Re: [dpdk-dev] [PATCH v3 0/6] use IOVAs check based on DMA mask

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 29-Oct-18 2:18 PM, Thomas Monjalon wrote:
> 29/10/2018 14:40, Alejandro Lucero:
>> On Mon, Oct 29, 2018 at 1:18 PM Yao, Lei A <lei.a.yao@intel.com> wrote:
>>> *From:* Alejandro Lucero [mailto:alejandro.lucero@netronome.com]
>>> On Mon, Oct 29, 2018 at 11:46 AM Thomas Monjalon <thomas@monjalon.net>
>>> wrote:
>>>
>>> 29/10/2018 12:39, Alejandro Lucero:
>>>> I got a patch that solves a bug when calling rte_eal_dma_mask using the
>>>> mask instead of the maskbits. However, this does not solves the
>>> deadlock.
>>>
>>> The deadlock is a bigger concern I think.
>>>
>>> I think once the call to rte_eal_check_dma_mask uses the maskbits instead
>>> of the mask, calling rte_memseg_walk_thread_unsafe avoids the deadlock.
>>>
>>> Yao, can you try with the attached patch?
>>>
>>> Hi, Lucero
>>>
>>> This patch can fix the issue at my side. Thanks a lot
>>> for you quick action.
>>
>> Great!
>>
>> I will send an official patch with the changes.
> 
> Please, do not forget my other request to better comment functions.
> 
> 
>> I have to say that I tested the patchset, but I think it was where
>> legacy_mem was still there and therefore dynamic memory allocation code not
>> used during memory initialization.
>>
>> There is something that concerns me though. Using
>> rte_memseg_walk_thread_unsafe could be a problem under some situations
>> although those situations being unlikely.
>>
>> Usually, calling rte_eal_check_dma_mask happens during initialization. Then
>> it is safe to use the unsafe function for walking memsegs, but with device
>> hotplug and dynamic memory allocation, there exists a potential race
>> condition when the primary process is allocating more memory and
>> concurrently a device is hotplugged and a secondary process does the device
>> initialization. By now, this is just a problem with the NFP, and the
>> potential race condition window really unlikely, but I will work on this
>> asap.
> 
> Yes, this is what concerns me.
> You can add a comment explaining the unsafe which is not handled.

The issue here is that this code is called from both memory-locked and 
memory-unlocked context. Virtio had a similar issue with their mem table 
update code - they solved it by manually locking the memory before doing 
everything else, and using thread_unsafe version of the walk.

Could something like that be done here?

> 
> 
>>>> Interestingly, the problem looks like a compiler one. Calling
>>>> rte_memseg_walk does not return when calling inside rt_eal_dma_mask,
>>> but if
>>>> you modify the call like this:
>>>>
>>>> -       if (rte_memseg_walk(check_iova, &mask))
>>>> +       if (!rte_memseg_walk(check_iova, &mask))
>>>>
>>>> it works, although the value returned to the invoker changes, of course.
>>>> But the point here is it should be the same behaviour when calling
>>>> rte_memseg_walk than before and it is not.
>>>
>>> Anyway, the coding style requires to save the return value in a variable,
>>> instead of nesting the call in an "if" condition.
>>> And the "if" check should be explicitly != 0 because it is not a real
>>> boolean.
>>>
>>> PS: please do not top post and avoid HTML emails, thanks
>>>
>>>
>>
> 
> 
> 
> 
> 
> 


-- 
Thanks,
Anatoly