Do not allow more than one merging snapshot.
It is pointless to merge two snapshot simultaneously.
Allowing multiple merging snapshots would complicate things later.

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

---
 drivers/md/dm-snap.c |    7 +++++++
 1 file changed, 7 insertions(+)

Index: linux-2.6.28-rc5-devel/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.28-rc5-devel.orig/drivers/md/dm-snap.c	2008-11-25 16:11:27.000000000 +0100
+++ linux-2.6.28-rc5-devel/drivers/md/dm-snap.c	2008-11-25 16:11:31.000000000 +0100
@@ -234,6 +234,13 @@ static int register_snapshot(struct dm_s
 		return -ENOMEM;
 
 	down_write(&_origins_lock);
+
+	/* Do not allow more than one merging snapshot */
+	if (is_merge(snap->ti) && __find_merging_snapshot(bdev)) {
+		up_write(&_origins_lock);
+		return -EINVAL;
+	}
+
 	o = __lookup_origin(bdev);
 
 	if (o)