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

[Stefan Hajnoczi <stefanha@redhat.com>]

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)
  ...
  handlers:
  [<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
   virtqueue_get_buf().

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(-)

-- 
2.31.1