block: remove rcu_read_lock() from part_stat_lock()

The RCU lock is required only in disk_map_sector_rcu() to lookup the
partition.  After that request holds reference to related hd_struct.

Replace get_cpu() with preempt_disable() - returned cpu index is unused.

[hch: rebased]

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -321,11 +321,12 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 	struct hd_struct *part;
 	int i;
 
+	rcu_read_lock();
 	ptbl = rcu_dereference(disk->part_tbl);
 
 	part = rcu_dereference(ptbl->last_lookup);
 	if (part && sector_in_part(part, sector) && hd_struct_try_get(part))
-		return part;
+		goto out_unlock;
 
 	for (i = 1; i < ptbl->len; i++) {
 		part = rcu_dereference(ptbl->part[i]);
@@ -339,10 +340,14 @@ struct hd_struct *disk_map_sector_rcu(struct gendisk *disk, sector_t sector)
 			if (!hd_struct_try_get(part))
 				break;
 			rcu_assign_pointer(ptbl->last_lookup, part);
-			return part;
+			goto out_unlock;
 		}
 	}
-	return &disk->part0;
+
+	part = &disk->part0;
+out_unlock:
+	rcu_read_unlock();
+	return part;
 }
 
 /**

include/linux/part_stat.h

@@ -21,8 +21,8 @@ struct disk_stats {
  *
  * part_stat_read() can be called at any time.
  */
-#define part_stat_lock()	({ rcu_read_lock(); get_cpu(); })
-#define part_stat_unlock()	do { put_cpu(); rcu_read_unlock(); } while (0)
+#define part_stat_lock()	preempt_disable()
+#define part_stat_unlock()	preempt_enable()
 
 #define part_stat_get_cpu(part, field, cpu)				\
 	(per_cpu_ptr((part)->dkstats, (cpu))->field)