An empty, never-called function.

Just to keep this test in __generic_make_request satisfied.

if ((bio_empty_barrier(bio) && !q->prepare_flush_fn) ||
    (bio_discard(bio) && !q->prepare_discard_fn)) {
	err = -EOPNOTSUPP;
	goto end_io;
}

This should be better changed in generic block-layer code.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm.c |    9 +++++++++
 1 file changed, 9 insertions(+)

Index: linux-2.6.28-clean/drivers/md/dm.c
===================================================================
--- linux-2.6.28-clean.orig/drivers/md/dm.c	2009-01-12 15:19:46.000000000 +0100
+++ linux-2.6.28-clean/drivers/md/dm.c	2009-01-12 21:19:21.000000000 +0100
@@ -1026,6 +1026,14 @@ out:
 	return r;
 }
 
+/*
+ * This does nothing and is never called, just keeps zero-sized barriers
+ * pass through __generic_make_request
+ */
+static void dummy_prepare_flush_fn(struct request_queue *q, struct request *rq)
+{
+}
+
 static struct block_device_operations dm_blk_dops;
 
 static void dm_wq_work(struct work_struct *work);
@@ -1073,6 +1081,7 @@ static struct mapped_device *alloc_dev(i
 	md->queue->queuedata = md;
 	md->queue->backing_dev_info.congested_fn = dm_any_congested;
 	md->queue->backing_dev_info.congested_data = md;
+	md->queue->prepare_flush_fn = dummy_prepare_flush_fn;
 	blk_queue_make_request(md->queue, dm_request);
 	blk_queue_bounce_limit(md->queue, BLK_BOUNCE_ANY);
 	md->queue->unplug_fn = dm_unplug_all;