[PATCH] xen-blkback: Convert from atomic_t to refcount_t on xen_blkif->refcnt

[Xiyu Yang]

refcount_t type and corresponding API can protect refcounters from
accidental underflow and overflow and further use-after-free situations.

Signed-off-by: Xiyu Yang <xiyuyang19@xxxxxxxxxxxx>
Signed-off-by: Xin Tan <tanxin.ctf@xxxxxxxxx>
---
 drivers/block/xen-blkback/common.h | 7 ++++---
 drivers/block/xen-blkback/xenbus.c | 2 +-
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index bda5c815e441..d074cf41b4bc 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -27,6 +27,7 @@
 #ifndef __XEN_BLKIF__BACKEND__COMMON_H__
 #define __XEN_BLKIF__BACKEND__COMMON_H__
 
+#include <linux/refcount.h>
 #include <linux/module.h>
 #include <linux/interrupt.h>
 #include <linux/slab.h>
@@ -306,7 +307,7 @@ struct xen_blkif {
 	struct xen_vbd		vbd;
 	/* Back pointer to the backend_info. */
 	struct backend_info	*be;
-	atomic_t		refcnt;
+	refcount_t		refcnt;
 	/* for barrier (drain) requests */
 	struct completion	drain_complete;
 	atomic_t		drain;
@@ -359,10 +360,10 @@ struct pending_req {
 
 #define vbd_sz(_v)	bdev_nr_sectors((_v)->bdev)
 
-#define xen_blkif_get(_b) (atomic_inc(&(_b)->refcnt))
+#define xen_blkif_get(_b) (refcount_inc(&(_b)->refcnt))
 #define xen_blkif_put(_b)				\
 	do {						\
-		if (atomic_dec_and_test(&(_b)->refcnt))	\
+		if (refcount_dec_and_test(&(_b)->refcnt))	\
 			schedule_work(&(_b)->free_work);\
 	} while (0)
 
diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c
index 125b22205d38..b0bd5faa3865 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -168,7 +168,7 @@ static struct xen_blkif *xen_blkif_alloc(domid_t domid)
 		return ERR_PTR(-ENOMEM);
 
 	blkif->domid = domid;
-	atomic_set(&blkif->refcnt, 1);
+	refcount_set(&blkif->refcnt, 1);
 	init_completion(&blkif->drain_complete);
 
 	/*
-- 
2.7.4