[PATCH 2/2] virtio_balloon: quiesce balloon work before device shutdown

["Denis V. Lunev" <den@openvz.org>]

Commit 8bd2fa086a04 ("virtio: break and reset virtio devices on
device_shutdown()") added a generic virtio bus .shutdown handler that
breaks and resets every virtio device during device_shutdown(), i.e. on
reboot and kexec.

virtio_balloon provides no .shutdown of its own, so that generic path
runs while the balloon's asynchronous work is still armed. Once the
device has been broken, virtqueue_add_inbuf() in
virtballoon_free_page_report() returns -EIO and trips its
WARN_ON_ONCE(). On a kernel booted with panic_on_warn that turns an
ordinary reboot, for example a kexec based upgrade, into a fatal panic
in the middle of device_shutdown(), so the machine never reaches the
new kernel.

Relaxing that single WARN_ON_ONCE() would only hide the symptom: the
inflate/deflate and OOM paths do not warn, they call
wait_event(vb->acked, ...) and would instead block forever on a broken
queue that can no longer complete. The device has to be quiesced, not
just kept quiet.

Add a .shutdown handler that quiesces the balloon via the shared
virtballoon_quiesce() helper while the device is still alive, and only
then breaks and resets it. The break and reset are repeated here rather
than reused from virtio_dev_shutdown(): drv->shutdown replaces the
generic handler rather than augmenting it, so that drivers such as
virtio-gpu can opt out of the reset. Unlike virtballoon_remove() the
balloon workqueue is not destroyed, as shutdown does not free the
device and cancel_work_sync() together with stop_update already prevent
any further work from being queued.

Fixes: 8bd2fa086a04 ("virtio: break and reset virtio devices on device_shutdown()")
Signed-off-by: Denis V. Lunev <den@openvz.org>
---
 drivers/virtio/virtio_balloon.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/drivers/virtio/virtio_balloon.c b/drivers/virtio/virtio_balloon.c
index 5b02d9191ac6..e35ada767b4b 100644
--- a/drivers/virtio/virtio_balloon.c
+++ b/drivers/virtio/virtio_balloon.c
@@ -1137,6 +1137,15 @@ static void virtballoon_remove(struct virtio_device *vdev)
 	kfree(vb);
 }
 
+static void virtballoon_shutdown(struct virtio_device *vdev)
+{
+	virtballoon_quiesce(vdev->priv);
+
+	virtio_break_device(vdev);
+	virtio_synchronize_cbs(vdev);
+	vdev->config->reset(vdev);
+}
+
 #ifdef CONFIG_PM_SLEEP
 static int virtballoon_freeze(struct virtio_device *vdev)
 {
@@ -1202,6 +1211,7 @@ static struct virtio_driver virtio_balloon_driver = {
 	.validate =	virtballoon_validate,
 	.probe =	virtballoon_probe,
 	.remove =	virtballoon_remove,
+	.shutdown =	virtballoon_shutdown,
 	.config_changed = virtballoon_changed,
 #ifdef CONFIG_PM_SLEEP
 	.freeze	=	virtballoon_freeze,
-- 
2.53.0