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

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

[Abhin Parekadan Jose]

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

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

[sashiko-bot]

> 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
> [ ... ]
> 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>

Sashiko has reviewed this patch and found no issues. It looks great!

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260929070714.169412-1-abhinjoses@gmail.com?part=1

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

[Abhin Parekadan Jose]

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)
> 
> base-commit: 72d3fcf802c45d00b300f25b848a93c3a2bd7c7e
> -- 
> 2.51.1
> 

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

[Dmitry Osipenko]

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

-- 
Best regards,
Dmitry