Re: [dpdk-dev] [PATCH] lib: move rte_ring read barrier to correct location

[Olivier Matz <olivier.matz@6wind.com>]

Hi,

On 07/11/2016 01:22 PM, Kuusisaari, Juhamatti wrote:
> 
> Hi,
>  
>>> -----Original Message-----
>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Juhamatti
>>> Kuusisaari
>>> Sent: Monday, July 11, 2016 11:21 AM
>>> To: dev@dpdk.org
>>> Subject: [dpdk-dev] [PATCH] lib: move rte_ring read barrier to correct
>>> location
>>>
>>> Fix the location of the rte_ring data dependency read barrier.
>>> It needs to be called before accessing indexed data to ensure that the
>>> data itself is guaranteed to be correctly updated.
>>>
>>> See more details at kernel/Documentation/memory-barriers.txt
>>> section 'Data dependency barriers'.
>>
>>
>> Any explanation why?
>> From my point smp_rmb()s are on the proper places here :) Konstantin
> 
> The problem here is that on a weak memory model system the CPU is
> allowed to load the address data out-of-order in advance.
> If the read barrier is after the DEQUEUE, you might end up having the old 
> data there on a race situation when the buffer is continuously full.
> Having it before the DEQUEUE guarantees that the load is not done
> in advance. 
> 
> On Intel, it should not matter due to different memory model, so this is 
> limited to weak memory model systems.


I agree with Juhamatti. To me, the reading of consumer_head must occur
before the reading of objects ptrs.

That was the case before, and this is something I already noticed when I
sent that mail:
http://dpdk.org/ml/archives/dev/2014-March/001742.html

At that time, only Intel CPUs were supported, so it did not make any
difference.

Juhamatti, do you have a setup where you can trigger the issue or is it
something you've seen by code review?

Thanks,
Olivier