rbd: protect read of snapshot sequence number

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@dreamhost.com>
Reviewed-by: Alex Elder <elder@dreamhost.com>
Reviewed-by: Yehuda Sadeh <yehuda@hq.newdream.net>

1 files changed, 3 insertions, 1 deletions

diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index a75fe93a25b1..5ab9f55d3e0c 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -1684,7 +1684,9 @@ static int rbd_header_add_snap(struct rbd_device *dev,
         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: