nvme/core: Fix race kicking freed request_queue

If a namespace has already been marked dead, we don't want to kick the
request_queue again since we may have just freed it from another thread.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>

drivers/nvme/host/core.c

@@ -2333,9 +2333,9 @@ void nvme_kill_queues(struct nvme_ctrl *ctrl)
 		 * Revalidating a dead namespace sets capacity to 0. This will
 		 * end buffered writers dirtying pages that can't be synced.
 		 */
-		if (ns->disk && !test_and_set_bit(NVME_NS_DEAD, &ns->flags))
-			revalidate_disk(ns->disk);
-
+		if (!ns->disk || test_and_set_bit(NVME_NS_DEAD, &ns->flags))
+			continue;
+		revalidate_disk(ns->disk);
 		blk_set_queue_dying(ns->queue);
 		blk_mq_abort_requeue_list(ns->queue);
 		blk_mq_start_stopped_hw_queues(ns->queue, true);