block: add a __disk_get_part helper

This helper allows looking up a partion under RCU protection without
grabbing a reference to it.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk.h

@@ -204,6 +204,8 @@ static inline void elv_deactivate_rq(struct request_queue *q, struct request *rq
 		e->type->ops.sq.elevator_deactivate_req_fn(q, rq);
 }
 
+struct hd_struct *__disk_get_part(struct gendisk *disk, int partno);
+
 #ifdef CONFIG_FAIL_IO_TIMEOUT
 int blk_should_fake_timeout(struct request_queue *);
 ssize_t part_timeout_show(struct device *, struct device_attribute *, char *);

block/genhd.c

@@ -82,6 +82,15 @@ void part_in_flight(struct request_queue *q, struct hd_struct *part,
 	}
 }
 
+struct hd_struct *__disk_get_part(struct gendisk *disk, int partno)
+{
+	struct disk_part_tbl *ptbl = rcu_dereference(disk->part_tbl);
+
+	if (unlikely(partno < 0 || partno >= ptbl->len))
+		return NULL;
+	return rcu_dereference(ptbl->part[partno]);
+}
+
 /**
  * disk_get_part - get partition
  * @disk: disk to look partition from
@@ -98,21 +107,12 @@ void part_in_flight(struct request_queue *q, struct hd_struct *part,
  */
 struct hd_struct *disk_get_part(struct gendisk *disk, int partno)
 {
-	struct hd_struct *part = NULL;
-	struct disk_part_tbl *ptbl;
-
-	if (unlikely(partno < 0))
-		return NULL;
+	struct hd_struct *part;
 
 	rcu_read_lock();
-
-	ptbl = rcu_dereference(disk->part_tbl);
-	if (likely(partno < ptbl->len)) {
-		part = rcu_dereference(ptbl->part[partno]);
+	part = __disk_get_part(disk, partno);
 	if (part)
 		get_device(part_to_dev(part));
-	}
-
 	rcu_read_unlock();
 
 	return part;