linux-kernel - Re: [PATCH V3 4/7] ublk_drv: requeue rqs with recovery feature enabled

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Date:   Tue, 20 Sep 2022 11:34:32 +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 4/7] ublk_drv: requeue rqs with recovery feature
 enabled

On 2022/9/20 11:18, Ming Lei wrote:
> On Tue, Sep 20, 2022 at 11:04:30AM +0800, Ziyang Zhang wrote:
>> On 2022/9/20 10:39, Ming Lei wrote:
>>> On Tue, Sep 20, 2022 at 09:31:54AM +0800, Ziyang Zhang wrote:
>>>> On 2022/9/19 20:39, Ming Lei wrote:
>>>>> On Mon, Sep 19, 2022 at 05:12:21PM +0800, Ziyang Zhang wrote:
>>>>>> On 2022/9/19 11:55, Ming Lei wrote:
>>>>>>> On Tue, Sep 13, 2022 at 12:17:04PM +0800, ZiyangZhang wrote:
>>>>>>>> With recovery feature enabled, in ublk_queue_rq or task work
>>>>>>>> (in exit_task_work or fallback wq), we requeue rqs instead of
>>>>>>>> ending(aborting) them. Besides, No matter recovery feature is enabled
>>>>>>>> or disabled, we schedule monitor_work immediately.
>>>>>>>>
>>>>>>>> Signed-off-by: ZiyangZhang <ZiyangZhang@...ux.alibaba.com>
>>>>>>>> ---
>>>>>>>>  drivers/block/ublk_drv.c | 34 ++++++++++++++++++++++++++++++++--
>>>>>>>>  1 file changed, 32 insertions(+), 2 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
>>>>>>>> index 23337bd7c105..b067f33a1913 100644
>>>>>>>> --- a/drivers/block/ublk_drv.c
>>>>>>>> +++ b/drivers/block/ublk_drv.c
>>>>>>>> @@ -682,6 +682,21 @@ static void ubq_complete_io_cmd(struct ublk_io *io, int res)
>>>>>>>>  
>>>>>>>>  #define UBLK_REQUEUE_DELAY_MS	3
>>>>>>>>  
>>>>>>>> +static inline void __ublk_abort_rq_in_task_work(struct ublk_queue *ubq,
>>>>>>>> +		struct request *rq)
>>>>>>>> +{
>>>>>>>> +	pr_devel("%s: %s q_id %d tag %d io_flags %x.\n", __func__,
>>>>>>>> +			(ublk_queue_can_use_recovery(ubq)) ? "requeue" : "abort",
>>>>>>>> +			ubq->q_id, rq->tag, ubq->ios[rq->tag].flags);
>>>>>>>> +	/* We cannot process this rq so just requeue it. */
>>>>>>>> +	if (ublk_queue_can_use_recovery(ubq)) {
>>>>>>>> +		blk_mq_requeue_request(rq, false);
>>>>>>>> +		blk_mq_delay_kick_requeue_list(rq->q, UBLK_REQUEUE_DELAY_MS);
>>>>>>>
>>>>>>> Here you needn't to kick requeue list since we know it can't make
>>>>>>> progress. And you can do that once before deleting gendisk
>>>>>>> or the queue is recovered.
>>>>>>
>>>>>> No, kicking rq here is necessary.
>>>>>>
>>>>>> Consider USER_RECOVERY is enabled and everything goes well.
>>>>>> User sends STOP_DEV, and we have kicked requeue list in
>>>>>> ublk_stop_dev() and are going to call del_gendisk().
>>>>>> However, a crash happens now. Then rqs may be still requeued
>>>>>> by ublk_queue_rq() because ublk_queue_rq() sees a dying
>>>>>> ubq_daemon. So del_gendisk() will hang because there are
>>>>>> rqs leaving in requeue list and no one kicks them.
>>>>>
>>>>> Why can't you kick requeue list before calling del_gendisk().
>>>>
>>>> Yes, we can kick requeue list once before calling del_gendisk().
>>>> But a crash may happen just after kicking but before del_gendisk().
>>>> So some rqs may be requeued at this moment. But we have already
>>>> kicked the requeue list! Then del_gendisk() will hang, right?
>>>
>>> ->force_abort is set before kicking in ublk_unquiesce_dev(), so
>>> all new requests are failed immediately instead of being requeued,
>>> right?
>>>
>>
>> ->force_abort is not heplful here because there may be fallback wq running
>> which can requeue rqs after kicking requeue list.
> 
> After ublk_wait_tagset_rqs_idle() returns, there can't be any
> pending requests in fallback wq or task work, can there
Please consider this case: a crash happens while ublk_stop_dev() is
calling. In such case I cannot schedule quiesce_work or call
ublk_wait_tagset_rqs_idle(). This is because quiesce_work has to
accquire ub_mutex to quiesce request queue.

> 
> Please look at the 2nd point of the comment log, and I did ask you
> to add the words for fallback wq and task work.
> 
>>
>> In ublk_unquiesce_dev(), I simply disable recovery feature
>> if ub's state is UBLK_S_DEV_LIVE while stopping ublk_dev.
> 
> monitor work will provide forward progress in case of not applying
> user recovery.

Yes, that's why I disable recovery feature in ublk_stop_dev if quiesce_work
has not run. In this case nonitor_work can abort rqs.

> 
>>
>> Note: We can make sure fallback wq does not run if we wait until all rqs with
>> ACTIVE flag set are requeued. This is done in quiesce_work(). But it cannot
>> run while ublk_stop_dev() is running...
>  
> Please take a look at the delta patch I just sent, which is supposed to be
> simpler for addressing this corner case.

I saw your patch, it is for rqs with ACTIVE unset, but I am concerning on rqs
with ACTIVE set.

Regards,
Zhang.