[PATCH v6 08/18] drm/virtio: rework virtio_gpu_execbuffer_ioctl fencing

[Gerd Hoffmann]

  Hi,

> >         if (fence)
> >                 virtio_gpu_fence_emit(vgdev, hdr, fence);
> > +       if (vbuf->objs) {
> > +               virtio_gpu_array_add_fence(vbuf->objs, &fence->f);
> > +               virtio_gpu_array_unlock_resv(vbuf->objs);
> > +       }
> This is with the spinlock held.  Maybe we should move the
> virtio_gpu_array_unlock_resv call out of the critical section.

That would bring back the race ...

> I am actually more concerned about virtio_gpu_array_add_fence, but it
> is also harder to move.  Should we add a kref to the object array?

Yep, refcounting would be the other way to fix the race.

> This bothers me because I recently ran into a CPU-bound game with very
> bad lock contention here.

Hmm.  Any clue where this comes from?  Multiple threads competing for
virtio buffers I guess?  Maybe we should have larger virtqueues?

cheers,
  Gerd