[PATCH v2] drm/virtio: fix deadlock in display_info_cb by removing hotplug from dequeue worker

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 removing the hotplug notification from
virtio_gpu_cmd_get_display_info_cb(). The display data (outputs[i].info)
is still updated synchronously in the callback.

For the init path, drm_client_register() already fires an initial
hotplug when the client is registered, which picks up the connector
state updated by display_info_cb.

For the runtime config_changed path, add a wait_event_timeout() in
config_changed_work_func() so that display_info_cb updates the connector
data before the hotplug notification is sent. Also replace
drm_helper_hpd_irq_event() with drm_kms_helper_hotplug_event() since
virtio-gpu never calls drm_kms_helper_poll_init() and thus
drm_helper_hpd_irq_event() always returns false without doing anything.

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
Suggested-by: Dmitry Osipenko <[email protected]>
Signed-off-by: Ryosuke Yasuoka <[email protected]>
---
I checked whether drm_helper_hpd_irq_event() is needed in
virtio_gpu_init(), as Dmitry suggested. AFAIS, it is not needed because:

1. drm_helper_hpd_irq_event() is always a no-op in virtio-gpu.
   It returns false immediately probe_helper.c:1088 because
   dev->mode_config.poll_enabled is false — virtio-gpu never calls
   drm_kms_helper_poll_init(). Even if it passed that gate, no
   virtio-gpu connectors set DRM_CONNECTOR_POLL_HPD.

1082 bool drm_helper_hpd_irq_event(struct drm_device *dev)
1083 {
...
1088         if (!dev->mode_config.poll_enabled)
1089                 return false;

2. virtio_gpu_init() runs before drm_dev_register() and
   drm_client_setup(), so no DRM clients are registered yet.
   drm_kms_helper_hotplug_event() would iterate an empty client list.
   The initial hotplug is handled by drm_client_register(), which fires
   a hotplug callback to the newly registered client. By that time,
   display_info_cb has already updated the connector data.

For the same reason, drm_helper_hpd_irq_event() in
config_changed_work_func() was also a no-op. The actual runtime hotplug
notification was always delivered by display_info_cb's call to
drm_kms_helper_hotplug_event(). This patch replaces it with a direct
drm_kms_helper_hotplug_event() call after waiting for the display info
response.
---
Changes in v2:
- Dropped the work_struct approach from v1.
- Instead, removed the hotplug calls from display_info_cb entirely, as
suggested by Dmitry.
- Added wait_event_timeout() in config_changed_work_func() so that the
display info response is received before sending the hotplug
notification.
- Replaced drm_helper_hpd_irq_event() with drm_kms_helper_hotplug_event()
in config_changed_work_func() since drm_helper_hpd_irq_event() is
always a no-op in virtio-gpu (poll_enabled is never set).
- No changes to virtio_gpu_init() — drm_client_register() already
handles the initial hotplug and hotplug event does nothing before DRM
device/client has been registered.
- Link to v1: https://lore.kernel.org/r/[email protected]
---
 drivers/gpu/drm/virtio/virtgpu_kms.c | 5 ++++-
 drivers/gpu/drm/virtio/virtgpu_vq.c  | 3 ---
 2 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
index cfde9f573df6..b4329f28e976 100644
--- a/drivers/gpu/drm/virtio/virtgpu_kms.c
+++ b/drivers/gpu/drm/virtio/virtgpu_kms.c
@@ -49,7 +49,10 @@ 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);
-			drm_helper_hpd_irq_event(vgdev->ddev);
+			wait_event_timeout(vgdev->resp_wq,
+					   !vgdev->display_info_pending,
+					   5 * HZ);
+			drm_kms_helper_hotplug_event(vgdev->ddev);
 		}
 		events_clear |= VIRTIO_GPU_EVENT_DISPLAY;
 	}
diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
index c8b9475a7472..e5e1af8b8e8a 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -840,9 +840,6 @@ 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,

---
base-commit: a13c140cc289c0b7b3770bce5b3ad42ab35074aa
change-id: 20260619-virtiogpu_syzbot-bdab508ffcd5

Best regards,
-- 
Ryosuke Yasuoka <[email protected]>