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: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=3590d97d8a586fa955c2
> Assisted-by: LLM
> Signed-off-by: Abhin Parekadan Jose <[email protected]>
> ---
> 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)
>
> base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
> --
> 2.51.1
>
