[PATCH 3.5 23/90] xen/blkback: fix reference counting

[Luis Henriques]

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

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

From: Vegard Nossum <vegard.nossum at oracle.com>

commit ea5ec76d76da9279d12027c1828544c5ccbe7932 upstream.

If the permission check fails, we drop a reference to the blkif without
having taken it in the first place. The bug was introduced in commit
604c499cbbcc3d5fe5fb8d53306aa0fae1990109 (xen/blkback: Check device
permissions before allowing OP_DISCARD).

Cc: Jan Beulich <JBeulich at suse.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
Signed-off-by: Vegard Nossum <vegard.nossum at oracle.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk at oracle.com>
Signed-off-by: Jens Axboe <axboe at kernel.dk>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/block/xen-blkback/blkback.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index b83b40e..ae1b0c4 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -406,6 +406,8 @@ static int dispatch_discard_io(struct xen_blkif *blkif,
 	unsigned long secure;
 	struct phys_req preq;
 
+	xen_blkif_get(blkif);
+
 	preq.sector_number = req->u.discard.sector_number;
 	preq.nr_sects      = req->u.discard.nr_sectors;
 
@@ -418,7 +420,6 @@ static int dispatch_discard_io(struct xen_blkif *blkif,
 	}
 	blkif->st_ds_req++;
 
-	xen_blkif_get(blkif);
 	secure = (blkif->vbd.discard_secure &&
 		 (req->u.discard.flag & BLKIF_DISCARD_SECURE)) ?
 		 BLKDEV_DISCARD_SECURE : 0;
-- 
1.8.3.2