Re: [PATCH v2] virtio_net: fix infinite loop in virtnet_poll_cleantx when device is broken

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 16, 2026 at 11:52:01AM +0800, Jinqian Yang wrote:
> 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 <yangjinqian1@huawei.com>


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

> ---
> Changes in v2:
>   - Moved vq->broken check to virtqueue_enable_cb_delayed().
> 
> v1: https://lore.kernel.org/lkml/20260713132025.703147-1-yangjinqian1@huawei.com/
> ---
>  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