Re: [PATCH net-next v2 2/2] ptr_ring: make __ptr_ring_empty() checking more reliable

[Yunsheng Lin]

On 2021/6/27 14:03, Michael S. Tsirkin wrote:
>>>>>
>>>>>
>>>>> So if now we need this to be reliable then
>>>>> we also need smp_wmb before writing r->queue[consumer_head],
>>>>> there could be other gotchas.
>>>>
>>>> Yes, This patch does not make it strictly reliable.
>>>> T think I could mention that in the commit log?
>>>
>>> OK so it's not that it makes it more reliable - this patch simply makes
>>> a possible false positive less likely while making  a false negative
>>> more likely. Our assumption is that a false negative is cheaper then?
>>>
>>> How do we know that it is?
>>>
>>> And even if we prove the ptr_ring itself is faster now,
>>> how do we know what affects callers in a better way a
>>> false positive or a false negative?
>>>
>>> I would rather we worked on actually making it reliable
>>> e.g. if we can guarantee no false positives, that would be
>>> a net win.
>> I thought deeper about the case you mentioned above, it
>> seems for the above to happen, the consumer_head need to
>> be rolled back to zero and incremented to the point when
>> caller of __ptr_ring_empty() is still *not* able to see the
>> r->queue[] which has been set to NULL in __ptr_ring_discard_one().
>>
>> It seems smp_wmb() only need to be done once when consumer_head
>> is rolled back to zero, and maybe that is enough to make sure the
>> case you mentioned is fixed too?
>>
>> And the smp_wmb() is only done once in a round of producing/
>> consuming, so the performance impact should be minimized?(of
>> course we need to test it too).
> 
> 
> Sorry I don't really understand the question here.
> I think I agree it's enough to do one smp_wmb between
> the write of r->queue and write of consumer_head
> to help guarantee no false positives.
> What other code changes are necessary I can't yet say
> without more a deeper code review.
> 

Ok, thanks for the reviewing.
Will add handling the case you mentioned above in V3 if there
is no noticable performanc impact for handling the above case.