Re: [PATCH v2 5/9] bpf: simplify callback inuse tracking to boolean flag

[Marat Khalili <qm2k21@gmail.com>]

On 25/10/2026 10:50, Konstantin Ananyev wrote:
>>>        /* in use, busy wait till current RX/TX iteration is finished */
>>>         if ((puse & BPF_ETH_CBI_INUSE) != 0) {
>>>                 RTE_WAIT_UNTIL_MASKED((__rte_atomic uint32_t 
>>> *)(uintptr_t)&cbi->use,
>>>                         UINT32_MAX, !=, puse, 
>>> rte_memory_order_acquire);
>>>         }
>>>
>>> Here, we are waiting when 'puse' will become not equal to 'cbi->use'.
>>> So, any value except current puse is good.
>>
>> If you look a the sequence diagram, there is no way to arrive here 
>> without the callback being removed and `cb` cleared in the DP thread, 
>> no matter how you move the arrows around.  So the DP thread will 
>> increment `use` once to make it even, and then will never ever reach 
>> the callback or increment anything again. We allow any value except 
>> current puse, but there are never any values except the current puse 
>> and the one after it. It's just these two values we can possibly see.
>
> DP can (and most likely will) call rx/tx_burst again.
> Just to give you an example:
> --- DP thread:
> cbi->use = 1;
>                          --- CP thread:
>                             read cbi->use (1)
By this time the callback is removed and `cb` is cleared.
> --DP thread:
> cbi->use = 2;
> ...
> next rx burst:
> cbi->use = 3;
How did we reach hear if `cb` is NULL?
>                 read cbi->use (3)
>
> So CP sees an odd value (3), but it is ok to proceed, as previoud RX 
> iteration (quiscent period) had already finished,
> and in new one rx_burst will not see removed call-back.