linux-kernel - Re: [PATCH 1/8] blk-mq: get rid of the synchronize_rcu in __blk_mq_update_nr_hw

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Date:   Sun, 17 Mar 2019 14:14:06 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     Jianchao Wang <jianchao.w.wang@...cle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        Hannes Reinecke <hare@...e.de>,
        Josef Bacik <josef@...icpanda.com>,
        Bart Van Assche <bvanassche@....org>,
        Sagi Grimberg <sagi@...mberg.me>,
        Keith Busch <keith.busch@...el.com>, jsmart2021@...il.com,
        linux-block <linux-block@...r.kernel.org>,
        linux-nvme <linux-nvme@...ts.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/8] blk-mq: get rid of the synchronize_rcu in __blk_mq_update_nr_hw_queues

On Fri, Mar 15, 2019 at 5:07 PM Jianchao Wang
<jianchao.w.wang@...cle.com> wrote:
>
> In commit 530ca2c (blk-mq: Allow blocking queue tag iter callbacks),
> we try to get a non-zero q_usage_counter to avoid access hctxs that
> being modified. So the synchronize_rcu is useless and should be
> removed.
>
> Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
> ---
>  block/blk-mq-tag.c | 4 +---
>  block/blk-mq.c     | 4 ----
>  2 files changed, 1 insertion(+), 7 deletions(-)
>
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 2089c6c..b792537 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -384,9 +384,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
>         /*
>          * __blk_mq_update_nr_hw_queues() updates nr_hw_queues and queue_hw_ctx
>          * while the queue is frozen. So we can use q_usage_counter to avoid
> -        * racing with it. __blk_mq_update_nr_hw_queues() uses
> -        * synchronize_rcu() to ensure this function left the critical section
> -        * below.
> +        * racing with it.
>          */
>         if (!percpu_ref_tryget(&q->q_usage_counter))
>                 return;
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 9437a5e..07584a9 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -3208,10 +3208,6 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set,
>         list_for_each_entry(q, &set->tag_list, tag_set_list)
>                 blk_mq_freeze_queue(q);
>         /*
> -        * Sync with blk_mq_queue_tag_busy_iter.
> -        */
> -       synchronize_rcu();
> -       /*
>          * Switch IO scheduler to 'none', cleaning up the data associated
>          * with the previous scheduler. We will switch back once we are done
>          * updating the new sw to hw queue mappings.
> --
> 2.7.4
>

Reviewed-by: Ming Lei <ming.lei@...hat.com>

Thanks,
Ming Lei