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Date:   Tue, 20 Sep 2022 10:39:42 +0800
From:   Ming Lei <ming.lei@redhat.com>
To:     Ziyang Zhang <ZiyangZhang@linux.alibaba.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
Message-ID: <Yykn7q/T9CUzZpxH@T590>
References: <20220913041707.197334-1-ZiyangZhang@linux.alibaba.com>
 <20220913041707.197334-5-ZiyangZhang@linux.alibaba.com>
 <YyfoQuw18kOynxcC@T590>
 <ff61718d-da2d-f754-5e56-b58a3e57820f@linux.alibaba.com>
 <Yyhi/kavaq1aLAQY@T590>
 <84b99294-6859-f49f-d529-c6e3899f2aa2@linux.alibaba.com>
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In-Reply-To: <84b99294-6859-f49f-d529-c6e3899f2aa2@linux.alibaba.com>
Precedence: bulk

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@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?

->force_abort is set before kicking in ublk_unquiesce_dev(), so
all new requests are failed immediately instead of being requeued,
right?


Thanks, 
Ming