[24/99] dm snapshot: lock snapshot while supplying status

[Greg KH]

2.6.31-stable review patch.  If anyone has any objections, please let us know.

------------------
From: Mikulas Patocka <mpatocka@xxxxxxxxxx>

commit 4c6fff445d7aa753957856278d4d93bcad6e2c14 upstream.

This patch locks the snapshot when returning status.  It fixes a race
when it could return an invalid number of free chunks if someone
was simultaneously modifying it.

Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
Signed-off-by: Alasdair G Kergon <agk@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

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

--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1147,6 +1147,8 @@ 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:
 		if (!snap->valid)
@@ -1178,6 +1180,8 @@ static int snapshot_status(struct dm_tar
 		break;
 	}
 
+	up_write(&snap->lock);
+
 	return 0;
 }
 


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