Re: [PATCH] drm/virtio: Destroy obj_restore_lock on the final drm_dev_put()

[Abhin Parekadan Jose <abhinjoses@gmail.com>]

On Wed, Oct 14, 2026 at 01:16:57AM +0300, Dmitry Osipenko wrote:
> On 10/12/26 08:44, Abhin Parekadan Jose wrote:
> > Gentle ping on this fix for the syzbot report.
> > 
> > Dmitry, Gerd, do you have a preference between moving mutex_destroy()
> > to virtio_gpu_release() (this patch) and using drmm_mutex_init()? I can
> > send a v2 with the other option if you prefer it.
> > 
> > Thanks,
> > Abhin
> > 
> > On Tue, Sep 29, 2026 at 07:07:14AM +0000, Abhin Parekadan Jose wrote:
> >> virtio_gpu_deinit() calls mutex_destroy() on obj_restore_lock, but
> >> deinit runs from virtio_gpu_remove() at unbind or PCI remove, and GEM
> >> objects can outlive that. An open /dev/fb0 keeps the fbdev buffer
> >> alive, and the fbdev DRM client holds a drm_device reference. When the
> >> fd is finally closed, the buffer is freed and takes the destroyed lock:
> >>
> >>   fb_release()
> >>     drm_fbdev_shmem_fb_destroy()
> >>       drm_client_buffer_delete()
> >>         virtio_gpu_free_object()
> >>           virtio_gpu_remove_from_restore_list()
> >>             mutex_lock(&vgdev->obj_restore_lock)
> >>
> >>   DEBUG_LOCKS_WARN_ON(lock->magic != lock)
> >>   WARNING: kernel/locking/mutex.c:625 at __mutex_lock+0xf2c/0x12ec
> >>
> >> virtio_gpu_deinit() should only do hardware teardown in this case
> >> related virtio, its queues and others. Software state that GEM callbacks
> >> still use belongs in virtio_gpu_release(), which runs on the final
> >> drm_dev_put().
> >>
> >> There are two equivalent ways to tie the mutex lifetime to the
> >> drm_device:
> >>
> >>  1) Move mutex_destroy() from virtio_gpu_deinit() to the end of
> >>     virtio_gpu_release(). This is what this patch does. It is the
> >>     smallest change and keeps the teardown next to the rest of the
> >>     final-put software cleanup.
> >>
> >>  2) Initialise the lock with drmm_mutex_init() (checking its return
> >>     value) and drop mutex_destroy() from virtio_gpu_deinit(), so DRM's
> >>     managed release destroys it. This also covers the virtio_gpu_init()
> >>     error path, where release returns early because dev_private is
> >>     NULL. Skipping mutex_destroy() there is harmless, since it only
> >>     matters for mutex debugging.
> >>
> >> Either fix removes the WARN. Option 1 is sent as the smaller change,
> >> and option 2 can be done instead if preferred.
> >>
> >> Reproduced on arm64 QEMU (-device virtio-gpu-pci) with DEBUG_MUTEXES
> >> and PROVE_LOCKING: open /dev/fb0, write 1 to
> >> /sys/bus/pci/devices/0000:00:01.0/remove, then close the fd. With this
> >> patch the WARN is gone, and rebinding brings back /dev/fb0 and
> >> /dev/dri/card0.
> >>
> >> Fixes: 54a970048296 ("drm/virtio: Add support for saving and restoring virtio_gpu_objects")
> >> Reported-by: syzbot+3590d97d8a586fa955c2@syzkaller.appspotmail.com
> >> Closes: https://syzkaller.appspot.com/bug?extid=3590d97d8a586fa955c2
> >> Assisted-by: LLM
> >> Signed-off-by: Abhin Parekadan Jose <abhinjoses@gmail.com>
> >> ---
> >>  drivers/gpu/drm/virtio/virtgpu_kms.c | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
> >> index 1d4d3bf46a20..0ec755c35050 100644
> >> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
> >> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
> >> @@ -354,7 +354,6 @@ void virtio_gpu_deinit(struct drm_device *dev)
> >>  	virtio_reset_device(vgdev->vdev);
> >>  	virtio_gpu_reclaim_vbufs(vgdev);
> >>  	vgdev->vdev->config->del_vqs(vgdev->vdev);
> >> -	mutex_destroy(&vgdev->obj_restore_lock);
> >>  }
> >>  
> >>  void virtio_gpu_release(struct drm_device *dev)
> >> @@ -370,6 +369,8 @@ void virtio_gpu_release(struct drm_device *dev)
> >>  
> >>  	if (vgdev->has_host_visible)
> >>  		drm_mm_takedown(&vgdev->host_visible_mm);
> >> +
> >> +	mutex_destroy(&vgdev->obj_restore_lock);
> >>  }
> >>  
> >>  int virtio_gpu_driver_open(struct drm_device *dev, struct drm_file *file)
> Calling virtio_gpu_deinit() from virtio_gpu_release() might be a better
> option

Wouldn't that end up freeing the virtqueues twice, or using them after
they are freed? virtio_gpu_deinit() would then run from both the
virtio_gpu_remove() path and the virtio_gpu_release() path.
or do you mean removing it from virtio_gpu_remove() and calling it only
from virtio_gpu_release()?
This is only from a high-level look so far. I'll look into it more
closely and get back to you.

> 
> -- 
> Best regards,
> Dmitry