Re: [PATCH] net/vhost: fix segfault when creating vdev dynamically

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 03/27/2018 04:01 PM, Tan, Jianfeng wrote:
> 
> 
> On 3/27/2018 7:28 PM, Maxime Coquelin wrote:
>>
>>
>> On 03/27/2018 11:42 AM, Tan, Jianfeng wrote:
>>>
>>>
>>> On 3/27/2018 5:24 PM, Chen, Junjie J wrote:
>>>>>>> On 3/28/2018 12:05 AM, Junjie Chen wrote:
>>>>>>>> when creating vdev dynamically, vhost pmd driver start directly
>>>>>>>> without checking TX/RX queues ready or not, and thus cause
>>>>>>>> segmentation fault when vhost library accessing queues. This patch
>>>>>>>> add flag to check whether queues setup or not, and add driver start
>>>>>>>> call into dev_start to allow user start it after setting up queue.
>>>>>>> The issue is clear now. But this patch just puts the situation 
>>>>>>> before below
>>>>> fix:
>>>>>>> "it doesn't create the actual datagram socket until you call 
>>>>>>> .dev_start()."
>>>>>> No, if the queue exist, the datagram socket still get created in 
>>>>>> vhost_create
>>>>> API, since the vhost_driver_register still exist in vhost_create.
>>>>>
>>>>> The queue can never be created, as it's still not probed.
>>>> I think we need to separate this into two cases:
>>>>     Statically create vdev, the datagram recreate logical is still 
>>>> there since queues are exist already, this patch doesn't change 
>>>> anything.
>>>>     Dynamic create vdev, as you pointed out, queue can never be 
>>>> created, while this should be not valid since In normal process of 
>>>> creating vdev dynamically, we always need to config queues. Correct 
>>>> me if I'm wrong.
>>>
>>> My point is, either vdev is created statically or dynamically, when 
>>> probe(), queues are not setup yet definitely, then *the unix socket 
>>> will not be created* until we set up the queues and do dev_start(). 
>>> If the unix socket is not created, then VM cannot connect to it.
>>
>> FYI, I think I reproduced such an issue with the vdev statically created
>> in the past, while doing some experiments. I didn't went further into 
>> the analysis at that time, but it looks like the issue Junjie is trying
>> to address with this patch for dynamically created vdev.
> 
> Yes, I have noticed that this issue mostly happens at dynamic case. Just 
> try to suggest a proper way to fix. Please check if my suggestion in 
> another email makes sense.

Yes, it makes sense, that's the right thing to do I think.

Thanks,
Maxime
> 
> Thanks,
> Jianfeng
> 
>>
>> Cheers,
>> Maxime
>