block: move hctx debugfs/sysfs registering out of freezing queue

Move hctx debugfs/sysfs register out of freezing queue in
__blk_mq_update_nr_hw_queues(), so that the following lockdep dependency
can be killed:

	#2 (&q->q_usage_counter(io)#16){++++}-{0:0}:
	#1 (fs_reclaim){+.+.}-{0:0}:
	#0 (&sb->s_type->i_mutex_key#3){+.+.}-{4:4}: //debugfs

And registering/un-registering hctx debugfs/sysfs does not require queue to
be frozen:

- hctx sysfs attributes show() are drained when removing kobject, and
  there isn't store() implementation for hctx sysfs attributes

- debugfs entry read() is drained too when removing debugfs directory,
  and there isn't write() implementation for hctx debugfs too

- so it is safe to register/unregister hctx sysfs/debugfs without
  freezing queue because the cod paths changes nothing, and we just
  need to keep hctx live

Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250505141805.2751237-23-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 7 insertions, 8 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 263bbe34d3f0..a4ab7779e020 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -5005,14 +5005,14 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 		return;
 
 	memflags = memalloc_noio_save();
-	list_for_each_entry(q, &set->tag_list, tag_set_list)
-		blk_mq_freeze_queue_nomemsave(q);
-
 	list_for_each_entry(q, &set->tag_list, tag_set_list) {
 		blk_mq_debugfs_unregister_hctxs(q);
 		blk_mq_sysfs_unregister_hctxs(q);
 	}
 
+	list_for_each_entry(q, &set->tag_list, tag_set_list)
+		blk_mq_freeze_queue_nomemsave(q);
+
 	if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0)
 		goto reregister;
 
@@ -5035,16 +5035,15 @@ fallback:
 		blk_mq_map_swqueue(q);
 	}
 
+	/* elv_update_nr_hw_queues() unfreeze queue for us */
+	list_for_each_entry(q, &set->tag_list, tag_set_list)
+		elv_update_nr_hw_queues(q);
+
 reregister:
 	list_for_each_entry(q, &set->tag_list, tag_set_list) {
 		blk_mq_sysfs_register_hctxs(q);
 		blk_mq_debugfs_register_hctxs(q);
 	}
-
-	/* elv_update_nr_hw_queues() unfreeze queue for us */
-	list_for_each_entry(q, &set->tag_list, tag_set_list)
-		elv_update_nr_hw_queues(q);
-
 	memalloc_noio_restore(memflags);
 
 	/* Free the excess tags when nr_hw_queues shrink. */