block: make queue stat accounting a reference

kyber turns on IO statistics when it is loaded on a queue, which means
that even if kyber is then later unloaded, we're still stuck with stats
enabled on the queue.

Change the account enabled from a bool to an int, and pair the enable call
with the equivalent disable call. This ensures that stats gets turned off
again appropriately.
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-stat.c

@@ -15,7 +15,7 @@
 struct blk_queue_stats {
 	struct list_head callbacks;
 	spinlock_t lock;
-	bool enable_accounting;
+	int accounting;
 };
 
 void blk_rq_stat_init(struct blk_rq_stat *stat)
@@ -161,7 +161,7 @@ void blk_stat_remove_callback(struct request_queue *q,
 
 	spin_lock_irqsave(&q->stats->lock, flags);
 	list_del_rcu(&cb->list);
-	if (list_empty(&q->stats->callbacks) && !q->stats->enable_accounting)
+	if (list_empty(&q->stats->callbacks) && !q->stats->accounting)
 		blk_queue_flag_clear(QUEUE_FLAG_STATS, q);
 	spin_unlock_irqrestore(&q->stats->lock, flags);
 
@@ -184,13 +184,24 @@ void blk_stat_free_callback(struct blk_stat_callback *cb)
 		call_rcu(&cb->rcu, blk_stat_free_callback_rcu);
 }
 
+void blk_stat_disable_accounting(struct request_queue *q)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&q->stats->lock, flags);
+	if (!--q->stats->accounting)
+		blk_queue_flag_clear(QUEUE_FLAG_STATS, q);
+	spin_unlock_irqrestore(&q->stats->lock, flags);
+}
+EXPORT_SYMBOL_GPL(blk_stat_disable_accounting);
+
 void blk_stat_enable_accounting(struct request_queue *q)
 {
 	unsigned long flags;
 
 	spin_lock_irqsave(&q->stats->lock, flags);
-	q->stats->enable_accounting = true;
-	blk_queue_flag_set(QUEUE_FLAG_STATS, q);
+	if (!q->stats->accounting++)
+		blk_queue_flag_set(QUEUE_FLAG_STATS, q);
 	spin_unlock_irqrestore(&q->stats->lock, flags);
 }
 EXPORT_SYMBOL_GPL(blk_stat_enable_accounting);
@@ -205,7 +216,7 @@ struct blk_queue_stats *blk_alloc_queue_stats(void)
 
 	INIT_LIST_HEAD(&stats->callbacks);
 	spin_lock_init(&stats->lock);
-	stats->enable_accounting = false;
+	stats->accounting = 0;
 
 	return stats;
 }

block/blk-stat.h

@@ -70,6 +70,7 @@ void blk_stat_add(struct request *rq, u64 now);
 
 /* record time/size info in request but not add a callback */
 void blk_stat_enable_accounting(struct request_queue *q);
+void blk_stat_disable_accounting(struct request_queue *q);
 
 /**
  * blk_stat_alloc_callback() - Allocate a block statistics callback.

block/kyber-iosched.c

@@ -433,6 +433,7 @@ static void kyber_exit_sched(struct elevator_queue *e)
 	int i;
 
 	del_timer_sync(&kqd->timer);
+	blk_stat_disable_accounting(kqd->q);
 
 	for (i = 0; i < KYBER_NUM_DOMAINS; i++)
 		sbitmap_queue_free(&kqd->domain_tokens[i]);