drm/virtio: fix deadlock in display_info_cb by removing hotplug from dequeue worker

[ Upstream commit d1b894c ]

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: 27655b9 ("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 <dmitry.osipenko@collabora.com>
Signed-off-by: Ryosuke Yasuoka <ryasuoka@redhat.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Link: https://patch.msgid.link/20260713-virtiogpu_syzbot-v2-1-2958fa37d46d@redhat.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: YsuOS

drivers/gpu/drm/virtio/virtgpu_kms.c

@@ -48,7 +48,10 @@ 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;
 	}

drivers/gpu/drm/virtio/virtgpu_vq.c

@@ -669,9 +669,6 @@ 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,