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: Mon, 19 Sep 2022 17:12:21 +0800	[thread overview]
Message-ID: <ff61718d-da2d-f754-5e56-b58a3e57820f@linux.alibaba.com> (raw)
In-Reply-To: <YyfoQuw18kOynxcC@T590>

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.

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.

> 
>> +	} else {
>> +		blk_mq_end_request(rq, BLK_STS_IOERR);
>> +	}
>> +}
>> +
>>  static inline void __ublk_rq_task_work(struct request *req)
>>  {
>>  	struct ublk_queue *ubq = req->mq_hctx->driver_data;
>> @@ -704,7 +719,7 @@ static inline void __ublk_rq_task_work(struct request *req)
>>  	 * (2) current->flags & PF_EXITING.
>>  	 */
>>  	if (unlikely(current != ubq->ubq_daemon || current->flags & PF_EXITING)) {
>> -		blk_mq_end_request(req, BLK_STS_IOERR);
>> +		__ublk_abort_rq_in_task_work(ubq, req);
>>  		mod_delayed_work(system_wq, &ub->monitor_work, 0);
>>  		return;
>>  	}
>> @@ -779,6 +794,21 @@ static void ublk_rq_task_work_fn(struct callback_head *work)
>>  	__ublk_rq_task_work(req);
>>  }
>>  
>> +static inline blk_status_t __ublk_abort_rq(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);
> 
> Same with above.
> 
> 
> Thanks,
> Ming