Re: [Qemu-devel] [Qemu-block] [PATCH 4/4] virtio-blk: Clean up start/stop with mutex and BH

[tu bo]

Hi Christian:

On 03/23/2016 05:12 PM, Christian Borntraeger wrote:
> On 03/23/2016 10:08 AM, Paolo Bonzini wrote:
> > On 23/03/2016 09:10, Cornelia Huck wrote:
> > > > -    /* Kick right away to begin processing requests already in vring */
> > > > -    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
> > > > +    vblk->dataplane_started = true;
> > > >
> > > > -    /* Get this show started by hooking up our callbacks */
> > > > +    /* Get this show started by hooking up our callbacks.  */
> > > >       aio_context_acquire(s->ctx);
> > > >       virtio_queue_aio_set_host_notifier_handler(s->vq, s->ctx, true, true);
> > > >       aio_context_release(s->ctx);
> > > > +    atomic_dec(&s->starting);
> > > > +
> > > > +    /* Kick right away to begin processing requests already in vring */
> > > > +    event_notifier_set(virtio_queue_get_host_notifier(s->vq));
> > >
> > > I'm wondering whether moving this event_notifier_set() masks something?
> > > IOW, may we run into trouble if the event notifier is set from some
> > > other path before the callbacks are set up properly?
> >
> > The reentrancy check should catch that...  But:
> >
> > 1) the patch really makes no difference, your fix is enough for me
>
>
> Tu Bo, can you test with master + Cornelias 6 refactoring patches and nothing 
> on top?

With qemu master + Cornelia's 6 refactoring patches and nothing on top, 
I did NOT see crash so far.

> > 2) vblk->dataplane_started becomes true before the callbacks are set;
> > that should be enough.
> >
> > 3) this matches what I tested, but it would of course be better if the
> > assertions on s->starting suffice
> >
> > > > -    if (!vblk->dataplane_started || s->stopping) {
> > > > +    if (!vblk->dataplane_started) {
> > >
> > > No fear of reentrancy here?
> >
> > No, because this is only invoked from reset, hence only from the CPU
> > thread and only under the BQL.
> >
> > On start, reentrancy happens between iothread (running outside BQL) and
> > a CPU thread (running within BQL).
> >
> > Paolo
> >
> > > >           return;
> > > >       }
> > > >
> > > > @@ -255,7 +251,7 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
> > > >           vblk->dataplane_started = false;
> > > >           return;
> > > >       }
> > > > -    s->stopping = true;
> > > > +
> > > >       trace_virtio_blk_data_plane_stop(s);
> > > >
> > > >       aio_context_acquire(s->ctx);
> > > > @@ -274,5 +270,4 @@ void virtio_blk_data_plane_stop(VirtIOBlockDataPlane *s)
> > > >       k->set_guest_notifiers(qbus->parent, 1, false);
> > > >
> > > >       vblk->dataplane_started = false;
> > > > -    s->stopping = false;
> > > >   }