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

[Johannes Berg]

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.

johannes