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

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Message-Id: <A88698A8-4334-4521-BEE9-39910D37C3DF@linux.dev>
Date: Tue, 22 Oct 2024 16:02:11 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Chengming Zhou <chengming.zhou@...ux.dev>
Cc: Muchun Song <songmuchun@...edance.com>,
 axboe@...nel.dk,
 josef@...icpanda.com,
 oleg@...hat.com,
 linux-block@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] block: remove redundant explicit memory barrier from
 rq_qos waiter and waker



> On Oct 22, 2024, at 15:53, Chengming Zhou <chengming.zhou@...ux.dev> wrote:
> 
> 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?

I also noticed about this and I am working on this. Will send a separate
patch for this refactoring later.

Thanks.

> 
> Thanks.
> 
>>   io_schedule();
>>   has_sleeper = true;