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Date:   Wed, 22 May 2019 11:44:26 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>
Cc:     netdev@...r.kernel.org, Stefan Hajnoczi <stefanha@...hat.com>
Subject: Re: Question about IRQs during the .remove() of virtio-vsock driver


On 2019/5/21 下午9:56, Michael S. Tsirkin wrote:
> On Tue, May 21, 2019 at 03:49:20PM +0200, Stefano Garzarella wrote:
>> 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
>
> instead of rcu tricks I would just have rx_run and tx_run and check it
> within the queued work - presumably under tx or rx lock.
>
> then queueing an extra work becomes harmless,
> and you flush it after del vqs which flushes everything for you.
>
>

It looks to me that we need guarantee no work queued or scheduled before 
del_vqs. Otherwise it may lead use after free? (E.g net disable NAPI 
before del_vqs).

Thanks

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