Hi,
On 2026/7/14 21:17, Michael S. Tsirkin wrote:
On Mon, Jul 13, 2026 at 09:20:25PM +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 <[email protected]>
I'd expect lots of drivers have this issue? Wouldn't it make more sense
to check virtqueue_is_broken in
virtqueue_enable_cb_delayed/virtqueue_enable_cb? This way it works for
all drivers.
In virtqueue_enable_cb->virtqueue_poll, a check for vq->broken is
performed, so other devices do not have this issue.
Indeed, it is more reasonable to check vq->broken inside
virtqueue_enable_cb_delayed. I will make the changes in v2.
Thanks,
Jinqian
---
drivers/net/virtio_net.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 7d2eeb9b1226..c8d2d420c31d 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -2970,7 +2970,8 @@ static void virtnet_poll_cleantx(struct receive_queue
*rq, int budget)
do {
virtqueue_disable_cb(sq->vq);
free_old_xmit(sq, txq, !!budget);
- } while (unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
+ } while (!virtqueue_is_broken(sq->vq) &&
+ unlikely(!virtqueue_enable_cb_delayed(sq->vq)));
if (sq->vq->num_free >= MAX_SKB_FRAGS + 2)
virtnet_tx_wake_queue(vi, sq);
--
2.33.0
