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

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Date:   Fri, 15 Jun 2018 13:10:00 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Ming Lei <tom.leiming@...il.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

Hi Ming

Thanks for your kindly response

On 06/15/2018 12:03 PM, Ming Lei wrote:
> 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',

Yes, I used to read that mail thread.

> 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().

Yes

> 
> So what is your real concern about blk-mq's timeout?
I concern whether the current drivers have bee ready for taking this task currently.
At least, for scsi, if I try to trigger timeout and completion path concurrently, system would crash.
4.17.rc7 or 4.18 with a patch that change state in blk_mq_check_expired will survive.

Thanks
jianchao 

> 
> Thanks,
> Ming
>