virtio_blk: Drop unused request tracking list

Benchmark shows small performance improvement on fusion io device.

Before:
  seq-read : io=1,024MB, bw=19,982KB/s, iops=39,964, runt= 52475msec
  seq-write: io=1,024MB, bw=20,321KB/s, iops=40,641, runt= 51601msec
  rnd-read : io=1,024MB, bw=15,404KB/s, iops=30,808, runt= 68070msec
  rnd-write: io=1,024MB, bw=14,776KB/s, iops=29,552, runt= 70963msec

After:
  seq-read : io=1,024MB, bw=20,343KB/s, iops=40,685, runt= 51546msec
  seq-write: io=1,024MB, bw=20,803KB/s, iops=41,606, runt= 50404msec
  rnd-read : io=1,024MB, bw=16,221KB/s, iops=32,442, runt= 64642msec
  rnd-write: io=1,024MB, bw=15,199KB/s, iops=30,397, runt= 68991msec

Signed-off-by: Asias He <asias@redhat.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/block/virtio_blk.c

@@ -29,9 +29,6 @@ struct virtio_blk
 	/* The disk structure for the kernel. */
 	struct gendisk *disk;
 
-	/* Request tracking. */
-	struct list_head reqs;
-
 	mempool_t *pool;
 
 	/* Process context for config space updates */
@@ -55,7 +52,6 @@ struct virtio_blk
 
 struct virtblk_req
 {
-	struct list_head list;
 	struct request *req;
 	struct virtio_blk_outhdr out_hdr;
 	struct virtio_scsi_inhdr in_hdr;
@@ -99,7 +95,6 @@ static void blk_done(struct virtqueue *vq)
 		}
 
 		__blk_end_request_all(vbr->req, error);
-		list_del(&vbr->list);
 		mempool_free(vbr, vblk->pool);
 	}
 	/* In case queue is stopped waiting for more buffers. */
@@ -184,7 +179,6 @@ static bool do_req(struct request_queue *q, struct virtio_blk *vblk,
 		return false;
 	}
 
-	list_add_tail(&vbr->list, &vblk->reqs);
 	return true;
 }
 
@@ -437,7 +431,6 @@ static int __devinit virtblk_probe(struct virtio_device *vdev)
 		goto out_free_index;
 	}
 
-	INIT_LIST_HEAD(&vblk->reqs);
 	spin_lock_init(&vblk->lock);
 	vblk->vdev = vdev;
 	vblk->sg_elems = sg_elems;
@@ -591,9 +584,6 @@ static void __devexit virtblk_remove(struct virtio_device *vdev)
 	vblk->config_enable = false;
 	mutex_unlock(&vblk->config_lock);
 
-	/* Nothing should be pending. */
-	BUG_ON(!list_empty(&vblk->reqs));
-
 	/* Stop all the virtqueues. */
 	vdev->config->reset(vdev);