[Xen-devel] [PATCH v2 3/3] xen/blkback: Use refcount_t for refcount

[Xen-devel] [PATCH v2 3/3] xen/blkback: Use refcount_t for refcount

[Chuhong Yuan]

Reference counters are preferred to use refcount_t instead of
atomic_t.
This is because the implementation of refcount_t can prevent
overflows and detect possible use-after-free.
So convert atomic_t ref counters to refcount_t.

Signed-off-by: Chuhong Yuan <hslester96@gmail.com>
---
Changes in v2:
  - Also convert pending_req::pendcnt to refcount_t.

 drivers/block/xen-blkback/blkback.c | 6 +++---
 drivers/block/xen-blkback/common.h  | 9 +++++----
 drivers/block/xen-blkback/xenbus.c  | 2 +-
 3 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/drivers/block/xen-blkback/blkback.c b/drivers/block/xen-blkback/blkback.c
index fd1e19f1a49f..b24bb0aea35f 100644
--- a/drivers/block/xen-blkback/blkback.c
+++ b/drivers/block/xen-blkback/blkback.c
@@ -1098,7 +1098,7 @@ static void __end_block_io_op(struct pending_req *pending_req,
 	 * the grant references associated with 'request' and provide
 	 * the proper response on the ring.
 	 */
-	if (atomic_dec_and_test(&pending_req->pendcnt))
+	if (refcount_dec_and_test(&pending_req->pendcnt))
 		xen_blkbk_unmap_and_respond(pending_req);
 }
 
@@ -1395,7 +1395,7 @@ static int dispatch_rw_block_io(struct xen_blkif_ring *ring,
 		bio_set_op_attrs(bio, operation, operation_flags);
 	}
 
-	atomic_set(&pending_req->pendcnt, nbio);
+	refcount_set(&pending_req->pendcnt, nbio);
 	blk_start_plug(&plug);
 
 	for (i = 0; i < nbio; i++)
@@ -1424,7 +1424,7 @@ static int dispatch_rw_block_io(struct xen_blkif_ring *ring,
  fail_put_bio:
 	for (i = 0; i < nbio; i++)
 		bio_put(biolist[i]);
-	atomic_set(&pending_req->pendcnt, 1);
+	refcount_set(&pending_req->pendcnt, 1);
 	__end_block_io_op(pending_req, BLK_STS_RESOURCE);
 	msleep(1); /* back off a bit */
 	return -EIO;
diff --git a/drivers/block/xen-blkback/common.h b/drivers/block/xen-blkback/common.h
index 1d3002d773f7..824d64a8339b 100644
--- a/drivers/block/xen-blkback/common.h
+++ b/drivers/block/xen-blkback/common.h
@@ -35,6 +35,7 @@
 #include <linux/wait.h>
 #include <linux/io.h>
 #include <linux/rbtree.h>
+#include <linux/refcount.h>
 #include <asm/setup.h>
 #include <asm/pgalloc.h>
 #include <asm/hypervisor.h>
@@ -309,7 +310,7 @@ 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;
@@ -343,7 +344,7 @@ struct pending_req {
 	struct xen_blkif_ring   *ring;
 	u64			id;
 	int			nr_segs;
-	atomic_t		pendcnt;
+	refcount_t		pendcnt;
 	unsigned short		operation;
 	int			status;
 	struct list_head	free_list;
@@ -362,10 +363,10 @@ struct pending_req {
 			 (_v)->bdev->bd_part->nr_sects : \
 			  get_capacity((_v)->bdev->bd_disk))
 
-#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 3ac6a5d18071..ecc5f9c5bf3f 100644
--- a/drivers/block/xen-blkback/xenbus.c
+++ b/drivers/block/xen-blkback/xenbus.c
@@ -169,7 +169,7 @@ 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);
 	INIT_WORK(&blkif->free_work, xen_blkif_deferred_free);
 
-- 
2.20.1


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Re: [Xen-devel] [PATCH v2 3/3] xen/blkback: Use refcount_t for refcount

[Roger Pau Monné]

On Tue, Aug 13, 2019 at 02:16:50PM +0800, Chuhong Yuan wrote:
> Reference counters are preferred to use refcount_t instead of
> atomic_t.
> This is because the implementation of refcount_t can prevent
> overflows and detect possible use-after-free.
> So convert atomic_t ref counters to refcount_t.
> 
> Signed-off-by: Chuhong Yuan <hslester96@gmail.com>

Acked-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks, Roger.

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