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

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

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)

base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
-- 
2.51.1