Hi,

On 2026/7/16 13:14, Michael S. Tsirkin wrote:

On Thu, Jul 16, 2026 at 11:52:01AM +0800, Jinqian Yang wrote:

quoted

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 virtqueue_is_broken() check to the loop condition, so
that the loop exits immediately when the device is broken, allowing
the device shutdown to proceed.

Signed-off-by: Jinqian Yang <redacted>

Good thanks! Just the subject needs change so it's clear
we are changing virtio core not virtio net.

Thanks for the catch. Will change the subject prefix to "virtio_ring:"
and send v3 shortly.

Thanks,
Jinqian

quoted

---
Changes in v2:
   - Moved vq->broken check to virtqueue_enable_cb_delayed().

v1: https://lore.kernel.org/lkml/20260713132025.703147-1-yangjinqian1@huawei.com/ (local)
---
  drivers/virtio/virtio_ring.c | 8 ++++++++
  1 file changed, 8 insertions(+)

diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index b438dc2ce1b8..5c169fbb418a 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/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);
  

-- 
2.33.0