Re: Question about IRQs during the .remove() of virtio-vsock driver

[Stefano Garzarella <sgarzare@redhat.com>]

On Tue, May 21, 2019 at 06:05:31AM -0400, Michael S. Tsirkin wrote:
> On Tue, May 21, 2019 at 11:44:07AM +0200, Stefano Garzarella wrote:
> > Hi Micheal, Jason,
> > as suggested by Stefan, I'm checking if we have some races in the
> > virtio-vsock driver. We found some races in the .probe() and .remove()
> > with the upper layer (socket) and I'll fix it.
> > 
> > Now my attention is on the bottom layer (virtio device) and my question is:
> > during the .remove() of virtio-vsock driver (virtio_vsock_remove), could happen
> > that an IRQ comes and one of our callback (e.g. virtio_vsock_rx_done()) is
> > executed, queueing new works?
> > 
> > I tried to follow the code in both cases (device unplugged or module removed)
> > and maybe it couldn't happen because we remove it from bus's knowledge,
> > but I'm not sure and your advice would be very helpful.
> > 
> > Thanks in advance,
> > Stefano
> 
> 
> Great question! This should be better documented: patches welcome!

When I'm clear, I'll be happy to document this.

> 
> Here's my understanding:
> 
> 
> A typical removal flow works like this:
> 
> - prevent linux from sending new kick requests to device
>   and flush such outstanding requests if any
>   (device can still send notifications to linux)
> 
> - call
>           vi->vdev->config->reset(vi->vdev);
>   this will flush all device writes and interrupts.
>   device will not use any more buffers.
>   previously outstanding callbacks might still be active.
> 
> - Then call
>           vdev->config->del_vqs(vdev);
>   to flush outstanding callbacks if any.

Thanks for sharing these useful information.

So, IIUC between step 1 (e.g. in virtio-vsock we flush all work-queues) and
step 2, new IRQs could happen, and in the virtio-vsock driver new work
will be queued.

In order to handle this case, I'm thinking to add a new variable
'work_enabled' in the struct virtio_vsock, put it to false at the start
of the .remove(), then call synchronize_rcu() before to flush all work
queues and use an helper function virtio_transport_queue_work() to queue
a new work, where the check of work_enabled and the queue_work are in the
RCU read critical section.

Here a pseudo code to explain better the idea:

virtio_vsock_remove() {
    vsock->work_enabled = false;

    /* Wait for other CPUs to finish to queue works */
    synchronize_rcu();

    flush_works();

    vdev->config->reset(vdev);

    ...

    vdev->config->del_vqs(vdev);
}

virtio_vsock_queue_work(vsock, work) {
    rcu_read_lock();

    if (!vsock->work_enabled) {
        goto out;
    }

    queue_work(virtio_vsock_workqueue, work);

out:
    rcu_read_unlock();
}


Do you think can work?
Please tell me if there is a better way to handle this case.

Thanks,
Stefano