blk-mq: Swap two calls in blk_mq_exit_queue()

If a tag set is shared across request queues (e.g. SCSI LUNs) then the
block layer core keeps track of the number of active request queues in
tags->active_queues. blk_mq_tag_busy() and blk_mq_tag_idle() update that
atomic counter if the hctx flag BLK_MQ_F_TAG_QUEUE_SHARED is set. Make
sure that blk_mq_exit_queue() calls blk_mq_tag_idle() before that flag is
cleared by blk_mq_del_queue_tag_set().

Cc: Christoph Hellwig <hch@infradead.org>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Fixes: 0d2602c ("blk-mq: improve support for shared tags maps")
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20210513171529.7977-1-bvanassche@acm.org
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk-mq.c

@@ -3286,10 +3286,12 @@ EXPORT_SYMBOL(blk_mq_init_allocated_queue);
 /* tags can _not_ be used after returning from blk_mq_exit_queue */
 void blk_mq_exit_queue(struct request_queue *q)
 {
-	struct blk_mq_tag_set	*set = q->tag_set;
+	struct blk_mq_tag_set *set = q->tag_set;
 
-	blk_mq_del_queue_tag_set(q);
+	/* Checks hctx->flags & BLK_MQ_F_TAG_QUEUE_SHARED. */
 	blk_mq_exit_hw_queues(q, set, set->nr_hw_queues);
+	/* May clear BLK_MQ_F_TAG_QUEUE_SHARED in hctx->flags. */
+	blk_mq_del_queue_tag_set(q);
 }
 
 static int __blk_mq_alloc_rq_maps(struct blk_mq_tag_set *set)