[PATCH v2] um: vector: Replace locks guarding queue depth with atomics

[Anton Ivanov]

On 04/07/2024 23:55, Johannes Berg wrote:
> On Thu, 2024-07-04 at 21:16 +0100, Anton Ivanov wrote:
>
>>>> -	qi->queue_depth = 0;
>>>> +	wmb(); /* Ensure that RX processing elsewhere sees the changes */
>>>> +	atomic_set(&qi->queue_depth, 0);
>>>>    }
>>> I don't understand this.
>>>
>>> prep_queue_for_rx() is called by vector_mmsg_rx(), not in parallel or in
>>> a different thread or something, inside the NAPI polling. All it does is
>>> reset the queue to empty with all the SKBs allocated.
>>>
>>> After that, prep_queue_for_rx() calls uml_vector_recvmmsg() [1] which
>>> fills the SKBs, and then vector_mmsg_rx() itself consumes them by going
>>> over it and calling napi_gro_receive() (or whatever else).
>>>
>>> There's no parallelism here? The RX queue wouldn't even need locking or
>>> atomic instructions at all at this point, since it's just "refill
>>> buffers, fill buffers, release buffers to network stack".
>>>
>>> What am I missing?
>> You are not missing anything.
>>
>> The rx and tx are using the same infra to map vectors to skbs arrays.
>>
>> I can make the RX use a set of lockless and atomicless functions, but
>> this means duplicating some of the code.
> Right. No need to duplicate I'd say, but I don't see then a need to add
> a wmb() barrier here? It's confusing me because there's nothing
> accessing it in parallel.

Ack. Will clean it up for v3.

>
> johannes
>

-- 
Anton R. Ivanov
https://www.kot-begemot.co.uk/