[Devel] [PATCH RHEL10 COMMIT] ms/drm/virtio: fix deadlock in display_info_cb by removing hotplug from dequeue worker

[Konstantin Khorenko <khorenko@virtuozzo.com>]

The commit is pushed to "branch-rh10-6.12.0-211.39.1.16.x.vz10-ovz" and will appear at git at bitbucket.org:openvz/vzkernel.git
after rh10-6.12.0-211.39.1.16.3.vz10
------>
commit ff8aae3c80cecda20c3a48f3fa00b837c614943c
Author: Ryosuke Yasuoka <ryasuoka@redhat.com>
Date:   Wed Aug 12 16:18:57 2026 +0200

    ms/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 at redhat.com
    
    (cherry picked from commit d1b894c5bbb3fee0012bd14356286dc2384e8213)
    
    VHI hit the runtime variant of this deadlock in a compute test run.
    An almalinux-10 guest froze mid-boot, roughly once in 20 boots, with
    the serial console and the VGA output both stuck a moment after fbcon
    came up. The dequeue worker gets past clientlist_mutex there and
    blocks one frame further in, on the modeset ww_mutex held by the fbdev
    damage worker:
    
      kworker/0:1  Workqueue: events drm_fb_helper_damage_work
        drm_fb_helper_damage_work -> drm_fbdev_shmem_helper_fb_dirty
          -> drm_atomic_helper_dirtyfb      holds the modeset ww_mutexes
            -> drm_atomic_commit -> virtio_gpu_primary_plane_update
              -> virtio_gpu_queue_ctrl_sgs  waits for ctrl-vq descriptors
    
      kworker/0:3  Workqueue: events virtio_gpu_dequeue_ctrl_func
        virtio_gpu_dequeue_ctrl_func        the only code that frees them
          -> drm_client_dev_hotplug -> drm_fb_helper_hotplug_event
            -> drm_client_modeset_probe -> drm_modeset_lock_all_ctx
                                          waits for the same ww_mutexes
    
    Both waits are uninterruptible, so the guest never recovers. The
    console dies with it: setfont blocks in drm_fb_helper_pan_display
    while holding console_lock, and systemd blocks on console_sem in
    tty_open, which is where the boot stops.
    
    Cherry-pick applied cleanly, no conflicts.
    
    https://virtuozzo.atlassian.net/browse/VSTOR-141131
    Feature: fix ms/drm/virtio
    Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 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 1c15cbf326b78..4a25347734fda 100644
--- a/drivers/gpu/drm/virtio/virtgpu_kms.c
+++ b/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;
 	}
diff --git a/drivers/gpu/drm/virtio/virtgpu_vq.c b/drivers/gpu/drm/virtio/virtgpu_vq.c
index 8181b22b9b46a..412384e16daea 100644
--- a/drivers/gpu/drm/virtio/virtgpu_vq.c
+++ b/drivers/gpu/drm/virtio/virtgpu_vq.c
@@ -839,9 +839,6 @@ 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,