NVMe: Memory barrier before queue_count is incremented

Protects against reordering and/or preempting which would allow the
kthread to access the queue descriptor before it is set up
Signed-off-by: Jon Derrick <jonathan.derrick@intel.com>
Acked-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/block/nvme-core.c

@@ -1453,9 +1453,12 @@ static struct nvme_queue *nvme_alloc_queue(struct nvme_dev *dev, int qid,
 	nvmeq->q_db = &dev->dbs[qid * 2 * dev->db_stride];
 	nvmeq->q_depth = depth;
 	nvmeq->qid = qid;
-	dev->queue_count++;
 	dev->queues[qid] = nvmeq;
 
+	/* make sure queue descriptor is set before queue count, for kthread */
+	mb();
+	dev->queue_count++;
+
 	return nvmeq;
 
  free_cqdma: