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 <[email protected]>
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/[email protected]/ > --- > 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
