(int rw, long timeout)
 {
         long ret;
         DEFINE_WAIT(wait);
         wait_queue_head_t *wqh = &congestion_wqh[rw];
 
         prepare_to_wait(wqh, &wait, TASK_UNINTERRUPTIBLE);
         ret = io_schedule_timeout(timeout);
         finish_wait(wqh, &wait);
         return ret;
 }
 
 EXPORT_SYMBOL(blk_congestion_wait);
 
 /*
  * Has to be called with the request spinlock acquired
  */
 static int attempt_merge(request_queue_t *q, struct request *req,
                           struct request *next)
 {
         if (!rq_mergeable(req) || !rq_mergeable(next))
                 return 0;
 
         /*
          * not contigious
          */
         if (req->sector + req->nr_sectors != next->sector)
                 return 0;
 
         if (rq_data_dir(req) != rq_data_dir(next)
             || req->rq_disk != next->rq_disk
             || next->waiting || next->special)
                 return 0;
 
         /*
          * If we are allowed to merge, then append bio list
          * from next to rq and release next. merge_requests_fn
          * will have updated segment counts, update sector
          * counts here.
          */
         if (!q->merge_requests_fn(q, req, next))
                 return 0;
 
         /*
          * At this point we have either done a back merge
          * or front merge. We need the smaller start_time of
          * the merged requests to be the current request
          * for accounting purposes.
          */
         if (time_after(req->start_time, next->start_time))
                 req->start_time = next->start_time;
 
         req->biotail->bi_next = next->bio;
         req->biotail = next->biotail;
 
         req->nr_sectors = req->hard_nr_sectors += next->hard_nr_sectors;
 
         elv_merge_requests(q, req, next);
 
         if (req->rq_disk) {
                 disk_round_stats(req->rq_disk);
                 req->rq_disk->in_flight--;
         }
 
         __blk_put_request(q, next);
         return 1;
 }
 
 static inline int attempt_back_merge(request_queue_t *q, struct request *rq)
 {
         struct request *next = elv_latter_request(q, rq);
 
         if (next)
                 return attempt_merge(q, rq, next);
 
         return 0;
 }
 
 static inline int attempt_front_merge(request_queue_t *q, struct request *rq)
 {
         struct request *prev = elv_former_request(q, rq);
 
         if (prev)
                 return attempt_merge(q, prev, rq);
 
         return 0;
 }
 
 /**
  * blk_attempt_remerge  - attempt to remerge active head with next request
  * @q:    The &request_queue_t belonging to the device
  * @rq:   The head request (usually)
  *
  * Description:
  *    For head-active devices, the queue can easily be unplugged so quickly
  *    that proper merging is not done on the front request. This may hurt
  *    performance greatly for some devices. The block layer cannot safely
  *    do merging on that first request for these queues, but the driver can
  *    call this function and make it happen any way. Only the driver knows
  *    when it is safe to do so.
  **/
 void blk_attempt_remerge(request_queue_t *q, struct request *rq)
 {
         unsigned long flags;
 
         spin_lock_irqsave(q->queue_lock, flags);
         attempt_back_merge(q, rq);
         spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
 EXPORT_SYMBOL(blk_attempt_remerge);
 
 /*
  * Non-locking blk_attempt_remerge variant.
  */
 void __blk_attempt_remerge(request_queue_t *q, struct request *rq)
 {
         attempt_back_merge(q, rq);
 }
 
 EXPORT_SYMBOL(__blk_attempt_remerge);
 
 static int __make_request(request_queue_t *q, struct bio *bio)
 {
         struct request *req, *freereq = NULL;
         int el_ret, rw, nr_sectors, cur_nr_sectors, barrier, err;
         sector_t sector;
 
         sector = bio->bi_sector;
         nr_sectors = bio_sectors(bio);
         cur_nr_sectors = bio_cur_sectors(bio);
 
         rw = bio_data_dir(bio);
 
         /*
          * low level driver can indicate that it wants pages above a
          * certain limit bounced to low memory (ie for highmem, or even
          * ISA dma in theory)
          */
         blk_queue_bounce(q, &bio);
 
         spin_lock_prefetch(q->queue_lock);
 
         barrier = bio_barrier(bio);
         if (barrier && !(q->queue_flags & (1 << QUEUE_FLAG_ORDERED))) {
                 err = -EOPNOTSUPP;
                 goto end_io;
         }
 
 again:
         spin_lock_irq(q->queue_lock);
 
         if (elv_queue_empty(q)) {
                 blk_plug_device(q);
                 goto get_rq;
         }
         if (barrier)
                 goto get_rq;
 
         el_ret = elv_merge(q, &req, bio);
         switch (el_ret) {
                 case ELEVATOR_BACK_MERGE:
                         BUG_ON(!rq_mergeable(req));
 
                         if (!q->back_merge_fn(q, req, bio))
                                 break;
 
                         req->biotail->bi_next = bio;
                         req->biotail = bio;
                         req->nr_sectors = req->hard_nr_sectors += nr_sectors;
                         drive_stat_acct(req, nr_sectors, 0);
                         if (!attempt_back_merge(q, req))
                                 elv_merged_request(q, req);
                         goto out;
 
                 case ELEVATOR_FRONT_MERGE:
                         BUG_ON(!rq_mergeable(req));
 
                         if (!q->front_merge_fn(q, req, bio))
                                 break;
 
                         bio->bi_next = req->bio;
                         req->bio = bio;
 
                         /*
                          * may not be valid. if the low level driver said
                          * it didn't need a bounce buffer then it better
                          * not touch req->buffer either...
                          */
                         req->buffer = bio_data(bio);
                         req->current_nr_sectors = cur_nr_sectors;
                         req->hard_cur_sectors = cur_nr_sectors;
                         req->sector = req->hard_sector = sector;
                         req->nr_sectors = req->hard_nr_sectors += nr_sectors;
                         drive_stat_acct(req, nr_sectors, 0);
                         if (!attempt_front_merge(q, req))
                                 elv_merged_request(q, req);
                         goto out;
 
                 /*
                  * elevator says don't/can't merge. get new request
                  */
                 case ELEVATOR_NO_MERGE:
                         break;
 
                 default:
                         printk("elevator returned crap (%d)\n", el_ret);
                         BUG();
         }
 
         /*
          * Grab a free request from the freelist - if that is empty, check
          * if we are doing read ahead and abort instead of blocking for
          * a free slot.
          */
 get_rq:
         if (freereq) {
                 req = freereq;
                 freereq = NULL;
         } else {
                 spin_unlock_irq(q->queue_lock);
                 if ((freereq = get_request(q, rw, GFP_ATOMIC)) == NULL) {
                         /*
                          * READA bit set
                          */
                         err = -EWOULDBLOCK;
                         if (bio_rw_ahead(bio))
                                 goto end_io;
         
                         freereq = get_request_wait(q, rw);
                 }
                 goto again;
         }
 
         req->flags |= REQ_CMD;
 
         /*
          * inherit FAILFAST from bio (for read-ahead, and explicit FAILFAST)
          */
         if (bio_rw_ahead(bio) || bio_failfast(bio))
                 req->flags |= REQ_FAILFAST;
 
         /*
          * REQ_BARRIER implies no merging, but lets make it explicit
          */
         if (barrier)
                 req->flags |= (REQ_HARDBARRIER | REQ_NOMERGE);
 
         req->errors = 0;
         req->hard_sector = req->sector = sector;
         req->hard_nr_sectors = req->nr_sectors = nr_sectors;
         req->current_nr_sectors = req->hard_cur_sectors = cur_nr_sectors;
         req->nr_phys_segments = bio_phys_segments(q, bio);
         req->nr_hw_segments = bio_hw_segments(q, bio);
         req->buffer = bio_data(bio);    /* see ->buffer comment above */
         req->waiting = NULL;
         req->bio = req->biotail = bio;
         req->rq_disk = bio->bi_bdev->bd_disk;
         req->start_time = jiffies;
 
         add_request(q, req);
 out:
         if (freereq)
                 __blk_put_request(q, freereq);
         if (bio_sync(bio))
                 __generic_unplug_device(q);
 
         spin_unlock_irq(q->queue_lock);
         return 0;
 
 end_io:
         bio_endio(bio, nr_sectors << 9, err);
         return 0;
 }
 
 /*
  * If bio->bi_dev is a partition, remap the location
  */
 static inline void blk_partition_remap(struct bio *bio)
 {
         struct block_device *bdev = bio->bi_bdev;
 
         if (bdev != bdev->bd_contains) {
                 struct hd_struct *p = bdev->bd_part;
 
                 switch (bio->bi_rw) {
                 case READ:
                         p->read_sectors += bio_sectors(bio);
                         p->reads++;
                         break;
                 case WRITE:
                         p->write_sectors += bio_sectors(bio);
                         p->writes++;
                         break;
                 }
                 bio->bi_sector += p->start_sect;
                 bio->bi_bdev = bdev->bd_contains;
         }
 }
 
 void blk_finish_queue_drain(request_queue_t *q)
 {
         struct request_list *rl = &q->rq;
         struct request *rq;
 
         spin_lock_irq(q->queue_lock);
         clear_bit(QUEUE_FLAG_DRAIN, &q->queue_flags);
 
         while (!list_empty(&q->drain_list)) {
                 rq = list_entry_rq(q->drain_list.next);
 
                 list_del_init(&rq->queuelist);
                 __elv_add_request(q, rq, ELEVATOR_INSERT_BACK, 1);
         }
 
         spin_unlock_irq(q->queue_lock);
 
         wake_up(&rl->wait[0]);
         wake_up(&rl->wait[1]);
         wake_up(&rl->drain);
 }
 
 static int wait_drain(request_queue_t *q, struct request_list *rl, int dispatch)
 {
         int wait = rl->count[READ] + rl->count[WRITE];
 
         if (dispatch)
                 wait += !list_empty(&q->queue_head);
 
         return wait;
 }
 
 /*
  * We rely on the fact that only requests allocated through blk_alloc_request()
  * have io scheduler private data structures associated with them. Any other
  * type of request (allocated on stack or through kmalloc()) should not go
  * to the io scheduler core, but be attached to the queue head instead.
  */
 void blk_wait_queue_drained(request_queue_t *q, int wait_dispatch)
 {
         struct request_list *rl = &q->rq;
         DEFINE_WAIT(wait);
 
         spin_lock_irq(q->queue_lock);
         set_bit(QUEUE_FLAG_DRAIN, &q->queue_flags);
 
         while (wait_drain(q, rl, wait_dispatch)) {
                 prepare_to_wait(&rl->drain, &wait, TASK_UNINTERRUPTIBLE);
 
                 if (wait_drain(q, rl, wait_dispatch)) {
                         __generic_unplug_device(q);
                         spin_unlock_irq(q->queue_lock);
                         io_schedule();
                         spin_lock_irq(q->queue_lock);
                 }
 
                 finish_wait(&rl->drain, &wait);
         }
 
         spin_unlock_irq(q->queue_lock);
 }
 
 /*
  * block waiting for the io scheduler being started again.
  */
 static inline void block_wait_queue_running(request_queue_t *q)
 {
         DEFINE_WAIT(wait);
 
         while (test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags)) {
                 struct request_list *rl = &q->rq;
 
                 prepare_to_wait_exclusive(&rl->drain, &wait,
                                 TASK_UNINTERRUPTIBLE);
 
                 /*
                  * re-check the condition. avoids using prepare_to_wait()
                  * in the fast path (queue is running)
                  */
                 if (test_bit(QUEUE_FLAG_DRAIN, &q->queue_flags))
                         io_schedule();
 
                 finish_wait(&rl->drain, &wait);
         }
 }
 
 static void handle_bad_sector(struct bio *bio)
 {
         char b[BDEVNAME_SIZE];
 
         printk(KERN_INFO "attempt to access beyond end of device\n");
         printk(KERN_INFO "%s: rw=%ld, want=%Lu, limit=%Lu\n",
                         bdevname(bio->bi_bdev, b),
                         bio->bi_rw,
                         (unsigned long long)bio->bi_sector + bio_sectors(bio),
                         (long long)(bio->bi_bdev->bd_inode->i_size >> 9));
 
         set_bit(BIO_EOF, &bio->bi_flags);
 }
 
 /**
  * generic_make_request: hand a buffer to its device driver for I/O
  * @bio:  The bio describing the location in memory and on the device.
  *
  * generic_make_request() is used to make I/O requests of block
  * devices. It is passed a &struct bio, which describes the I/O that needs
  * to be done.
  *
  * generic_make_request() does not return any status.  The
  * success/failure status of the request, along with notification of
  * completion, is delivered asynchronously through the bio->bi_end_io
  * function described (one day) else where.
  *
  * The caller of generic_make_request must make sure that bi_io_vec
  * are set to describe the memory buffer, and that bi_dev and bi_sector are
  * set to describe the device address, and the
  * bi_end_io and optionally bi_private are set to describe how
  * completion notification should be signaled.
  *
  * generic_make_request and the drivers it calls may use bi_next if this
  * bio happens to be merged with someone else, and may change bi_dev and
  * bi_sector for remaps as it sees fit.  So the values of these fields
  * should NOT be depended on after the call to generic_make_request.
  */
 void generic_make_request(struct bio *bio)
 {
         request_queue_t *q;
         sector_t maxsector;
         int ret, nr_sectors = bio_sectors(bio);
 
         might_sleep();
         /* Test device or partition size, when known. */
         maxsector = bio->bi_bdev->bd_inode->i_size >> 9;
         if (maxsector) {
                 sector_t sector = bio->bi_sector;
 
                 if (maxsector < nr_sectors || maxsector - nr_sectors < sector) {
                         /*
                          * This may well happen - the kernel calls bread()
                          * without checking the size of the device, e.g., when
                          * mounting a device.
                          */
                         handle_bad_sector(bio);
                         goto end_io;
                 }
         }
 
         /*
          * Resolve the mapping until finished. (drivers are
          * still free to implement/resolve their own stacking
          * by explicitly returning 0)
          *
          * NOTE: we don't repeat the blk_size check for each new device.
          * Stacking drivers are expected to know what they are doing.
          */
         do {
                 char b[BDEVNAME_SIZE];
 
                 q = bdev_get_queue(bio->bi_bdev);
                 if (!q) {
                         printk(KERN_ERR
                                "generic_make_request: Trying to access "
                                 "nonexistent block-device %s (%Lu)\n",
                                 bdevname(bio->bi_bdev, b),
                                 (long long) bio->bi_sector);
 end_io:
                         bio_endio(bio, bio->bi_size, -EIO);
                         break;
                 }
 
                 if (unlikely(bio_sectors(bio) > q->max_hw_sectors)) {
                         printk("bio too big device %s (%u > %u)\n", 
                                 bdevname(bio->bi_bdev, b),
                                 bio_sectors(bio),
                                 q->max_hw_sectors);
                         goto end_io;
                 }
 
                 if (test_bit(QUEUE_FLAG_DEAD, &q->queue_flags))
                         goto end_io;
 
                 block_wait_queue_running(q);
 
                 /*
                  * If this device has partitions, remap block n
                  * of partition p to block n+start(p) of the disk.
                  */
                 blk_partition_remap(bio);
 
                 ret = q->make_request_fn(q, bio);
         } while (ret);
 }
 
 EXPORT_SYMBOL(generic_make_request);
 
 /**
  * submit_bio: submit a bio to the block device layer for I/O
  * @rw: whether to %READ or %WRITE, or maybe to %READA (read ahead)
  * @bio: The &struct bio which describes the I/O
  *
  * submit_bio() is very similar in purpose to generic_make_request(), and
  * uses that function to do most of the work. Both are fairly rough
  * interfaces, @bio must be presetup and ready for I/O.
  *
  */
 void submit_bio(int rw, struct bio *bio)
 {
         int count = bio_sectors(bio);
 
         BIO_BUG_ON(!bio->bi_size);
         BIO_BUG_ON(!bio->bi_io_vec);
         bio->bi_rw = rw;
         if (rw & WRITE)
                 mod_page_state(pgpgout, count);
         else
                 mod_page_state(pgpgin, count);
 
         if (unlikely(block_dump)) {
                 char b[BDEVNAME_SIZE];
                 printk(KERN_DEBUG "%s(%d): %s block %Lu on %s\n",
                         current->comm, current->pid,
                         (rw & WRITE) ? "WRITE" : "READ",
                         (unsigned long long)bio->bi_sector,
                         bdevname(bio->bi_bdev,b));
         }
 
         generic_make_request(bio);
 }
 
 EXPORT_SYMBOL(submit_bio);
 
 void blk_recalc_rq_segments(struct request *rq)
 {
         struct bio *bio, *prevbio = NULL;
         int nr_phys_segs, nr_hw_segs;
         unsigned int phys_size, hw_size;
         request_queue_t *q = rq->q;
 
         if (!rq->bio)
                 return;
 
         phys_size = hw_size = nr_phys_segs = nr_hw_segs = 0;
         rq_for_each_bio(bio, rq) {
                 /* Force bio hw/phys segs to be recalculated. */
                 bio->bi_flags &= ~(1 << BIO_SEG_VALID);
 
                 nr_phys_segs += bio_phys_segments(q, bio);
                 nr_hw_segs += bio_hw_segments(q, bio);
                 if (prevbio) {
                         int pseg = phys_size + prevbio->bi_size + bio->bi_size;
                         int hseg = hw_size + prevbio->bi_size + bio->bi_size;
 
                         if (blk_phys_contig_segment(q, prevbio, bio) &&
                             pseg <= q->max_segment_size) {
                                 nr_phys_segs--;
                                 phys_size += prevbio->bi_size + bio->bi_size;
                         } else
                                 phys_size = 0;
 
                         if (blk_hw_contig_segment(q, prevbio, bio) &&
                             hseg <= q->max_segment_size) {
                                 nr_hw_segs--;
                                 hw_size += prevbio->bi_size + bio->bi_size;
                         } else
                                 hw_size = 0;
                 }
                 prevbio = bio;
         }
 
         rq->nr_phys_segments = nr_phys_segs;
         rq->nr_hw_segments = nr_hw_segs;
 }
 
 void blk_recalc_rq_sectors(struct request *rq, int nsect)
 {
         if (blk_fs_request(rq)) {
                 rq->hard_sector += nsect;
                 rq->hard_nr_sectors -= nsect;
 
                 /*
                  * Move the I/O submission pointers ahead if required.
                  */
                 if ((rq->nr_sectors >= rq->hard_nr_sectors) &&
                     (rq->sector <= rq->hard_sector)) {
                         rq->sector = rq->hard_sector;
                         rq->nr_sectors = rq->hard_nr_sectors;
                         rq->hard_cur_sectors = bio_cur_sectors(rq->bio);
                         rq->current_nr_sectors = rq->hard_cur_sectors;
                         rq->buffer = bio_data(rq->bio);
                 }
 
                 /*
                  * if total number of sectors is less than the first segment
                  * size, something has gone terribly wrong
                  */
                 if (rq->nr_sectors < rq->current_nr_sectors) {
                         printk("blk: request botched\n");
                         rq->nr_sectors = rq->current_nr_sectors;
                 }
         }
 }
 
 static int __end_that_request_first(struct request *req, int uptodate,
                                     int nr_bytes)
 {
         int total_bytes, bio_nbytes, error, next_idx = 0;
         struct bio *bio;
 
         /*
          * extend uptodate bool to allow < 0 value to be direct io error
          */
         error = 0;
         if (end_io_error(uptodate))
                 error = !uptodate ? -EIO : uptodate;
 
         /*
          * for a REQ_BLOCK_PC request, we want to carry any eventual
          * sense key with us all the way through
          */
         if (!blk_pc_request(req))
                 req->errors = 0;
 
         if (!uptodate) {
                 if (blk_fs_request(req) && !(req->flags & REQ_QUIET))
                         printk("end_request: I/O error, dev %s, sector %llu\n",
                                 req->rq_disk ? req->rq_disk->disk_name : "?",
                                 (unsigned long long)req->sector);
         }
 
         total_bytes = bio_nbytes = 0;
         while ((bio = req->bio) != NULL) {
                 int nbytes;
 
                 if (nr_bytes >= bio->bi_size) {
                         req->bio = bio->bi_next;
                         nbytes = bio->bi_size;
                         bio_endio(bio, nbytes, error);
                         next_idx = 0;
                         bio_nbytes = 0;
                 } else {
                         int idx = bio->bi_idx + next_idx;
 
                         if (unlikely(bio->bi_idx >= bio->bi_vcnt)) {
                                 blk_dump_rq_flags(req, "__end_that");
                                 printk("%s: bio idx %d >= vcnt %d\n",
                                                 __FUNCTION__,
                                                 bio->bi_idx, bio->bi_vcnt);
                                 break;
                         }
 
                         nbytes = bio_iovec_idx(bio, idx)->bv_len;
                         BIO_BUG_ON(nbytes > bio->bi_size);
 
                         /*
                          * not a complete bvec done
                          */
                         if (unlikely(nbytes > nr_bytes)) {
                                 bio_nbytes += nr_bytes;
                                 total_bytes += nr_bytes;
                                 break;
                         }
 
                         /*
                          * advance to the next vector
                          */
                         next_idx++;
                         bio_nbytes += nbytes;
                 }
 
                 total_bytes += nbytes;
                 nr_bytes -= nbytes;
 
                 if ((bio = req->bio)) {
                         /*
                          * end more in this run, or just return 'not-done'
                          */
                         if (unlikely(nr_bytes <= 0))
                                 break;
                 }
         }
 
         /*
          * completely done
          */
         if (!req->bio)
                 return 0;
 
         /*
          * if the request wasn't completed, update state
          */
         if (bio_nbytes) {
                 bio_endio(bio, bio_nbytes, error);
                 bio->bi_idx += next_idx;
                 bio_iovec(bio)->bv_offset += nr_bytes;
                 bio_iovec(bio)->bv_len -= nr_bytes;
         }
 
         blk_recalc_rq_sectors(req, total_bytes >> 9);
         blk_recalc_rq_segments(req);
         return 1;
 }
 
 /**
  * end_that_request_first - end I/O on a request
  * @req:      the request being processed
  * @uptodate: 1 for success, 0 for I/O error, < 0 for specific error
  * @nr_sectors: number of sectors to end I/O on
  *
  * Description:
  *     Ends I/O on a number of sectors attached to @req, and sets it up
  *     for the next range of segments (if any) in the cluster.
  *
  * Return:
  *     0 - we are done with this request, call end_that_request_last()
  *     1 - still buffers pending for this request
  **/
 int end_that_request_first(struct request *req, int uptodate, int nr_sectors)
 {
         return __end_that_request_first(req, uptodate, nr_sectors << 9);
 }
 
 EXPORT_SYMBOL(end_that_request_first);
 
 /**
  * end_that_request_chunk - end I/O on a request
  * @req:      the request being processed
  * @uptodate: 1 for success, 0 for I/O error, < 0 for specific error
  * @nr_bytes: number of bytes to complete
  *
  * Description:
  *     Ends I/O on a number of bytes attached to @req, and sets it up
  *     for the next range of segments (if any). Like end_that_request_first(),
  *     but deals with bytes instead of sectors.
  *
  * Return:
  *     0 - we are done with this request, call end_that_request_last()
  *     1 - still buffers pending for this request
  **/
 int end_that_request_chunk(struct request *req, int uptodate, int nr_bytes)
 {
         return __end_that_request_first(req, uptodate, nr_bytes);
 }
 
 EXPORT_SYMBOL(end_that_request_chunk);
 
 /*
  * queue lock must be held
  */
 void end_that_request_last(struct request *req)
 {
         struct gendisk *disk = req->rq_disk;
         struct completion *waiting = req->waiting;
 
         if (unlikely(laptop_mode) && blk_fs_request(req))
                 laptop_io_completion();
 
         if (disk && blk_fs_request(req)) {
                 unsigned long duration = jiffies - req->start_time;
                 switch (rq_data_dir(req)) {
                     case WRITE:
                         __disk_stat_inc(disk, writes);
                         __disk_stat_add(disk, write_ticks, duration);
                         break;
                     case READ:
                         __disk_stat_inc(disk, reads);
                         __disk_stat_add(disk, read_ticks, duration);
                         break;
                 }
                 disk_round_stats(disk);
                 disk->in_flight--;
         }
         __blk_put_request(req->q, req);
         /* Do this LAST! The structure may be freed immediately afterwards */
         if (waiting)
                 complete(waiting);
 }
 
 EXPORT_SYMBOL(end_that_request_last);
 
 void end_request(struct request *req, int uptodate)
 {
         if (!end_that_request_first(req, uptodate, req->hard_cur_sectors)) {
                 add_disk_randomness(req->rq_disk);
                 blkdev_dequeue_request(req);
                 end_that_request_last(req);
         }
 }
 
 EXPORT_SYMBOL(end_request);
 
 void blk_rq_bio_prep(request_queue_t *q, struct request *rq, struct bio *bio)
 {
         /* first three bits are identical in rq->flags and bio->bi_rw */
         rq->flags |= (bio->bi_rw & 7);
 
         rq->nr_phys_segments = bio_phys_segments(q, bio);
         rq->nr_hw_segments = bio_hw_segments(q, bio);
         rq->current_nr_sectors = bio_cur_sectors(bio);
         rq->hard_cur_sectors = rq->current_nr_sectors;
         rq->hard_nr_sectors = rq->nr_sectors = bio_sectors(bio);
         rq->buffer = bio_data(bio);
 
         rq->bio = rq->biotail = bio;
 }
 
 EXPORT_SYMBOL(blk_rq_bio_prep);
 
 int kblockd_schedule_work(struct work_struct *work)
 {
         return queue_work(kblockd_workqueue, work);
 }
 
 EXPORT_SYMBOL(kblockd_schedule_work);
 
 void kblockd_flush(void)
 {
         flush_workqueue(kblockd_workqueue);
 }
 EXPORT_SYMBOL(kblockd_flush);
 
 int __init blk_dev_init(void)
 {
         kblockd_workqueue = create_workqueue("kblockd");
         if (!kblockd_workqueue)
                 panic("Failed to create kblockd\n");
 
         request_cachep = kmem_cache_create("blkdev_requests",
                         sizeof(struct request), 0, SLAB_PANIC, NULL, NULL);
 
         requestq_cachep = kmem_cache_create("blkdev_queue",
                         sizeof(request_queue_t), 0, SLAB_PANIC, NULL, NULL);
 
         iocontext_cachep = kmem_cache_create("blkdev_ioc",
                         sizeof(struct io_context), 0, SLAB_PANIC, NULL, NULL);
 
         blk_max_low_pfn = max_low_pfn;
         blk_max_pfn = max_pfn;
 
         return 0;
 }
 
 /*
  * IO Context helper functions
  */
 void put_io_context(struct io_context *ioc)
 {
         if (ioc == NULL)
                 return;
 
         BUG_ON(atomic_read(&ioc->refcount) == 0);
 
         if (atomic_dec_and_test(&ioc->refcount)) {
                 if (ioc->aic && ioc->aic->dtor)
                         ioc->aic->dtor(ioc->aic);
                 if (ioc->cic && ioc->cic->dtor)
                         ioc->cic->dtor(ioc->cic);
 
                 kmem_cache_free(iocontext_cachep, ioc);
         }
 }
 EXPORT_SYMBOL(put_io_context);
 
 /* Called by the exitting task */
 void exit_io_context(void)
 {
         unsigned long flags;
         struct io_context *ioc;
 
         local_irq_save(flags);
         ioc = current->io_context;
         current->io_context = NULL;
         local_irq_restore(flags);
 
         if (ioc->aic && ioc->aic->exit)
                 ioc->aic->exit(ioc->aic);
         if (ioc->cic && ioc->cic->exit)
                 ioc->cic->exit(ioc->cic);
 
         put_io_context(ioc);
 }
 
 /*
  * If the current task has no IO context then create one and initialise it.
  * If it does have a context, take a ref on it.
  *
  * This is always called in the context of the task which submitted the I/O.
  * But weird things happen, so we disable local interrupts to ensure exclusive
  * access to *current.
  */
 struct io_context *get_io_context(int gfp_flags)
 {
         struct task_struct *tsk = current;
         unsigned long flags;
         struct io_context *ret;
 
         local_irq_save(flags);
         ret = tsk->io_context;
         if (ret)
                 goto out;
 
         local_irq_restore(flags);
 
         ret = kmem_cache_alloc(iocontext_cachep, gfp_flags);
         if (ret) {
                 atomic_set(&ret->refcount, 1);
                 ret->pid = tsk->pid;
                 ret->last_waited = jiffies; /* doesn't matter... */
                 ret->nr_batch_requests = 0; /* because this is 0 */
                 ret->aic = NULL;
                 ret->cic = NULL;
                 spin_lock_init(&ret->lock);
 
                 local_irq_save(flags);
 
                 /*
                  * very unlikely, someone raced with us in setting up the task
                  * io context. free new context and just grab a reference.
                  */
                 if (!tsk->io_context)
                         tsk->io_context = ret;
                 else {
                         kmem_cache_free(iocontext_cachep, ret);
                         ret = tsk->io_context;
                 }
 
 out:
                 atomic_inc(&ret->refcount);
                 local_irq_restore(flags);
         }
 
         return ret;
 }
 EXPORT_SYMBOL(get_io_context);
 
 void copy_io_context(struct io_context **pdst, struct io_context **psrc)
 {
         struct io_context *src = *psrc;
         struct io_context *dst = *pdst;
 
         if (src) {
                 BUG_ON(atomic_read(&src->refcount) == 0);
                 atomic_inc(&src->refcount);
                 put_io_context(dst);
                 *pdst = src;
         }
 }
 EXPORT_SYMBOL(copy_io_context);
 
 void swap_io_context(struct io_context **ioc1, struct io_context **ioc2)
 {
         struct io_context *temp;
         temp = *ioc1;
         *ioc1 = *ioc2;
         *ioc2 = temp;
 }
 EXPORT_SYMBOL(swap_io_context);
 
 /*
  * sysfs parts below
  */
 struct queue_sysfs_entry {
         struct attribute attr;
         ssize_t (*show)(struct request_queue *, char *);
         ssize_t (*store)(struct request_queue *, const char *, size_t);
 };
 
 static ssize_t
 queue_var_show(unsigned int var, char *page)
 {
         return sprintf(page, "%d\n", var);
 }
 
 static ssize_t
 queue_var_store(unsigned long *var, const char *page, size_t count)
 {
         char *p = (char *) page;
 
         *var = simple_strtoul(p, &p, 10);
         return count;
 }
 
 static ssize_t queue_requests_show(struct request_queue *q, char *page)
 {
         return queue_var_show(q->nr_requests, (page));
 }
 
 static ssize_t
 queue_requests_store(struct request_queue *q, const char *page, size_t count)
 {
         struct request_list *rl = &q->rq;
 
         int ret = queue_var_store(&q->nr_requests, page, count);
         if (q->nr_requests < BLKDEV_MIN_RQ)
                 q->nr_requests = BLKDEV_MIN_RQ;
         blk_queue_congestion_threshold(q);
 
         if (rl->count[READ] >= queue_congestion_on_threshold(q))
                 set_queue_congested(q, READ);
         else if (rl->count[READ] < queue_congestion_off_threshold(q))
                 clear_queue_congested(q, READ);
 
         if (rl->count[WRITE] >= queue_congestion_on_threshold(q))
                 set_queue_congested(q, WRITE);
         else if (rl->count[WRITE] < queue_congestion_off_threshold(q))
                 clear_queue_congested(q, WRITE);
 
         if (rl->count[READ] >= q->nr_requests) {
                 blk_set_queue_full(q, READ);
         } else if (rl->count[READ]+1 <= q->nr_requests) {
                 blk_clear_queue_full(q, READ);
                 wake_up(&rl->wait[READ]);
         }
 
         if (rl->count[WRITE] >= q->nr_requests) {
                 blk_set_queue_full(q, WRITE);
         } else if (rl->count[WRITE]+1 <= q->nr_requests) {
                 blk_clear_queue_full(q, WRITE);
                 wake_up(&rl->wait[WRITE]);
         }
         return ret;
 }
 
 static ssize_t queue_ra_show(struct request_queue *q, char *page)
 {
         int ra_kb = q->backing_dev_info.ra_pages << (PAGE_CACHE_SHIFT - 10);
 
         return queue_var_show(ra_kb, (page));
 }
 
 static ssize_t
 queue_ra_store(struct request_queue *q, const char *page, size_t count)
 {
         unsigned long ra_kb;
         ssize_t ret = queue_var_store(&ra_kb, page, count);
 
         spin_lock_irq(q->queue_lock);
         if (ra_kb > (q->max_sectors >> 1))
                 ra_kb = (q->max_sectors >> 1);
 
         q->backing_dev_info.ra_pages = ra_kb >> (PAGE_CACHE_SHIFT - 10);
         spin_unlock_irq(q->queue_lock);
 
         return ret;
 }
 
 static ssize_t queue_max_sectors_show(struct request_queue *q, char *page)
 {
         int max_sectors_kb = q->max_sectors >> 1;
 
         return queue_var_show(max_sectors_kb, (page));
 }
 
 static ssize_t
 queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
 {
         unsigned long max_sectors_kb,
                         max_hw_sectors_kb = q->max_hw_sectors >> 1,
                         page_kb = 1 << (PAGE_CACHE_SHIFT - 10);
         ssize_t ret = queue_var_store(&max_sectors_kb, page, count);
         int ra_kb;
 
         if (max_sectors_kb > max_hw_sectors_kb || max_sectors_kb < page_kb)
                 return -EINVAL;
         /*
          * Take the queue lock to update the readahead and max_sectors
          * values synchronously:
          */
         spin_lock_irq(q->queue_lock);
         /*
          * Trim readahead window as well, if necessary:
          */
         ra_kb = q->backing_dev_info.ra_pages << (PAGE_CACHE_SHIFT - 10);
         if (ra_kb > max_sectors_kb)
                 q->backing_dev_info.ra_pages =
                                 max_sectors_kb >> (PAGE_CACHE_SHIFT - 10);
 
         q->max_sectors = max_sectors_kb << 1;
         spin_unlock_irq(q->queue_lock);
 
         return ret;
 }
 
 static ssize_t queue_max_hw_sectors_show(struct request_queue *q, char *page)
 {
         int max_hw_sectors_kb = q->max_hw_sectors >> 1;
 
         return queue_var_show(max_hw_sectors_kb, (page));
 }
 
 
 static struct queue_sysfs_entry queue_requests_entry = {
         .attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
         .show = queue_requests_show,
         .store = queue_requests_store,
 };
 
 static struct queue_sysfs_entry queue_ra_entry = {
         .attr = {.name = "read_ahead_kb", .mode = S_IRUGO | S_IWUSR },
         .show = queue_ra_show,
         .store = queue_ra_store,
 };
 
 static struct queue_sysfs_entry queue_max_sectors_entry = {
         .attr = {.name = "max_sectors_kb", .mode = S_IRUGO | S_IWUSR },
         .show = queue_max_sectors_show,
         .store = queue_max_sectors_store,
 };
 
 static struct queue_sysfs_entry queue_max_hw_sectors_entry = {
         .attr = {.name = "max_hw_sectors_kb", .mode = S_IRUGO },
         .show = queue_max_hw_sectors_show,
 };
 
 static struct queue_sysfs_entry queue_iosched_entry = {
         .attr = {.name = "scheduler", .mode = S_IRUGO | S_IWUSR },
         .show = elv_iosched_show,
         .store = elv_iosched_store,
 };
 
 static struct attribute *default_attrs[] = {
         &queue_requests_entry.attr,
         &queue_ra_entry.attr,
         &queue_max_hw_sectors_entry.attr,
         &queue_max_sectors_entry.attr,
         &queue_iosched_entry.attr,
         NULL,
 };
 
 #define to_queue(atr) container_of((atr), struct queue_sysfs_entry, attr)
 
 static ssize_t
 queue_attr_show(struct kobject *kobj, struct attribute *attr, char *page)
 {
         struct queue_sysfs_entry *entry = to_queue(attr);
         struct request_queue *q;
 
         q = container_of(kobj, struct request_queue, kobj);
         if (!entry->show)
                 return 0;
 
         return entry->show(q, page);
 }
 
 static ssize_t
 queue_attr_store(struct kobject *kobj, struct attribute *attr,
                     const char *page, size_t length)
 {
         struct queue_sysfs_entry *entry = to_queue(attr);
         struct request_queue *q;
 
         q = container_of(kobj, struct request_queue, kobj);
         if (!entry->store)
                 return -EINVAL;
 
         return entry->store(q, page, length);
 }
 
 static struct sysfs_ops queue_sysfs_ops = {
         .show   = queue_attr_show,
         .store  = queue_attr_store,
 };
 
 struct kobj_type queue_ktype = {
         .sysfs_ops      = &queue_sysfs_ops,
         .default_attrs  = default_attrs,
 };
 
 int blk_register_queue(struct gendisk *disk)
 {
         int ret;
 
         request_queue_t *q = disk->queue;
 
         if (!q || !q->request_fn)
                 return -ENXIO;
 
         q->kobj.parent = kobject_get(&disk->kobj);
         if (!q->kobj.parent)
                 return -EBUSY;
 
         snprintf(q->kobj.name, KOBJ_NAME_LEN, "%s", "queue");
         q->kobj.ktype = &queue_ktype;
 
         ret = kobject_register(&q->kobj);
         if (ret < 0)
                 return ret;
 
         ret = elv_register_queue(q);
         if (ret) {
                 kobject_unregister(&q->kobj);
                 return ret;
         }
 
         return 0;
 }
 
 void blk_unregister_queue(struct gendisk *disk)
 {
         request_queue_t *q = disk->queue;
 
         if (q && q->request_fn) {
                 elv_unregister_queue(q);
 
                 kobject_unregister(&q->kobj);
                 kobject_put(&disk->kobj);
         }
 }