block: drain queue after disk is removed from sysfs

Before removing disk from sysfs, userspace still may change queue via
sysfs, such as switching elevator or setting wbt latency, both may
reinitialize wbt, then the warning in blk_free_queue_stats() will be
triggered since rq_qos_exit() is moved to del_gendisk().

Fixes the issue by moving draining queue & tearing down after disk is
removed from sysfs, at that time no one can come into queue's
store()/show().
Reported-by: Yi Zhang <yi.zhang@redhat.com>
Tested-by: Yi Zhang <yi.zhang@redhat.com>
Fixes: 8e141f9e ("block: drain file system I/O on del_gendisk")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20211026101204.2897166-1-ming.lei@redhat.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

block/genhd.c

@@ -588,16 +588,6 @@ void del_gendisk(struct gendisk *disk)
 	 * Prevent new I/O from crossing bio_queue_enter().
 	 */
 	blk_queue_start_drain(q);
-	blk_mq_freeze_queue_wait(q);
-
-	rq_qos_exit(q);
-	blk_sync_queue(q);
-	blk_flush_integrity();
-	/*
-	 * Allow using passthrough request again after the queue is torn down.
-	 */
-	blk_queue_flag_clear(QUEUE_FLAG_INIT_DONE, q);
-	__blk_mq_unfreeze_queue(q, true);
 
 	if (!(disk->flags & GENHD_FL_HIDDEN)) {
 		sysfs_remove_link(&disk_to_dev(disk)->kobj, "bdi");
@@ -620,6 +610,18 @@ void del_gendisk(struct gendisk *disk)
 		sysfs_remove_link(block_depr, dev_name(disk_to_dev(disk)));
 	pm_runtime_set_memalloc_noio(disk_to_dev(disk), false);
 	device_del(disk_to_dev(disk));
+
+	blk_mq_freeze_queue_wait(q);
+
+	rq_qos_exit(q);
+	blk_sync_queue(q);
+	blk_flush_integrity();
+	/*
+	 * Allow using passthrough request again after the queue is torn down.
+	 */
+	blk_queue_flag_clear(QUEUE_FLAG_INIT_DONE, q);
+	__blk_mq_unfreeze_queue(q, true);
+
 }
 EXPORT_SYMBOL(del_gendisk);