[dm-devel] [PATCH] Lock only when querying status, not table

[Mikulas Patocka]

Hi

Please, apply this before 2.6.32, there is a possibility for regression 
(locking in snapshot_status was introduced in 2.6.32-rc).

Mikulas

---

Lock only when querying status, not table

LVM sometimes suspends the underlying device and asks for a table.

To prevent a possible deadlock (LVM suspends the snapshot store, snapshot
takes the lock and waits for I/O, LVM asks for table before unsuspend,
deadlock), lock only when querying the status. The lock is not needed for
table anyway, because the table doesn't change during snapshot operation.

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

---
 drivers/md/dm-snap.c |   10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

Index: linux-2.6.32-rc8-devel/drivers/md/dm-snap.c
===================================================================
--- linux-2.6.32-rc8-devel.orig/drivers/md/dm-snap.c	2009-11-30 17:56:06.000000000 +0100
+++ linux-2.6.32-rc8-devel/drivers/md/dm-snap.c	2009-11-30 18:00:44.000000000 +0100
@@ -1407,10 +1407,11 @@ static int snapshot_status(struct dm_tar
 	unsigned sz = 0;
 	struct dm_snapshot *snap = ti->private;
 
-	down_write(&snap->lock);
-
 	switch (type) {
 	case STATUSTYPE_INFO:
+
+		down_write(&snap->lock);
+
 		if (!snap->valid)
 			DMEMIT("Invalid");
 		else {
@@ -1429,6 +1430,9 @@ static int snapshot_status(struct dm_tar
 			else
 				DMEMIT("Unknown");
 		}
+
+		up_write(&snap->lock);
+
 		break;
 
 	case STATUSTYPE_TABLE:
@@ -1443,8 +1447,6 @@ static int snapshot_status(struct dm_tar
 		break;
 	}
 
-	up_write(&snap->lock);
-
 	return 0;
 }