linux-kernel - Re: [RFC V4 1/6] blk: prepare to make blk-rq-qos pluggable and modular

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Message-ID: <Yg4L67f96aQ2q5uy@infradead.org>
Date:   Thu, 17 Feb 2022 00:48:43 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Wang Jianchao (Kuaishou)" <jianchao.wan9@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Josef Bacik <jbacik@...com>,
        Tejun Heo <tj@...nel.org>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC V4 1/6] blk: prepare to make blk-rq-qos pluggable and
 modular

>  {
>  	struct request_queue *q = rqos->q;
> -	const char *dir_name = rq_qos_id_to_name(rqos->id);
> +	const char *dir_name;
> +
> +	dir_name = rqos->ops->name ? rqos->ops->name : rq_qos_id_to_name(rqos->id);

Overly long line here.  And it would be much more readable if you used
a good old if/else.

> +static DEFINE_IDA(rq_qos_ida);
> +static int nr_rqos_blkcg_pols;
> +static DEFINE_MUTEX(rq_qos_mutex);
> +static LIST_HEAD(rq_qos_list);

Please use an allocating xarray instead of an IDA plus list.

> +	/*
> +	 * queue must have been unregistered here, it is safe to iterate
> +	 * the list w/o lock
> +	 */

Please capitalize multi-line comments.

> + * After the pluggable blk-qos, rqos's life cycle become complicated,
> + * as we may modify the rqos list there. Except for the places where
> + * queue is not registered, there are following places may access rqos
> + * list concurrently:

Code comments are not the place to explain history.  PLease explain the
current situation.

> +struct rq_qos *rq_qos_get(struct request_queue *q, int id)
> +{
> +	struct rq_qos *rqos;
> +
> +	spin_lock_irq(&q->queue_lock);

Please don't use the grab all queue_lock for new code.  It badly needs
to be split and documented, and new code is the best place to start
that.

Also with all the new code please add a new config option that is
selected by all rq-pos implementations so that blk-rq-qos.c only gets
built when actually needed.

> +static inline struct rq_qos *rq_qos_by_id(struct request_queue *q, int id)
> +{
> +	struct rq_qos *rqos;
> +
> +	WARN_ON(!mutex_is_locked(&q->sysfs_lock) && !spin_is_locked(&q->queue_lock));

Another overly long line.  And in doubt split this into two helpers
so that you cna use lockdep_assert_held instead of doing the incorrect
asserts.