linux-kernel - Re: [PATCH] block: remove redundant explicit memory barrier from rq

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Message-ID: <c297cba9-5136-46b6-b2a4-5169a1a3f7cf@linux.dev>
Date: Tue, 22 Oct 2024 15:53:08 +0800
From: Chengming Zhou <chengming.zhou@...ux.dev>
To: Muchun Song <songmuchun@...edance.com>, axboe@...nel.dk
Cc: josef@...icpanda.com, oleg@...hat.com, linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org, muchun.song@...ux.dev
Subject: Re: [PATCH] block: remove redundant explicit memory barrier from
 rq_qos waiter and waker

On 2024/10/21 16:52, Muchun Song wrote:
> The memory barriers in list_del_init_careful() and list_empty_careful()
> in pairs already handle the proper ordering between data.got_token
> and data.wq.entry. So remove the redundant explicit barriers. And also
> change a "break" statement to "return" to avoid redundant calling of
> finish_wait().
> 
> Signed-off-by: Muchun Song <songmuchun@...edance.com>

Good catch! Just a small nit below, feel free to add:

Reviewed-by: Chengming Zhou <chengming.zhou@...ux.dev>

> ---
>   block/blk-rq-qos.c | 4 +---
>   1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/block/blk-rq-qos.c b/block/blk-rq-qos.c
> index dc510f493ba57..9b0aa7dd6779f 100644
> --- a/block/blk-rq-qos.c
> +++ b/block/blk-rq-qos.c
> @@ -218,7 +218,6 @@ static int rq_qos_wake_function(struct wait_queue_entry *curr,
>   		return -1;
>   
>   	data->got_token = true;
> -	smp_wmb();
>   	wake_up_process(data->task);
>   	list_del_init_careful(&curr->entry);
>   	return 1;
> @@ -274,10 +273,9 @@ void rq_qos_wait(struct rq_wait *rqw, void *private_data,
>   			 * which means we now have two. Put our local token
>   			 * and wake anyone else potentially waiting for one.
>   			 */
> -			smp_rmb();
>   			if (data.got_token)
>   				cleanup_cb(rqw, private_data);
> -			break;
> +			return;
>   		}

Would it be better to move this acquire_inflight_cb() above out of
the do-while(1) since we rely on the waker to get inflight counter
for us?

Thanks.

>   		io_schedule();
>   		has_sleeper = true;