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

JIRA: https://issues.redhat.com/browse/RHEL-106845

commit 9dc7a88
Author: Ming Lei <[email protected]>
Date:   Mon May 5 22:18:00 2025 +0800

    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 <[email protected]>
    Reviewed-by: Nilay Shroff <[email protected]>
    Signed-off-by: Ming Lei <[email protected]>
    Link: https://lore.kernel.org/r/[email protected]
    Signed-off-by: Jens Axboe <[email protected]>

Signed-off-by: Ming Lei <[email protected]>

Author: Ming Lei

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 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
 		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. */