Re: [PATCH v3 01/29] bpf: replace deprecated SMP barriers with C11 fences

[Wathsala Vithanage <wathsala.vithanage@arm.com>]

On 10/9/26 10:48, Stephen Hemminger wrote:
> On Fri, 9 Oct 2026 01:05:56 -0500
> Wathsala Vithanage <wathsala.vithanage@arm.com> wrote:
>
>>>    /*
>>>     * Marks given callback as used by datapath.
>>>     */
>>>    static __rte_always_inline void
>>>    bpf_eth_cbi_inuse(struct bpf_eth_cbi *cbi)
>>>    {
>>> -	cbi->use++;
>>> -	/* make sure no store/load reordering could happen */
>>> -	rte_smp_mb();
>>> +	rte_atomic_store_explicit(&cbi->use,
>>> +		rte_atomic_load_explicit(&cbi->use, rte_memory_order_relaxed) + 1,
>>> +		rte_memory_order_relaxed);
>>> +
>>> +	/* full barrier: count must be visible before cb is read */
>>> +	rte_atomic_thread_fence(rte_memory_order_seq_cst);
>> This is correct but why use a barrier when the above store itself could
>> be SEQ_CST that
>> synchronizes with load in bpf_eth_cbi_wait?
> There are two different things be covered for safety here.
> The counter and the callback pointer. The counter uses atomic
> operations and the callback pointer is referenced without atomic.
> AI explains it as:
>
> A seq_cst store on the counter is not enough. This is a store/load
> pattern across two objects:
>
>    datapath: store use, load cb
>    unload:   store cb,  load use
>
> At least one side has to see the other's store. Making the store
> of use seq_cst does not order the datapath's later load of cb,
> which is a plain load. On arm64 that compiles to stlr followed by
> ldr, and the ldr can be satisfied before the stlr is visible.
> Only stlr/ldar pairs are kept in order.
>
> It can be done without fences by making cb atomic and using
> seq_cst for all four accesses (stlr + ldar on arm64). That touches
> every callback and is a bigger change than replacing the barriers,
> so I would rather do it as a follow-up if at all.
>
> There is no cost difference on x86. A seq_cst store is xchg and
> the fence is lock addl, one locked instruction per burst either
> way.


Agreed, barrier is the better implementation than converting
everything into seq_cst.

Reviewed-by: Wathsala Vithanage <wathsala.vithanage@arm.com>

--wathsala