[ 089/171] rbd: protect read of snapshot sequence number

[Greg Kroah-Hartman]

3.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Josh Durgin <josh.durgin@xxxxxxxxxxxxx>

(cherry picked from commit 403f24d3d51760a8b9368d595fa5f48c309f1a0f)

This is updated whenever a snapshot is added or deleted, and the
snapc pointer is changed with every refresh of the header.

Signed-off-by: Josh Durgin <josh.durgin@xxxxxxxxxxxxx>
Reviewed-by: Alex Elder <elder@xxxxxxxxxxxxx>
Reviewed-by: Yehuda Sadeh <yehuda@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/block/rbd.c |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -1683,7 +1683,9 @@ static int rbd_header_add_snap(struct rb
 	if (ret < 0)
 		return ret;
 
-	dev->header.snapc->seq =  new_snapid;
+	down_write(&dev->header_rwsem);
+	dev->header.snapc->seq = new_snapid;
+	up_write(&dev->header_rwsem);
 
 	return 0;
 bad:


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