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

From: Michael S. Tsirkin
Date: Tue Aug 24 2021 - 07:31:42 EST

On Tue, Aug 24, 2021 at 11:59:43AM +0100, Stefan Hajnoczi wrote:
> 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(-)

Fundamentally it is not a problem to have an unhandled IRQ
once in a while. It's only a problem if this happens time
after time.

Does the kernel you are testing include
commit 8d622d21d24803408b256d96463eac4574dcf067
Author: Michael S. Tsirkin <mst@xxxxxxxxxx>
Date: Tue Apr 13 01:19:16 2021 -0400

virtio: fix up virtio_disable_cb


If not it's worth checking whether the latest kernel still
has the issue.

Note cherry picking that commit isn't trivial there are
a bunch of dependencies.

If you want to try on an old kernel, disable event index.

> --
> 2.31.1