RE: [RFC 17/32] eal/x86: move optimized fence out of SMP barrier

> rte_atomic_thread_fence() implemented the seq_cst case by calling the
> deprecated rte_smp_mb(). Invert the dependency: the lock add based
> fence moves into rte_atomic_thread_fence() and rte_smp_mb() becomes a
> wrapper around it, so removing the deprecated barriers later is a pure
> deletion. The optimization itself must stay; a plain seq_cst fence is
> an mfence, about twice the cost. No change in generated code.
> 
> Drop no longer used rte_smp_mb().
> 
> Signed-off-by: Stephen Hemminger <[email protected]>
> ---
>  lib/eal/x86/include/rte_atomic.h | 37 ++++++++++++++------------------
>  1 file changed, 16 insertions(+), 21 deletions(-)
> 
> diff --git a/lib/eal/x86/include/rte_atomic.h b/lib/eal/x86/include/rte_atomic.h
> index e071e4234e..780fdce871 100644
> --- a/lib/eal/x86/include/rte_atomic.h
> +++ b/lib/eal/x86/include/rte_atomic.h
> @@ -60,23 +60,8 @@ extern "C" {
>   * Basic idea is to use lock prefixed add with some dummy memory location
>   * as the destination. From their experiments 128B(2 cache lines) below
>   * current stack pointer looks like a good candidate.
> - * So below we use that technique for rte_smp_mb() implementation.
>   */
> 
> -static __rte_always_inline void
> -rte_smp_mb(void)
> -{
> -#ifdef RTE_TOOLCHAIN_MSVC
> -	_mm_mfence();
> -#else
> -#ifdef RTE_ARCH_I686
> -	asm volatile("lock addl $0, -128(%%esp); " ::: "memory");
> -#else
> -	asm volatile("lock addl $0, -128(%%rsp); " ::: "memory");
> -#endif
> -#endif
> -}
> -
>  #define rte_io_mb() rte_mb()
> 
>  #define rte_io_wmb() rte_compiler_barrier()
> @@ -86,17 +71,27 @@ rte_smp_mb(void)
>  /**
>   * Synchronization fence between threads based on the specified memory order.
>   *
> - * On x86 the __rte_atomic_thread_fence(rte_memory_order_seq_cst)
> generates full 'mfence'
> - * which is quite expensive. The optimized implementation of rte_smp_mb is
> - * used instead.
> + * On x86 the __rte_atomic_thread_fence(rte_memory_order_seq_cst)
> generates
> + * a full 'mfence' which is quite expensive. The optimized lock add on a
> + * dummy stack location (see above) is used instead.
>   */
>  static __rte_always_inline void
>  rte_atomic_thread_fence(rte_memory_order memorder)
>  {
> -	if (memorder == rte_memory_order_seq_cst)
> -		rte_smp_mb();
> -	else
> +	if (memorder != rte_memory_order_seq_cst) {
>  		__rte_atomic_thread_fence(memorder);
> +		return;
> +	}
> +
> +#ifdef RTE_TOOLCHAIN_MSVC
> +	_mm_mfence();
> +#else
> +#ifdef RTE_ARCH_I686
> +	asm volatile("lock addl $0, -128(%%esp); " ::: "memory");
> +#else
> +	asm volatile("lock addl $0, -128(%%rsp); " ::: "memory");
> +#endif
> +#endif
>  }
> 
>  #ifdef __cplusplus
> --

Acked-by: Konstantin Ananyev <[email protected]>

> 2.53.0