[RFC 2/7] eal: reimplement rte_smp_*mb with rte_atomic_thread_fence

[Wathsala Vithanage]

Hi Stephen,

Suggesting minor changes to comments on acquire and
release fences..

> +/** @name SMP Memory Barrier
> + */
> +///@{
> +/**
> + * General memory barrier between lcores
> + *
> + * Guarantees that the LOAD and STORE operations that precede the
> + * rte_smp_mb() call are globally visible across the lcores
> + * before the LOAD and STORE operations that follows it.
> + */
> +static __rte_always_inline void
> +rte_smp_mb(void)
> +{
> +	rte_atomic_thread_fence(rte_memory_order_seq_cst);
> +}
> +
> +/**
> + * Write memory barrier between lcores
> + *
> + * Guarantees that the STORE operations that precede the
> + * rte_smp_wmb() call are globally visible across the lcores
> + * before the STORE operations that follows it.
> + */
> +static __rte_always_inline void
> +rte_smp_wmb(void)
> +{
> +	rte_atomic_thread_fence(rte_memory_order_release);
> +}

Release fences order STORE | STORE, and  LOAD | STORE.
Therefor, the comment should be "Guarantees that LOAD
and STORE operations that precede the rte_smp_wmb() call
are globally observed before STORE operations that follows it."

> +
> +/**
> + * Read memory barrier between lcores
> + *
> + * Guarantees that the LOAD operations that precede the
> + * rte_smp_rmb() call are globally visible across the lcores
> + * before the LOAD operations that follows it.
> + */
> +static __rte_always_inline void
> +rte_smp_rmb(void)
> +{
> +	rte_atomic_thread_fence(rte_memory_order_acquire);
> +}

Acquire fences order LOAD | LOAD and LOAD | STORE.
Thus, the comment should be "Guarantees that the LOAD
operations that precede the rte_smp_rmb() call observe
global  state before LOAD and STORE operations that
follows it"


--wathsala