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

From: Ziyang Zhang <ZiyangZhang@linux.alibaba.com>
To: Ming Lei <ming.lei@redhat.com>
Cc: axboe@kernel.dk, xiaoguang.wang@linux.alibaba.com,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
	joseph.qi@linux.alibaba.com
Subject: Re: [PATCH V3 4/7] ublk_drv: requeue rqs with recovery feature enabled
Date: Tue, 20 Sep 2022 09:31:54 +0800	[thread overview]
Message-ID: <84b99294-6859-f49f-d529-c6e3899f2aa2@linux.alibaba.com> (raw)
In-Reply-To: <Yyhi/kavaq1aLAQY@T590>

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@linux.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?

> 
>>
>> BTW, kicking requeue list after requeue rqs is really harmless
>> since we schedule quiesce_work immediately after finding a
>> dying ubq_daemon. So few rqs have chance to be re-dispatched.
> 
> Do you think it makes sense to kick requeue list when the queue
> can't handle any request?

I know it does not make sense while ubq_daemon is dying, but it is good
for handling the situation I discribed before.

Regards,
Zhang.