linux-kernel - Re: [PATCH v4 2/6] nbd: convert to use blk_mq_find_and_get

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Date:   Wed, 8 Sep 2021 19:03:25 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     Ming Lei <ming.lei@...hat.com>
CC:     <axboe@...nel.dk>, <josef@...icpanda.com>,
        <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <nbd@...er.debian.org>, <yi.zhang@...wei.com>
Subject: Re: [PATCH v4 2/6] nbd: convert to use blk_mq_find_and_get_req()

On 2021/09/08 17:27, Ming Lei wrote:
> On Wed, Sep 08, 2021 at 04:29:57PM +0800, yukuai (C) wrote:
>> On 2021/09/08 16:00, Ming Lei wrote:
>>> On Wed, Sep 08, 2021 at 03:37:06PM +0800, yukuai (C) wrote:
>>>> On 2021/09/08 15:30, Ming Lei wrote:
>>>>
>>>>>> +put_req:
>>>>>> +	if (req)
>>>>>> +		blk_mq_put_rq_ref(req);
>>>>>>     	return ret ? ERR_PTR(ret) : cmd;
>>>>>
>>>>> After the request's refcnt is dropped, it can be freed immediately, then
>>>>> one stale command is returned to caller.
>>>>
>>>> Hi, Ming
>>>>
>>>> It's right this patch leave this problem. Please take a look at patch 3
>>>> and patch 4, the problem should be fixed with these patches.
>>>
>>> Not see it is addressed in patch 3 & 4, and it is one obvious fault in
>>> patch 2, please fix it from beginning by moving the refcnt drop
>>> into recv_work().
>>
>> Hi, Ming
>>
>> With patch 3 & 4:
>>
>> if nbd_read_stat() return a valid cmd, then the refcnt should not drop
>> to 0 before blk_mq_complete_request() in recv_work().
> 
> The valid cmd won't be timed out just between nbd_read_stat() and
> calling blk_mq_complete_request()?
> 
> Yeah, the issue is addressed by patch 4, then please put 2 after
> 3 & 4, and suggest to add comment why request ref won't drop to zero
> in recv_work().

Hi, Ming

Thanks for the advice, will do this in next iteration.

Best regards
Kuai
> 
>>
>> if nbd_read_stat() failed, it won't be a problem if the request is freed
>> immediately when refcnt is dropped in nbd_read_stat().
>>
>> That's why I said that the problem will be fixed.
>>
>> BTW, if we move the refcnt drop into recv_work, we can only do that if
>> nbd_read_stat() return a valid cmd. If we get a valid rq and failed in
>> the following checkings in nbd_read_stat(), it's better to drop the
>> refcnt in nbd_read_stat().
> 
> The usual pattern is to drop the refcnt on when the object isn't used
> any more, so it is perfectly fine to hold the ref until that time.
> 
> 
> Thanks,
> Ming
> 
> .
>