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 <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

diff --git a/drivers/gpu/drm/virtio/virtgpu_kms.c b/drivers/gpu/drm/virtio/virtgpu_kms.c
index cfde9f573df6ecbcbb9d0e8cad777a987ebf477e..b4329f28e97665470ce95e3622afbb9a926e58d9 100644 (file)
--- 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 c8b9475a7472e09abc61085231a2629f2fad593d..e5e1af8b8e8a0a327a253b462fcf942f87c0c0c4 100644 (file)
--- 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,