Re: [PATCH] drm/virtio: defer hotplug event from dequeue worker to avoid deadlock

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

On 7/9/26 15:30, Ryosuke Yasuoka wrote:
> 
> 
> On 07/07/2026 18:10, Dmitry Osipenko wrote:
>> On 7/3/26 08:58, Ryosuke Yasuoka wrote:
>>>
>>>
>>> On 02/07/2026 14:26, Dmitry Osipenko wrote:
>>>> On 7/1/26 12:23, Ryosuke Yasuoka wrote:
>>>>>
>>>>>
>>>>> On 30/06/2026 16:46, Dmitry Osipenko wrote:
>>>>>> Hi,
>>>>>>
>>>>>> On 6/30/26 12:16, Ryosuke Yasuoka wrote:
>>>>>>> A probe-time deadlock can occur between the dequeue worker and
>>>>>>> drm_client_register(). During probe, drm_client_register() holds
>>>>>>> clientlist_mutex and calls the fbdev hotplug callback, which triggers an
>>>>>>> atomic commit that ends up sleeping in virtio_gpu_queue_ctrl_sgs()
>>>>>>> waiting for virtqueue space. The dequeue worker that would free that
>>>>>>> space calls virtio_gpu_cmd_get_display_info_cb(), which invokes
>>>>>>> drm_kms_helper_hotplug_event() -> drm_client_dev_hotplug(), attempting
>>>>>>> to acquire the same clientlist_mutex. Since wake_up() is only called
>>>>>>> after the resp_cb loop, the probe thread is never woken and both threads
>>>>>>> deadlock.
>>>>>>>
>>>>>>> Fix this by deferring the hotplug notification from
>>>>>>> virtio_gpu_cmd_get_display_info_cb() to a separate work item. The
>>>>>>> display data (outputs[i].info) is still updated synchronously in the
>>>>>>> callback, and the deferred work only triggers a re-probe notification to
>>>>>>> DRM clients.
>>>>>>>
>>>>>>> Fixes: 27655b9bb9f0 ("drm/client: Send hotplug event after registering a client")
>>>>>>> Closes: https://syzkaller.appspot.com/bug?id=d6dd6f86d3aaf7eebe7406e45c1c6e549453f224
>>>>>>> Closes: https://syzkaller.appspot.com/bug?id=908bd910da5dd79b88de4cf7baf376cc873a922e
>>>>>>> Signed-off-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
>>>>>>> ---
>>>>>>>  drivers/gpu/drm/virtio/virtgpu_drv.h |  3 +++
>>>>>>>  drivers/gpu/drm/virtio/virtgpu_kms.c |  3 +++
>>>>>>>  drivers/gpu/drm/virtio/virtgpu_vq.c  | 12 ++++++++++--
>>>>>>>  3 files changed, 16 insertions(+), 2 deletions(-)
>>>>>>>
>>>>>>> diff --git a/drivers/gpu/drm/virtio/virtgpu_drv.h b/drivers/gpu/drm/virtio/virtgpu_drv.h
>>>>>>> index 7449907754a4..27ffa4697ae9 100644
>>>>>>> --- a/drivers/gpu/drm/virtio/virtgpu_drv.h
>>>>>>> +++ b/drivers/gpu/drm/virtio/virtgpu_drv.h
>>>>>>> @@ -264,6 +264,8 @@ struct virtio_gpu_device {
>>>>>>>  
>>>>>>>  	struct work_struct config_changed_work;
>>>>>>>  
>>>>>>> +	struct work_struct hotplug_work;
>>>>>>> +
>>>>>>>  	struct work_struct obj_free_work;
>>>>>>>  	spinlock_t obj_free_lock;
>>>>>>>  	struct list_head obj_free_list;
>>>>>>> @@ -350,6 +352,7 @@ void virtio_gpu_cmd_transfer_to_host_2d(struct virtio_gpu_device *vgdev,
>>>>>>>  					uint32_t x, uint32_t y,
>>>>>>>  					struct virtio_gpu_object_array *objs,
>>>>>>>  					struct virtio_gpu_fence *fence);
>>>>>>> +void virtio_gpu_hotplug_work_func(struct work_struct *work);
>>>>>>>  void virtio_gpu_panic_cmd_resource_flush(struct virtio_gpu_device *vgdev,
>>>>>>>  					 uint32_t resource_id,
>>>>>>>  					 uint32_t x, uint32_t y,
>>>>>>> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
>>>>>>> index cfde9f573df6..cfb532ba43a4 100644
>>>>>>> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
>>>>>>> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
>>>>>>> @@ -154,6 +154,8 @@ int virtio_gpu_init(struct virtio_device *vdev, struct drm_device *dev)
>>>>>>>  	INIT_WORK(&vgdev->config_changed_work,
>>>>>>>  		  virtio_gpu_config_changed_work_func);
>>>>>>>  
>>>>>>> +	INIT_WORK(&vgdev->hotplug_work, virtio_gpu_hotplug_work_func);
>>>>>>> +
>>>>>>>  	INIT_WORK(&vgdev->obj_free_work,
>>>>>>>  		  virtio_gpu_array_put_free_work);
>>>>>>>  	INIT_LIST_HEAD(&vgdev->obj_free_list);
>>>>>>> @@ -293,6 +295,7 @@ void virtio_gpu_deinit(struct drm_device *dev)
>>>>>>>  	flush_work(&vgdev->obj_free_work);
>>>>>>>  	flush_work(&vgdev->ctrlq.dequeue_work);
>>>>>>>  	flush_work(&vgdev->cursorq.dequeue_work);
>>>>>>> +	flush_work(&vgdev->hotplug_work);
>>>>>>>  	flush_work(&vgdev->config_changed_work);
>>>>>>>  	virtio_reset_device(vgdev->vdev);
>>>>>>>  	vgdev->vdev->config->del_vqs(vgdev->vdev);
>>>>>>> diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
>>>>>>> index 67865810a2e7..084d98f5dc7b 100644
>>>>>>> --- a/drivers/gpu/drm/virtio/virtgpu_vq.c
>>>>>>> +++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
>>>>>>> @@ -816,6 +816,15 @@ virtio_gpu_cmd_resource_detach_backing(struct virtio_gpu_device *vgdev,
>>>>>>>  	virtio_gpu_queue_fenced_ctrl_buffer(vgdev, vbuf, fence);
>>>>>>>  }
>>>>>>>  
>>>>>>> +void virtio_gpu_hotplug_work_func(struct work_struct *work)
>>>>>>> +{
>>>>>>> +	struct virtio_gpu_device *vgdev =
>>>>>>> +		container_of(work, struct virtio_gpu_device, hotplug_work);
>>>>>>> +
>>>>>>> +	if (!drm_helper_hpd_irq_event(vgdev->ddev))
>>>>>>> +		drm_kms_helper_hotplug_event(vgdev->ddev);
>>>>>>> +}
>>>>>>> +
>>>>>>>  static void virtio_gpu_cmd_get_display_info_cb(struct virtio_gpu_device *vgdev,
>>>>>>>  					       struct virtio_gpu_vbuffer *vbuf)
>>>>>>>  {
>>>>>>> @@ -841,8 +850,7 @@ static void virtio_gpu_cmd_get_display_info_cb(struct virtio_gpu_device *vgdev,
>>>>>>>  	spin_unlock(&vgdev->display_info_lock);
>>>>>>>  	wake_up(&vgdev->resp_wq);
>>>>>>>  
>>>>>>> -	if (!drm_helper_hpd_irq_event(vgdev->ddev))
>>>>>>> -		drm_kms_helper_hotplug_event(vgdev->ddev);
>>>>>>> +	schedule_work(&vgdev->hotplug_work);
>>>>>>>  }
>>>>>>>  
>>>>>>>  static void virtio_gpu_cmd_get_capset_info_cb(struct virtio_gpu_device *vgdev,
>>>>>>
>>>>>
>>>>> Hi,
>>>>> Thank you for your review.
>>>>>
>>>>>> Could you please move drm_kms_helper_hotplug_event() to virtio_gpu_init(), placing it after wait_event_timeout(display_info_pending)? This will avoid additional work_struct that otherwise needs to be cancelled in virtio_gpu_init() on the timeout.
>>>>>
>>>>> IIUC, moving the drm_kms_helper_hotplug_event() and _hpd_irq_event()
>>>>> into virtio_gpu_init() after wait_event_timeout() would not prevent the
>>>>> issue.
>>>>>
>>>>> Looking at the syzbot call traces[1][2], the deadlock occurs during
>>>>> drm_client_setup(), which runs after virtio_gpu_init() has already
>>>>> returned. The display_info_cb that triggers the deadlock is called from
>>>>> the dequeue worker while drm_client_register() holds clientlist_mutex.
>>>>>
>>>>> Thread A:
>>>>> virtio_gpu_probe()
>>>>>  -> virtio_gpu_init()   // sends GET_DISPLAY_INFO and waits up to 5s
>>>>
>>>> You mean that the timeout happens and it's again syzkaller report for a
>>>> broken host. A day ago I applied [1] that should fix this "bogus"
>>>> syzkaller report.
>>>>
>>>> [1] https://patchwork.freedesktop.org/patch/735301/
>>>
>>> Thank you for [1] and the clarificaiton. I agree that [1] addresses the
>>> broken host scenario where virtio_gpu_init() times out and probe
>>> continues with a pending display_info response. I think there is another
>>> scenario causing the same deadlock reported by syzbot and my patch
>>> addresses both cases.
>>>
>>> The deadlock can also occur with the following scenario:
>>> after virtio_gpu_init() completes successfully (wait_event_timeout
>>> returns), if a config_changed event arrives before drm_client_setup()
>>> completes, config_changed_work_func() sends a new GET_DISPLAY_INFO. Its
>>> display_info_cb fires in the dequeue worker and blocks on
>>> clientlist_mutex held by drm_client_register(). This is the same
>>> deadlock path reported by syzbot.
>>>
>>> Since [1] only prevents the timeout case, I believe the schedule_work()
>>> approach is still needed to prevent display_info_cb from blocking on
>>> clientlist_mutex in the dequeue worker context, regardless of the
>>> trigger.
>>
>> The config_changed_work_func() already invokes
>> drm_helper_hpd_irq_event() by itself [1].
>>
>> [1]
>> https://elixir.bootlin.com/linux/v7.1.2/source/drivers/gpu/drm/virtio/virtgpu_kms.c#L52
>>
>> The drm_helper_hpd_irq_event() itself calls
>> drm_kms_helper_hotplug_event() [2].
>>
>> [2]
>> https://elixir.bootlin.com/linux/v7.1.2/source/drivers/gpu/drm/drm_probe_helper.c#L1113
>>
>> Isn't drm_helper_hpd_irq_event() call done by
>> virtio_gpu_cmd_get_display_info_cb() already partially redundant? I.e.
>> only it's needed by virtio_gpu_init() and not by
>> config_changed_work_func(). And then we can move
>> drm_helper_hpd_irq_event() to virtio_gpu_init():
>>
>> ```
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c
>> b/drivers/gpu/drm/virtio/virtgpu_kms.c
>> index 922782bdc9cd..9805283683c1 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
>> @@ -303,6 +303,7 @@ int virtio_gpu_init(struct virtio_device *vdev,
>> struct drm_device *dev)
>>                 virtio_gpu_notify(vgdev);
>>                 wait_event_timeout(vgdev->resp_wq,
>> !vgdev->display_info_pending,
>>                                    5 * HZ);
>> +               drm_helper_hpd_irq_event(vgdev->ddev);
>>         }
>>
>>         vgdev->pm_nb.notifier_call = virtio_gpu_pm_notifier;
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c
>> b/drivers/gpu/drm/virtio/virtgpu_vq.c
>> index 68d097ad9d1d..aa6440a9b81e 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_vq.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
>> @@ -880,9 +880,6 @@ static void
>> virtio_gpu_cmd_get_display_info_cb(struct virtio_gpu_device *vgdev,
>>         vgdev->display_info_pending = false;
>>         spin_unlock(&vgdev->display_info_lock);
>>         wake_up(&vgdev->resp_wq);
>> -
>> -       if (!drm_helper_hpd_irq_event(vgdev->ddev))
>> -               drm_kms_helper_hotplug_event(vgdev->ddev);
>>  }
>>
>>  static void virtio_gpu_cmd_get_capset_info_cb(struct virtio_gpu_device
>> *vgdev,
>> ```
>>
>> Isn't it a bug that config_changed_work_func() doesn't wait for
>> display_info_pending event?
>>
>>
>> ```
>> diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c
>> b/drivers/gpu/drm/virtio/virtgpu_kms.c
>> index 922782bdc9cd..dd2d5c896c39 100644
>> --- a/drivers/gpu/drm/virtio/virtgpu_kms.c
>> +++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
>> @@ -50,6 +50,8 @@ static void virtio_gpu_config_changed_work_func(struct
>> work_struct *work)
>>                                 virtio_gpu_cmd_get_edids(vgdev);
>>                         virtio_gpu_cmd_get_display_info(vgdev);
>>                         virtio_gpu_notify(vgdev);
>> +                       wait_event_timeout(vgdev->resp_wq,
>> !vgdev->display_info_pending,
>> +                                          5 * HZ);
>>                         drm_helper_hpd_irq_event(vgdev->ddev);
>>                 }
>>                 events_clear |= VIRTIO_GPU_EVENT_DISPLAY;
>>
>> ```
> 
> Thank you for your suggestion. I agree that removing the hotplug calls
> from display_info_cb and waiting for the response in
> config_changed_work_func() and virtio_gpu_init() is a cleaner approach.
> I'll send a v2 based on this.

Thanks, please also check whether drm_helper_hpd_irq_event() needed at
all in in virtio_gpu_init(). Suppose the hotplug event does nothing
before DRM device/client has been registered.

-- 
Best regards,
Dmitry