blk-cgroup: factor out a helper to read rwstat counter

Trying to break up the crazy statements to something readable.
Also switch to an unsigned counter as it can't ever turn negative.
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-cgroup.c

@@ -745,7 +745,7 @@ struct blkg_rwstat blkg_rwstat_recursive_sum(struct blkcg_gq *blkg,
 	struct blkcg_gq *pos_blkg;
 	struct cgroup_subsys_state *pos_css;
 	struct blkg_rwstat sum = { };
-	int i;
+	unsigned int i;
 
 	lockdep_assert_held(&blkg->q->queue_lock);
 
@@ -762,8 +762,7 @@ struct blkg_rwstat blkg_rwstat_recursive_sum(struct blkcg_gq *blkg,
 			rwstat = (void *)pos_blkg + off;
 
 		for (i = 0; i < BLKG_RWSTAT_NR; i++)
-			atomic64_add(atomic64_read(&rwstat->aux_cnt[i]) +
-				percpu_counter_sum_positive(&rwstat->cpu_cnt[i]),
+			atomic64_add(blkg_rwstat_read_counter(rwstat, i),
 				&sum.aux_cnt[i]);
 	}
 	rcu_read_unlock();

include/linux/blk-cgroup.h

@@ -198,6 +198,13 @@ int blkcg_activate_policy(struct request_queue *q,
 void blkcg_deactivate_policy(struct request_queue *q,
 			     const struct blkcg_policy *pol);
 
+static inline u64 blkg_rwstat_read_counter(struct blkg_rwstat *rwstat,
+		unsigned int idx)
+{
+	return atomic64_read(&rwstat->aux_cnt[idx]) +
+		percpu_counter_sum_positive(&rwstat->cpu_cnt[idx]);
+}
+
 const char *blkg_dev_name(struct blkcg_gq *blkg);
 void blkcg_print_blkgs(struct seq_file *sf, struct blkcg *blkcg,
 		       u64 (*prfill)(struct seq_file *,