linux-kernel - Re: [PATCH] block: Use non _rcu version of list functions for tag_set

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Date:   Tue, 28 Jul 2020 16:13:13 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     Daniel Wagner <dwagner@...e.de>, Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ming Lei <ming.lei@...hat.com>
Subject: Re: [PATCH] block: Use non _rcu version of list functions for
 tag_set_list

On 7/28/20 3:29 PM, Daniel Wagner wrote:
> tag_set_list is only accessed under the tag_set_lock lock. There is
> no need for using the _rcu list functions.
> 
> The _rcu list function were introduced to allow read access to the
> tag_set_list protected under RCU, see 705cda97ee3a ("blk-mq: Make it
> safe to use RCU to iterate over blk_mq_tag_set.tag_list") and
> 05b79413946d ("Revert "blk-mq: don't handle TAG_SHARED in restart"").
> Those changes got reverted later but the cleanup commit missed a
> couple of places to undo the changes.
> 
> Fixes: 97889f9ac24f ("blk-mq: remove synchronize_rcu() from blk_mq_del_queue_tag_set()"
> Cc: Ming Lei <ming.lei@...hat.com>
> Signed-off-by: Daniel Wagner <dwagner@...e.de>
> ---
>   block/blk-mq.c | 4 ++--
>   1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index e32cb0217135..14ee7506f32f 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2792,7 +2792,7 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>   	struct blk_mq_tag_set *set = q->tag_set;
>   
>   	mutex_lock(&set->tag_list_lock);
> -	list_del_rcu(&q->tag_set_list);
> +	list_del(&q->tag_set_list);
>   	if (list_is_singular(&set->tag_list)) {
>   		/* just transitioned to unshared */
>   		set->flags &= ~BLK_MQ_F_TAG_SHARED;
> @@ -2819,7 +2819,7 @@ static void blk_mq_add_queue_tag_set(struct blk_mq_tag_set *set,
>   	}
>   	if (set->flags & BLK_MQ_F_TAG_SHARED)
>   		queue_set_hctx_shared(q, true);
> -	list_add_tail_rcu(&q->tag_set_list, &set->tag_list);
> +	list_add_tail(&q->tag_set_list, &set->tag_list);
>   
>   	mutex_unlock(&set->tag_list_lock);
>   }
> 
Indeed.

Reviewed-by: Hannes Reinecke <hare@...e.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@...e.de                               +49 911 74053 688
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