Re: [PATCH V2 4/4] block: don't hold part0's refcount in IO path

[Ming Lei <ming.lei@redhat.com>]

On Thu, May 07, 2020 at 11:41:33PM -0700, Christoph Hellwig wrote:
> On Fri, May 08, 2020 at 12:44:07PM +0800, Ming Lei wrote:
> > gendisk can't be gone when there is IO activity, so not hold
> > part0's refcount in IO path.
> > 
> > Cc: Yufen Yu <yuyufen@huawei.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Hou Tao <houtao1@huawei.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> 
> This looks correct, although I'd still prefer to centralize the
> partno checks in the helpers.  Also hd_struct_get is unused with
> this patch isn't it?
 
OK, are you fine with the following patch?

diff --git a/block/blk.h b/block/blk.h
index 133fb0b99759..8efd1ca5c975 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -376,19 +376,18 @@ int bdev_resize_partition(struct block_device *bdev, int partno,
 int disk_expand_part_tbl(struct gendisk *disk, int target);
 int hd_ref_init(struct hd_struct *part);
 
-static inline void hd_struct_get(struct hd_struct *part)
-{
-	percpu_ref_get(&part->ref);
-}
-
+/* no need to get/put refcount of part0 */
 static inline int hd_struct_try_get(struct hd_struct *part)
 {
-	return percpu_ref_tryget_live(&part->ref);
+	if (part->partno)
+		return percpu_ref_tryget_live(&part->ref);
+	return 1;
 }
 
 static inline void hd_struct_put(struct hd_struct *part)
 {
-	percpu_ref_put(&part->ref);
+	if (part->partno)
+		percpu_ref_put(&part->ref);
 }
 
 static inline void hd_free_part(struct hd_struct *part)
diff --git a/block/genhd.c b/block/genhd.c
index bf8cbb033d64..d97b95d1a2fd 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -345,7 +345,8 @@ static inline int sector_in_part(struct hd_struct *part, sector_t sector)
  *
  * CONTEXT:
  * RCU read locked.  The returned partition pointer is always valid
- * because its refcount is grabbed.
+ * because its refcount is grabbed except for part0, which lifetime
+ * is same with the disk.
  *
  * RETURNS:
  * Found partition on success, part0 is returned if no partition matches
@@ -378,7 +379,6 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 			return part;
 		}
 	}
-	hd_struct_get(&disk->part0);
 	return &disk->part0;
 }
 


Thanks,
Ming