Re: [PATCH v2 5/5] vhost: add device started check in migration set log

[Jason Wang <jasowang@redhat.com>]

On 2020/5/12 下午5:35, Dima Stepanov wrote:
> On Tue, May 12, 2020 at 11:32:50AM +0800, Jason Wang wrote:
>> On 2020/5/11 下午5:25, Dima Stepanov wrote:
>>> On Mon, May 11, 2020 at 11:15:53AM +0800, Jason Wang wrote:
>>>> On 2020/4/30 下午9:36, Dima Stepanov wrote:
>>>>> If vhost-user daemon is used as a backend for the vhost device, then we
>>>>> should consider a possibility of disconnect at any moment. If such
>>>>> disconnect happened in the vhost_migration_log() routine the vhost
>>>>> device structure will be clean up.
>>>>> At the start of the vhost_migration_log() function there is a check:
>>>>>    if (!dev->started) {
>>>>>        dev->log_enabled = enable;
>>>>>        return 0;
>>>>>    }
>>>>> To be consistent with this check add the same check after calling the
>>>>> vhost_dev_set_log() routine. This in general help not to break a
>>>>> migration due the assert() message. But it looks like that this code
>>>>> should be revised to handle these errors more carefully.
>>>>>
>>>>> In case of vhost-user device backend the fail paths should consider the
>>>>> state of the device. In this case we should skip some function calls
>>>>> during rollback on the error paths, so not to get the NULL dereference
>>>>> errors.
>>>>>
>>>>> Signed-off-by: Dima Stepanov<dimastep@yandex-team.ru>
>>>>> ---
>>>>>   hw/virtio/vhost.c | 39 +++++++++++++++++++++++++++++++++++----
>>>>>   1 file changed, 35 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
>>>>> index 3ee50c4..d5ab96d 100644
>>>>> --- a/hw/virtio/vhost.c
>>>>> +++ b/hw/virtio/vhost.c
>>>>> @@ -787,6 +787,17 @@ static int vhost_dev_set_features(struct vhost_dev *dev,
>>>>>   static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
>>>>>   {
>>>>>       int r, i, idx;
>>>>> +
>>>>> +    if (!dev->started) {
>>>>> +        /*
>>>>> +         * If vhost-user daemon is used as a backend for the
>>>>> +         * device and the connection is broken, then the vhost_dev
>>>>> +         * structure will be reset all its values to 0.
>>>>> +         * Add additional check for the device state.
>>>>> +         */
>>>>> +        return -1;
>>>>> +    }
>>>>> +
>>>>>       r = vhost_dev_set_features(dev, enable_log);
>>>>>       if (r < 0) {
>>>>>           goto err_features;
>>>>> @@ -801,12 +812,19 @@ static int vhost_dev_set_log(struct vhost_dev *dev, bool enable_log)
>>>>>       }
>>>>>       return 0;
>>>>>   err_vq:
>>>>> -    for (; i >= 0; --i) {
>>>>> +    /*
>>>>> +     * Disconnect with the vhost-user daemon can lead to the
>>>>> +     * vhost_dev_cleanup() call which will clean up vhost_dev
>>>>> +     * structure.
>>>>> +     */
>>>>> +    for (; dev->started && (i >= 0); --i) {
>>>>>           idx = dev->vhost_ops->vhost_get_vq_index(
>>>> Why need the check of dev->started here, can started be modified outside
>>>> mainloop? If yes, I don't get the check of !dev->started in the beginning of
>>>> this function.
>>>>
>>> No dev->started can't change outside the mainloop. The main problem is
>>> only for the vhost_user_blk daemon. Consider the case when we
>>> successfully pass the dev->started check at the beginning of the
>>> function, but after it we hit the disconnect on the next call on the
>>> second or third iteration:
>>>       r = vhost_virtqueue_set_addr(dev, dev->vqs + i, idx, enable_log);
>>> The unix socket backend device will call the disconnect routine for this
>>> device and reset the structure. So the structure will be reset (and
>>> dev->started set to false) inside this set_addr() call.
>> I still don't get here. I think the disconnect can not happen in the middle
>> of vhost_dev_set_log() since both of them were running in mainloop. And even
>> if it can, we probably need other synchronization mechanism other than
>> simple check here.
> Disconnect isn't happened in the separate thread it is happened in this
> routine inside vhost_dev_set_log. When for instance vhost_user_write()
> call failed:
>    vhost_user_set_log_base()
>      vhost_user_write()
>        vhost_user_blk_disconnect()
>          vhost_dev_cleanup()
>            vhost_user_backend_cleanup()
> So the point is that if we somehow got a disconnect with the
> vhost-user-blk daemon before the vhost_user_write() call then it will
> continue clean up by running vhost_user_blk_disconnect() function. I
> wrote a more detailed backtrace stack in the separate thread, which is
> pretty similar to what we have here:
>    Re: [PATCH v2 4/5] vhost: check vring address before calling unmap
> The places are different but the problem is pretty similar.


Yes.


>
> So if vhost-user commands handshake then everything is fine and
> reconnect will work as expected. The only problem is how to handle
> reconnect properly between vhost-user command send/receive.
>
> As i wrote we have a test:
>    - run src VM with vhost-usr-blk daemon used
>    - run fio inside it
>    - perform reconnect every X seconds (just kill and restart daemon),
>      X is random
>    - run dst VM
>    - perform migration
>    - fio should complete in dst VM
> And we cycle this test like forever.
> So it fails once per ~25 iteration. By adding some delays inside qemu we
> were able to make the race window larger.


It would be better if we can draft some qtest for this.

Thanks


>