block: don't hold part0's refcount in IO path

gendisk can't be gone when there is IO activity, so not hold
part0's refcount in IO path.

Signed-off-by: Ming Lei <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Yufen Yu <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Hou Tao <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: Ming Lei

block/blk.h

@@ -378,19 +378,18 @@ 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)

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;
 }