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

[Xiyu Yang <xiyuyang19@xxxxxxxxxxxx>]

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