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

[Ming Lei]

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@xxxxxxxxxxxxxxxxx>
> >>>>>> ---
> >>>>>>  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 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.

> 
> 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.


Thanks,
Ming