linux-kernel - Re: [PATCH] drm/virtio: fix command submission with objects but without fence.

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Date:   Thu, 5 Sep 2019 07:23:40 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Chia-I Wu <olvaffe@...il.com>
Cc:     ML dri-devel <dri-devel@...ts.freedesktop.org>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] drm/virtio: fix command submission with objects but
 without fence.

On Wed, Sep 04, 2019 at 04:10:30PM -0700, Chia-I Wu wrote:
> On Wed, Sep 4, 2019 at 12:48 AM Gerd Hoffmann <kraxel@...hat.com> wrote:
> >
> > Only call virtio_gpu_array_add_fence if we actually have a fence.
> >
> > Fixes: da758d51968a ("drm/virtio: rework virtio_gpu_execbuffer_ioctl fencing")
> > Signed-off-by: Gerd Hoffmann <kraxel@...hat.com>
> > ---
> >  drivers/gpu/drm/virtio/virtgpu_vq.c | 9 +++++----
> >  1 file changed, 5 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
> > index 595fa6ec2d58..7fd2851f7b97 100644
> > --- a/drivers/gpu/drm/virtio/virtgpu_vq.c
> > +++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
> > @@ -339,11 +339,12 @@ static void virtio_gpu_queue_fenced_ctrl_buffer(struct virtio_gpu_device *vgdev,
> >                 goto again;
> >         }
> >
> > -       if (fence)
> > +       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);
> > +               if (vbuf->objs) {
> > +                       virtio_gpu_array_add_fence(vbuf->objs, &fence->f);
> > +                       virtio_gpu_array_unlock_resv(vbuf->objs);
> > +               }
> This leaks when fence == NULL and vbuf->objs != NULL (which can really
> happen IIRC... not at my desk to check).

Yes, it can happen, for example when flushing dumb buffers.

But I don't think we leak in this case.  The code paths which don't need
a fence also do not call virtio_gpu_array_lock_resv(), so things are
balanced.  The actual release of the objs happens in
virtio_gpu_dequeue_ctrl_func() via virtio_gpu_array_put_free_delayed().

cheers,
  Gerd