virtio_ring: fix infinite loop in virtnet_poll_cleantx when device is broken

virtnet_poll_cleantx() contains a do-while loop that cleans up
transmitted TX buffers and calls virtqueue_enable_cb_delayed() to check
whether more buffers need processing. When the virtio backend stops
responding during guest reboot, used->idx is never updated, so
virtqueue_enable_cb_delayed() always returns false and the loop never
terminates. Then it will block reboot process, and the guest will hang.

The problem occurs during guest reboot under network traffic:

  1. kernel_restart() -> device_shutdown() traverses the device list
  2. virtio_dev_shutdown() calls virtio_break_device() which sets
     vq->broken = true
  3. virtio_dev_shutdown() then calls virtio_synchronize_cbs() to wait
     for in-flight callbacks to complete
  4. A virtio interrupt fires, softirq is deferred to ksoftirqd which
     calls net_rx_action() -> virtnet_poll() -> virtnet_poll_cleantx()
  5. virtnet_poll_cleantx() enters the do-while loop and never exits
     because the QEMU backend has stopped updating used->idx, despite
     vq->broken having been set to true in step 2.

Since the loop runs inside ksoftirqd (a SCHED_OTHER kthread), it is
visible to the scheduler and does not trigger a hard lockup. However,
the kthread never leaves the loop, so RCU detects it as a CPU stall
and reports it periodically. Meanwhile, the reboot process remains
blocked in device_shutdown() because virtio_dev_shutdown() cannot
complete its synchronization step, and the guest hangs permanently.

This can be reproduced on a guest with a virtio-net device: run iperf3
traffic in the guest, then trigger reboot. The reboot occasionally hangs
permanently with RCU stall on ksoftirqd.

Observed on ARM64 KVM guest:

  CPU#1 RCU stall (ksoftirqd/1), repeated periodically:
    virtqueue_enable_cb_delayed_split <- virtnet_poll <- __napi_poll <-
    net_rx_action <- handle_softirqs <- run_ksoftirqd <-
    smpboot_thread_fn <- kthread

Fix by adding a vq->broken check in virtqueue_enable_cb_delayed(), so
that the loop exits immediately when the device is broken, allowing
the device shutdown to proceed.

Signed-off-by: Jinqian Yang <yangjinqian1@huawei.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260716115940.394832-1-yangjinqian1@huawei.com>

drivers/virtio/virtio_ring.c

@@ -3233,6 +3233,14 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
 {
 	struct vring_virtqueue *vq = to_vvq(_vq);
 
+	/*
+	 * When the device is broken there is no point in polling used->idx,
+	 * the backend will never update it. Return true to let callers
+	 * exit their cleanup loops instead of spinning forever.
+	 */
+	if (unlikely(vq->broken))
+		return true;
+
 	if (vq->event_triggered)
 		data_race(vq->event_triggered = false);