[RFC PATCH 0/1] virtio: false unhandled irqs from vring_interrupt()

From: Stefan Hajnoczi
Date: Tue Aug 24 2021 - 07:00:11 EST

While investigating an unhandled irq from vring_interrupt() with virtiofs I
stumbled onto a possible race that also affects virtio_gpu. This theory is
based on code inspection and hopefully you can point out something that makes
this a non-issue in practice :).

The vring_interrupt() function returns IRQ_NONE when an MSI-X interrupt is
taken with no used (completed) buffers in the virtqueue. The kernel disables
the irq and the driver is no longer receives irqs when this happens:

irq 77: nobody cared (try booting with the "irqpoll" option)
[<00000000a40a49bb>] vring_interrupt
Disabling IRQ #77

Consider the following:

1. An virtiofs irq is handled and the virtio_fs_requests_done_work() work
function is scheduled to dequeue used buffers:
vring_interrupt() -> virtio_fs_vq_done() -> schedule_work()

2. The device adds more used requests and just before...

3. ...virtio_fs_requests_done_work() empties the virtqueue with

4. The device raises the irq and vring_interrupt() is called after
virtio_fs_requests_done_work emptied the virtqueue:

irqreturn_t vring_interrupt(int irq, void *_vq)
struct vring_virtqueue *vq = to_vvq(_vq);

if (!more_used(vq)) {
pr_debug("virtqueue interrupt with no work for %p\n", vq);
return IRQ_NONE;

I have included a patch that switches virtiofs from spin_lock() to
spin_lock_irqsave() to prevent vring_interrupt() from running while the
virtqueue is processed from a work function.

virtio_gpu has a similar case where virtio_gpu_dequeue_ctrl_func() and
virtio_gpu_dequeue_cursor_func() work functions only use spin_lock().
I think this can result in the same false unhandled irq problem as virtiofs.

This race condition could in theory affect all drivers. The VIRTIO
specification says:

Neither of these notification suppression methods are reliable, as they are
not synchronized with the device, but they serve as useful optimizations.

If virtqueue_disable_cb() is just a hint and might not disable virtqueue irqs
then virtio_net and other drivers have a problem because because an irq could
be raised while the driver is dequeuing used buffers. I think we haven't seen
this because software VIRTIO devices honor virtqueue_disable_cb(). Hardware
devices might cache the value and not disable notifications for some time...

Have I missed something?

The virtiofs patch I attached is being stress tested to see if the unhandled
irqs still occur.

Stefan Hajnoczi (1):
fuse: disable local irqs when processing vq completions

fs/fuse/virtio_fs.c | 15 ++++++++++-----
1 file changed, 10 insertions(+), 5 deletions(-)