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 <[email protected]>
>>>>> ---
>>>>> 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;
```
--
Best regards,
Dmitry
