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

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Message-ID: <2cfb6701-6e58-3e13-be85-472437a5b2ed@huawei.com>
Date:   Wed, 8 Sep 2021 16:29:57 +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 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().

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().

> 
> BTW, the approach in patch 3 looks fine, which is very similar with
> SCSI's handling.

Thanks for taking time reviewing these patches.
Kuai
> 
> Thanks,
> Ming
> 
> .
>