2022-03-28 09:35:50

by Jakob Koschel

Subject: [PATCH] block: use dedicated list iterator variable

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 <[email protected]>
---
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;
}


base-commit: b47d5a4f6b8d42f8a8fbe891b36215e4fddc53be
--
2.25.1

2022-03-28 13:29:39

by Bart Van Assche

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 <[email protected]>
> ---
> 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);