Re: [PATCH 1/3] blk-ioc: add a new helper ioc_lookup_icq_rcu()

[Damien Le Moal <dlemoal@kernel.org>]

On 7/25/25 16:05, Yu Kuai wrote:
> From: Yu Kuai <yukuai3@huawei.com>
> 
> ioc_lookup_icq() is used by bfq to lookup bfqq from IO path, the helper
> have to be protected by queue_lock, which is too heavy. Hence add a new
> helper that is lookless, this is safe because both request_queue and ioc
> can be pinged by IO that is still issuing.
> 
> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> ---
>  block/blk-ioc.c | 34 ++++++++++++++++++++++++++++++++++
>  block/blk.h     |  1 +
>  2 files changed, 35 insertions(+)
> 
> diff --git a/block/blk-ioc.c b/block/blk-ioc.c
> index ce82770c72ab..4945b48dfdb6 100644
> --- a/block/blk-ioc.c
> +++ b/block/blk-ioc.c
> @@ -343,6 +343,40 @@ struct io_cq *ioc_lookup_icq(struct request_queue *q)
>  }
>  EXPORT_SYMBOL(ioc_lookup_icq);
>  
> +/**
> + * ioc_lookup_icq_rcu - lookup io_cq from ioc in io path
> + * @q: the associated request_queue
> + *
> + * Look up io_cq associated with @ioc - @q pair from @ioc. Must be called from
> + * io issue path, either return NULL if current issue io to @q for the first
> + * time, or return a valid icq.
> + */
> +struct io_cq *ioc_lookup_icq_rcu(struct request_queue *q)
> +{
> +	struct io_context *ioc = current->io_context;
> +	struct io_cq *icq;
> +
> +	WARN_ON_ONCE(percpu_ref_is_zero(&q->q_usage_counter));

I do not think this is necessary.

> +
> +	if (!ioc)
> +		return NULL;
> +
> +	icq = rcu_dereference(ioc->icq_hint);
> +	if (icq && icq->q == q)
> +		return icq;
> +
> +	icq = radix_tree_lookup(&ioc->icq_tree, q->id);
> +	if (!icq)
> +		return NULL;
> +
> +	if (WARN_ON_ONCE(icq->q != q))
> +		return NULL;
> +
> +	rcu_assign_pointer(ioc->icq_hint, icq);
> +	return icq;
> +}
> +EXPORT_SYMBOL(ioc_lookup_icq_rcu);

Patch 2 calls this function with the rcu_read_lock() held. Why not move that rcu
read lock here inside this function ? That is how ioc_lookup_icq() was doing
things, with code that is more compact than this.

And since ioc_lookup_icq() was already using RCU, it seems that the only change
you need is to remove the "lockdep_assert_held(&q->queue_lock);" from that
function to endup with the same above functionality. So why all the churn ?

Another question is: is it safe to call radix_tree_lookup() without any lock
held ? What if this races with a radix tree insertion ? (I may be wrong here as
I am not familiar with that code).

> +
>  /**
>   * ioc_create_icq - create and link io_cq
>   * @q: request_queue of interest
> diff --git a/block/blk.h b/block/blk.h
> index 468aa83c5a22..ef31b3ec1c69 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -461,6 +461,7 @@ static inline void req_set_nomerge(struct request_queue *q, struct request *req)
>   */
>  struct io_cq *ioc_find_get_icq(struct request_queue *q);
>  struct io_cq *ioc_lookup_icq(struct request_queue *q);
> +struct io_cq *ioc_lookup_icq_rcu(struct request_queue *q);
>  #ifdef CONFIG_BLK_ICQ
>  void ioc_clear_queue(struct request_queue *q);
>  #else


-- 
Damien Le Moal
Western Digital Research