Re: [RFC PATCH v2 2/4] blk-cgroup: use a request_queue rhashtable for blkg lookup

[Christoph Hellwig <hch@lst.de>]

On Tue, Aug 11, 2026 at 02:47:42PM +0800, Yu Kuai wrote:
> Keep q->blkg_list for ordered policy and scheduler walks. Initialize and
> destroy the hash with request_queue, and remove the radix-tree preload
> paths which are no longer needed.

Are these fast path operations?  Otherwise we can walk all rhashtable
entries without an extra list, but it might be slower.

> @@ -191,10 +198,15 @@ static void blkg_release(struct percpu_ref *ref)
>  {
>  	struct blkcg_gq *blkg = container_of(ref, struct blkcg_gq, refcnt);
>  	struct blkcg *blkcg = blkg->blkcg;
>  	int cpu;
>  
> +	if (!list_empty(&blkg->q_node))
> +		WARN_ON_ONCE(rhashtable_remove_fast(&blkg->q->blkg_hash,
> +						    &blkg->q_hash_node,
> +						    blkg_hash_params));
> +

The list_empty case is for initialization failure?  Or can we end up
with that by other means?

> + * Lookup a blkg for the @blkcg - @q pair, whether it is online or dying.
> + *
> + * Must be called in a RCU critical section.
> + */

Please add must_hold and/or lockdep annotations for this instead of just
a comment.

Also maybe mention that this does not acquire a reference and the caller
must already hold one?