From: Mikulas Patocka <[email-masked]>

register_snapshot() performs a GFP_KERNEL allocation while holding
_origins_lock for write, but that could write out dirty pages onto a
device that attempts to acquire _origins_lock for read, resulting in
deadlock.

So move the allocation up before taking the lock.

This path is not performance-critical, so it doesn't matter that we
allocate memory and free it if we find that we won't need it.

Signed-off-by: Mikulas Patocka <[email-masked]>
Signed-off-by: Alasdair G Kergon <[email-masked]>

---
 drivers/md/dm-snap.c |   16 +++++++++-------
 1 files changed, 9 insertions(+), 7 deletions(-)

Index: linux-2.6.28-rc2/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.28-rc2.orig/drivers/md/dm-snap.c	2008-10-30 13:26:33.000000000 +0000
+++ linux-2.6.28-rc2/drivers/md/dm-snap.c	2008-10-30 13:27:18.000000000 +0000
@@ -229,19 +229,21 @@ static void __insert_origin(struct origi
  */
 static int register_snapshot(struct dm_snapshot *snap)
 {
-	struct origin *o;
+	struct origin *o, *new_o;
 	struct block_device *bdev = snap->origin->bdev;
 
+	new_o = kmalloc(sizeof(*new_o), GFP_KERNEL);
+	if (!new_o)
+		return -ENOMEM;
+
 	down_write(&_origins_lock);
 	o = __lookup_origin(bdev);
 
-	if (!o) {
+	if (o)
+		kfree(new_o);
+	else {
 		/* New origin */
-		o = kmalloc(sizeof(*o), GFP_KERNEL);
-		if (!o) {
-			up_write(&_origins_lock);
-			return -ENOMEM;
-		}
+		o = new_o;
 
 		/* Initialise the struct */
 		INIT_LIST_HEAD(&o->snapshots);