Virtualization - Aug 2022 - [PATCH] virtio-blk: Fix WARN_ON_ONCE in virtio_queue_rq()

On Wed, Aug 24, 2022 at 10:16:10PM +0900, Kim Suwan
wrote:
> Hi Stefan,
> 
> On Wed, Aug 24, 2022 at 5:56 AM Stefan Hajnoczi <stefanha at
redhat.com> wrote:
> >
> > On Tue, Aug 23, 2022 at 11:50:05PM +0900, Suwan Kim wrote:
> > > @@ -409,6 +409,8 @@ static bool virtblk_add_req_batch(struct
virtio_blk_vq *vq,
> > >                       virtblk_unmap_data(req, vbr);
> > >                       virtblk_cleanup_cmd(req);
> > >                       rq_list_add(requeue_list, req);
> > > +             } else {
> > > +                     blk_mq_start_request(req);
> > >               }
> >
> > The device may see new requests as soon as virtblk_add_req() is called
> > above. Therefore the device may complete the request before
> > blk_mq_start_request() is called.
> >
> > rq->io_start_time_ns = ktime_get_ns() will be after the request was
> > actually submitted.
> >
> > I think blk_mq_start_request() needs to be called before
> > virtblk_add_req().
> >
> 
> But if blk_mq_start_request() is called before virtblk_add_req()
> and virtblk_add_req() fails, it can trigger WARN_ON_ONCE() at
> virtio_queue_rq().
> 
> With regard to the race condition between virtblk_add_req() and
> completion, I think that the race condition can not happen because
> virtblk_add_req() holds vq lock with irq saving and completion side
> (virtblk_done, virtblk_poll) need to acquire the vq lock also.
> Moreover, virtblk_done() is irq context so I think it can not be
> executed until virtblk_add_req() releases the lock.
I agree there is no race condition regarding the ordering of
blk_mq_start_request() and request completion. The spinlock prevents
that and I wasn't concerned about that part.

The issue is that the timestamp will be garbage. If we capture the
timestamp during/after the request is executing, then the collected
statistics will be wrong.

Can you look for another solution that doesn't break the timestamp?

FWIW I see that the rq->state state machine allows returning to the idle
state once the request has been started: __blk_mq_requeue_request().

Stefan
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