linux-kernel - Re: [PATCH 1/2] block: export __blk_complete

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Date:   Fri, 15 Jun 2018 12:03:12 +0800
From:   Ming Lei <tom.leiming@...il.com>
To:     "jianchao.wang" <jianchao.w.wang@...cle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Christoph Hellwig <hch@....de>,
        James Bottomley <jejb@...ux.vnet.ibm.com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        linux-block <linux-block@...r.kernel.org>,
        Linux SCSI List <linux-scsi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] block: export __blk_complete_request

On Fri, Jun 15, 2018 at 11:26 AM, jianchao.wang
<jianchao.w.wang@...cle.com> wrote:
> Hi Ming
>
> On 06/15/2018 11:20 AM, Ming Lei wrote:
>> On Fri, Jun 15, 2018 at 11:04 AM, jianchao.wang
>> <jianchao.w.wang@...cle.com> wrote:
>>> Hi Ming
>>>
>>> Thanks for your kindly response.
>>>
>>> On 06/15/2018 10:56 AM, Ming Lei wrote:
>>>>>> IMO, ref-counter is just to fix the blk-mq req life recycle issue.
>>>>>> It cannot replace the blk_mark_rq_complete which could avoid the race between
>>>>>> timeout and io completion path.
>>>>> The .timeout return BLK_EH_DONE doesn't always mean the request has been completed.
>>>>> Such as scsi-mid layer, its .timeout callback return BLK_EH_DONE but the timed out
>>>>> request is still in abort or eh process. What if a completion irq come during that ?
>>>> For blk-mq, it is avoided by the atomic state change in
>>>> __blk_mq_complete_request(),
>>>> that is why I mentioned the question in my last reply.
>>>>
>>>
>>> but blk_mq_check_expired doesn't do that.
>>> do I miss anything ?
>>
>> Right, that is the difference between blk-mq and legacy now,
>
> Sorry, I cannot follow your point.
> blk_mq_check_expired doesn't do a atmoc state change from IN-FLIGHT to COMPLETE.
> __blk_mq_complete_request could still proceed to complete a timed out request
> which is in scsi abort or eh process. Is it really OK ?

That is the idea of Christoph's patchset of 'complete requests from ->timeout',
then drivers need to cover race between timeout and normal completeion.

But at least the request won't be completed twice because of the atomic
state change in __blk_mq_complete_request().

So what is your real concern about blk-mq's timeout?

Thanks,
Ming