linux-kernel - Re: [PATCH] block: use dedicated list iterator variable

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Message-ID: <49f08642-8878-8574-ce86-d1baf41f1189@acm.org>
Date:   Sun, 27 Mar 2022 19:02:46 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Jakob Koschel <jakobkoschel@...il.com>,
        Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        Mike Rapoport <rppt@...nel.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: Re: [PATCH] block: use dedicated list iterator variable

On 3/27/22 14:47, Jakob Koschel wrote:
> To move the list iterator variable into the list_for_each_entry_*()
> macro in the future it should be avoided to use the list iterator
> variable after the loop body.
> 
> To *never* use the list iterator variable after the loop it was
> concluded to use a separate iterator variable instead of a
> found boolean [1].
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/
> Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
> ---
>   block/blk-mq.c | 10 ++++++----
>   1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 8e659dc5fcf3..455fdd488f3c 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -4466,12 +4466,14 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
>   static void blk_mq_elv_switch_back(struct list_head *head,
>   		struct request_queue *q)
>   {
> -	struct blk_mq_qe_pair *qe;
> +	struct blk_mq_qe_pair *qe = NULL;
> +	struct blk_mq_qe_pair *iter;
>   	struct elevator_type *t = NULL;
>   
> -	list_for_each_entry(qe, head, node)
> -		if (qe->q == q) {
> -			t = qe->type;
> +	list_for_each_entry(iter, head, node)
> +		if (iter->q == q) {
> +			t = iter->type;
> +			qe = iter;
>   			break;
>   		}

The current code looks much more readable to me than with the above
patch applied. How about replacing the above patch with the patch below?

Thanks,

Bart.


diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9c784262fd6b..c9435be54667 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -4451,21 +4451,28 @@ static bool blk_mq_elv_switch_none(struct list_head *head,
  	return true;
  }

-static void blk_mq_elv_switch_back(struct list_head *head,
-		struct request_queue *q)
+static struct blk_mq_qe_pair *blk_lookup_qe_pair(struct list_head *head,
+						 struct request_queue *q)
  {
  	struct blk_mq_qe_pair *qe;
-	struct elevator_type *t = NULL;

  	list_for_each_entry(qe, head, node)
-		if (qe->q == q) {
-			t = qe->type;
-			break;
-		}
+		if (qe->q == q)
+			return qe;
+
+	return NULL;
+}
+
+static void blk_mq_elv_switch_back(struct list_head *head,
+		struct request_queue *q)
+{
+	struct blk_mq_qe_pair *qe = blk_lookup_qe_pair(head, q);
+	struct elevator_type *t;

-	if (!t)
+	if (!qe)
  		return;

+	t = qe->type;
  	list_del(&qe->node);
  	kfree(qe);