Re: [PATCH v4] drm/virtio: use uninterruptible resv lock for plane updates

[Dmitry Osipenko <dmitry.osipenko@collabora.com>]

On 5/19/26 11:27, Christian König wrote:
> On 5/19/26 10:22, Deepanshu Kartikey wrote:
>> virtio_gpu_cursor_plane_update() and virtio_gpu_resource_flush() lock
>> the framebuffer BO's dma_resv via virtio_gpu_array_lock_resv() and
>> ignore its return value. The function can fail with -EINTR from
>> dma_resv_lock_interruptible() (signal during lock wait) or with
>> -ENOMEM from dma_resv_reserve_fences() (fence slot allocation),
>> leaving the resv lock not held. The queue path then walks the object
>> array and calls dma_resv_add_fence(), which requires the lock held;
>> with lockdep enabled this trips dma_resv_assert_held():
>>
>>   WARNING: drivers/dma-buf/dma-resv.c:296 at dma_resv_add_fence+0x71e/0x840
>>   Call Trace:
>>    virtio_gpu_array_add_fence
>>    virtio_gpu_queue_ctrl_sgs
>>    virtio_gpu_queue_fenced_ctrl_buffer
>>    virtio_gpu_cursor_plane_update
>>    drm_atomic_helper_commit_planes
>>    drm_atomic_helper_commit_tail
>>    commit_tail
>>    drm_atomic_helper_commit
>>    drm_atomic_commit
>>    drm_atomic_helper_update_plane
>>    __setplane_atomic
>>    drm_mode_cursor_universal
>>    drm_mode_cursor_common
>>    drm_mode_cursor_ioctl
>>    drm_ioctl
>>    __x64_sys_ioctl
>>
>> Beyond the WARN, mutating the dma_resv fence list without the lock
>> races with concurrent readers/writers and can corrupt the list.
> 
> Well why are you trying to add a fence on an atomic mode set in the first place?
> 
> That is usually an illegal operation here.
That is pre-existing in the driver. It performs draw operation and in
some cases waits for the completion during atomic. Whether all that
syncing is correct is hard to say immediately as some of it may be
historical edge cases.

-- 
Best regards,
Dmitry