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

[Chia-I Wu]

On Thu, Jul 4, 2019 at 4:25 AM Gerd Hoffmann <kraxel@xxxxxxxxxx> wrote:
>
>   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 ...
Right...
>
> > 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?
The game was single-threaded.  I guess it was the game and Xorg
competing for virtio buffers.  That was also on an older kernel
without explicit fences.  The userspace had to create dummy resources
frequently to do VIRTIO_IOCTL_RESOURCE_WAIT.

I think this is fine for now as far as I am concerned.  I can look
into this more closely after this series lands.


>
> cheers,
>   Gerd
>