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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Aug 24, 2021 at 07:31:29AM -0400, Michael S. Tsirkin wrote:
> 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@redhat.com>
> 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.

A new kernel with your patch doesn't have this issue.

Please disregard the patch I posted, your patch seems to be enough.

Stefan

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