Re: [PATCH V3 5/7] ublk_drv: consider recovery feature in aborting mechanism

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 5/7] ublk_drv: consider recovery feature in aborting mechanism
Date: Tue, 20 Sep 2022 12:01:50 +0800	[thread overview]
Message-ID: <Yyk7LnH9lj303DTj@T590> (raw)
In-Reply-To: <64492fad-e14a-c647-b490-cd1f53a475a8@linux.alibaba.com>

On Tue, Sep 20, 2022 at 11:24:12AM +0800, Ziyang Zhang wrote:
> On 2022/9/20 11:04, Ming Lei wrote:
> > On Tue, Sep 20, 2022 at 09:49:33AM +0800, Ziyang Zhang wrote:
> > 
> > Follows the delta patch against patch 5 for showing the idea:
> > 
> > 
> > diff --git a/drivers/block/ublk_drv.c b/drivers/block/ublk_drv.c
> > index 4409a130d0b6..60c5786c4711 100644
> > --- a/drivers/block/ublk_drv.c
> > +++ b/drivers/block/ublk_drv.c
> > @@ -656,7 +656,8 @@ static void ublk_complete_rq(struct request *req)
> >   * Also aborting may not be started yet, keep in mind that one failed
> >   * request may be issued by block layer again.
> >   */
> > -static void __ublk_fail_req(struct ublk_io *io, struct request *req)
> > +static void __ublk_fail_req(struct ublk_queue *ubq, struct ublk_io *io,
> > +		struct request *req)
> >  {
> >  	WARN_ON_ONCE(io->flags & UBLK_IO_FLAG_ACTIVE);
> >  
> > @@ -667,7 +668,10 @@ static void __ublk_fail_req(struct ublk_io *io, struct request *req)
> >  				req->tag,
> >  				io->flags);
> >  		io->flags |= UBLK_IO_FLAG_ABORTED;
> > -		blk_mq_end_request(req, BLK_STS_IOERR);
> > +		if (ublk_queue_can_use_recovery_reissue(ubq))
> > +			blk_mq_requeue_request(req, false);
> 
> Here is one problem:
> We reset io->flags to 0 in ublk_queue_reinit() and it is called before new

As we agreed, ublk_queue_reinit() will be moved to ublk_ch_release(), when there isn't
any inflight request, which is completed by either ublk server or __ublk_fail_req().

So clearing io->flags isn't related with quisceing device.

> ubq_daemon with FETCH_REQ is accepted. ublk_abort_queue() is not protected with
> ub_mutex and it is called many times in monitor_work. So same rq may be requeued
> multiple times.

UBLK_IO_FLAG_ABORTED is set for the slot, so one req is only ended or
requeued just once.

> 
> With recovery disabled, there is no such problem since io->flags does not change
> until ublk_dev is released.

But we have agreed that ublk_queue_reinit() can be moved to release
handler of /dev/ublkcN.

> 
> In my patch 5 I only requeue the same rq once. So re-using ublk_abort_queue() is
> hard for recovery feature.

No, the same rq is just requeued once. Here the point is:

1) reuse previous pattern in ublk_stop_dev(), which is proved as
workable reliably

2) avoid to stay in half-working state forever

3) the behind idea is more simpler.

Thanks.
Ming