linux-kernel - Re: [PATCH V3 5/7] ublk_drv: consider recovery feature in aborting mechanism

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Message-ID: <5af80188-c904-635a-242e-4bb1cd7f2e01@linux.alibaba.com>
Date:   Tue, 20 Sep 2022 12:39:31 +0800
From:   Ziyang Zhang <ZiyangZhang@...ux.alibaba.com>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     axboe@...nel.dk, xiaoguang.wang@...ux.alibaba.com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        joseph.qi@...ux.alibaba.com
Subject: Re: [PATCH V3 5/7] ublk_drv: consider recovery feature in aborting
 mechanism

On 2022/9/20 12:01, Ming Lei wrote:
> On Tue, Sep 20, 2022 at 11:24:12AM +0800, Ziyang Zhang wrote:
>> On 2022/9/20 11:04, Ming Lei wrote:
>>> On Tue, Sep 20, 2022 at 09:49:33AM +0800, Ziyang Zhang wrote:
>>>
>>> Follows the delta patch against patch 5 for showing the idea:
>>>
>>>
>>> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
>>> index 4409a130d0b6..60c5786c4711 100644
>>> --- a/drivers/block/ublk_drv.c
>>> +++ b/drivers/block/ublk_drv.c
>>> @@ -656,7 +656,8 @@ static void ublk_complete_rq(struct request *req)
>>>   * Also aborting may not be started yet, keep in mind that one failed
>>>   * request may be issued by block layer again.
>>>   */
>>> -static void __ublk_fail_req(struct ublk_io *io, struct request *req)
>>> +static void __ublk_fail_req(struct ublk_queue *ubq, struct ublk_io *io,
>>> +		struct request *req)
>>>  {
>>>  	WARN_ON_ONCE(io->flags & UBLK_IO_FLAG_ACTIVE);
>>>  
>>> @@ -667,7 +668,10 @@ static void __ublk_fail_req(struct ublk_io *io, struct request *req)
>>>  				req->tag,
>>>  				io->flags);
>>>  		io->flags |= UBLK_IO_FLAG_ABORTED;
>>> -		blk_mq_end_request(req, BLK_STS_IOERR);
>>> +		if (ublk_queue_can_use_recovery_reissue(ubq))
>>> +			blk_mq_requeue_request(req, false);
>>
>> Here is one problem:
>> We reset io->flags to 0 in ublk_queue_reinit() and it is called before new
> 
> As we agreed, ublk_queue_reinit() will be moved to ublk_ch_release(), when there isn't
> any inflight request, which is completed by either ublk server or __ublk_fail_req().
> 
> So clearing io->flags isn't related with quisceing device.
> 
>> ubq_daemon with FETCH_REQ is accepted. ublk_abort_queue() is not protected with
>> ub_mutex and it is called many times in monitor_work. So same rq may be requeued
>> multiple times.
> 
> UBLK_IO_FLAG_ABORTED is set for the slot, so one req is only ended or
> requeued just once.

Yes, we can move ublk_queue_reinit() into ublk_ch_release(), but monitor_work is scheduled
periodically so ublk_abort_queue() is called multiple times. As ublk_queue_reinit() clear
io->flags, ublk_abort_queue() can requeue the same rq twice. Note that monitor_work can be
scheduled after ublk_ch_release().
 
> 
>>
>> With recovery disabled, there is no such problem since io->flags does not change
>> until ublk_dev is released.
> 
> But we have agreed that ublk_queue_reinit() can be moved to release
> handler of /dev/ublkcN.
> 
>>
>> In my patch 5 I only requeue the same rq once. So re-using ublk_abort_queue() is
>> hard for recovery feature.
> 
> No, the same rq is just requeued once. Here the point is:
> 
> 1) reuse previous pattern in ublk_stop_dev(), which is proved as
> workable reliably
> 
> 2) avoid to stay in half-working state forever
> 
> 3) the behind idea is more simpler.

Ming, your patch requeue rqs with ACTVE unset. these rqs have been issued to the
dying ubq_daemon. What I concern about is inflight rqs with ACTIVE set.

Regards,
Zhang.