linux-kernel - Re: [PATCH v2 1/4] md/raid10: cleanup wait

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Date:   Thu, 15 Sep 2022 15:21:18 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Logan Gunthorpe <logang@...tatee.com>,
        Yu Kuai <yukuai1@...weicloud.com>, song@...nel.org,
        guoqing.jiang@...ux.dev, pmenzel@...gen.mpg.de,
        Paul Menzel <pmenzel@...gen.mpg.de>
Cc:     linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org,
        yi.zhang@...wei.com, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH v2 1/4] md/raid10: cleanup wait_barrier()

Hi,

在 2022/09/15 0:16, Logan Gunthorpe 写道:
> 
> 
> On 2022-09-13 19:49, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@...wei.com>
>>
>> Currently the nasty condition is wait_barrier() is hard to read. This
>> patch factor out the condition into a function.
>>
>> There are no functional changes.
>>
>> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
>> ---
>>   drivers/md/raid10.c | 56 ++++++++++++++++++++++++++-------------------
>>   1 file changed, 32 insertions(+), 24 deletions(-)
>>
>> diff --git a/drivers/md/raid10.c b/drivers/md/raid10.c
>> index 64d6e4cd8a3a..56458a53043d 100644
>> --- a/drivers/md/raid10.c
>> +++ b/drivers/md/raid10.c
>> @@ -957,44 +957,52 @@ static void lower_barrier(struct r10conf *conf)
>>   	wake_up(&conf->wait_barrier);
>>   }
>>   
>> +static bool stop_waiting_barrier(struct r10conf *conf)
>> +{
>> +	/* barrier is dropped */
>> +	if (!conf->barrier)
>> +		return true;
>> +
>> +	/*
>> +	 * If there are already pending requests (preventing the barrier from
>> +	 * rising completely), and the pre-process bio queue isn't empty, then
>> +	 * don't wait, as we need to empty that queue to get the nr_pending
>> +	 * count down.
>> +	 */
>> +	if (atomic_read(&conf->nr_pending)) {
>> +		struct bio_list *bio_list = current->bio_list;
> 
> I'd probably just put the bio_list declaration at the top of this
> function, then the nested if statements are not necessary. The compiler
> should be able to optimize the access just fine.
> 
>>   	if (conf->barrier) {
>> -		struct bio_list *bio_list = current->bio_list;
>> -		conf->nr_waiting++;
>> -		/* Wait for the barrier to drop.
>> -		 * However if there are already pending
>> -		 * requests (preventing the barrier from
>> -		 * rising completely), and the
>> -		 * pre-process bio queue isn't empty,
>> -		 * then don't wait, as we need to empty
>> -		 * that queue to get the nr_pending
>> -		 * count down.
>> -		 */
>>   		/* Return false when nowait flag is set */
>>   		if (nowait) {
>>   			ret = false;
>>   		} else {
>> +			conf->nr_waiting++;
> 
> Technically speaking, I think moving nr_waiting counts as a functional
> change. As best as I can see it is correct, but it should probably be at
> least mentioned in the commit message, or maybe done as a separate
> commit with it's own justification. That way if it causes problems down
> the road, a bisect will make the issue clearer.

Thanks for your advice, I just think increase and decrease nr_waiting in
the case 'nowait' is pointless, and I move it incidentally.

I'll post a separate clean up patch to do that.

Paul, can I still add your Acked-by for this patch?

Thanks,
Kuai
> 
> Thanks,
> 
> Logan
> .
>