Re: [PATCH v3 12/29] ring: replace SMP read barrier with C11 acquire fence

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

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

On 10/8/26 18:35, Stephen Hemminger wrote:
> rte_smp_rmb() is deprecated. This code is only used where
> RTE_USE_C11_MEM_MODEL is unset: x86, ppc64, loongarch and ThunderX.
> The acquire fence generates the same code on x86 (a compiler
> barrier) and on ThunderX (dmb ishld). On ppc64 the barrier becomes
> lwsync instead of sync, which still orders load against load.
>
> The gcc implementation itself is kept: unlike the lock-free stack,
> the in-tree comment records a 10% x86 performance drop with the
> C11 version, which needs re-measuring with current compilers
> before RTE_USE_C11_MEM_MODEL can be removed.
>
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> Acked-by: Konstantin Ananyev <konstantin.ananyev@huawei.com>
> Acked-by: Morten Brørup <mb@smartsharesystems.com>
> ---
>   lib/ring/rte_ring_gcc_pvt.h | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/lib/ring/rte_ring_gcc_pvt.h b/lib/ring/rte_ring_gcc_pvt.h
> index 340ece28c7..9c2c58824d 100644
> --- a/lib/ring/rte_ring_gcc_pvt.h
> +++ b/lib/ring/rte_ring_gcc_pvt.h
> @@ -55,10 +55,10 @@ __rte_ring_headtail_move_head_st(struct rte_ring_headtail *d,
>   
>   	*old_head = d->head;
>   
> -	/* add rmb barrier to avoid load/load reorder in weak
> -	 * memory model. It is noop on x86
> +	/* Acquire fence to avoid load/load reorder in weak
> +	 * memory model. It is noop on x86.
>   	 */
> -	rte_smp_rmb();
> +	rte_atomic_thread_fence(rte_memory_order_acquire);
>   
>   	/*
>   	 *  The subtraction is done between two unsigned 32bits value