[PATCH] eal: add notes to SMP memory barrier APIs

[Mattias Rönnblom]

On 2023-06-21 08:44, Ruifeng Wang wrote:
> The rte_smp_xx() APIs are deprecated. But it is not mentioned
> in the function header.
> Added notes in function header for clarification.
> 
> Signed-off-by: Ruifeng Wang <ruifeng.wang at arm.com>
> ---
>   lib/eal/include/generic/rte_atomic.h | 15 +++++++++++++++
>   1 file changed, 15 insertions(+)
> 
> diff --git a/lib/eal/include/generic/rte_atomic.h b/lib/eal/include/generic/rte_atomic.h
> index 58df843c54..542a2c16ff 100644
> --- a/lib/eal/include/generic/rte_atomic.h
> +++ b/lib/eal/include/generic/rte_atomic.h
> @@ -55,6 +55,11 @@ static inline void rte_rmb(void);
>    * 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.
> + *
> + * @note
> + *  This function is deprecated. It adds complexity to the memory model
> + *  used by this project. C11 memory model should always be used.
> + *  rte_atomic_thread_fence() should be used instead.

It's somewhat confusing to learn I should use the C11 memory model, and 
then in the next sentence that I should call a function which is not in C11.

I think it would be helpful to say which memory_model parameters should 
be used to replace the rte_smp_*mb() calls, and if there are any 
difference in semantics between the Linux kernel-style barriers and 
their C11 (near-)equivalents.

Is there some particular reason these functions aren't marked 
__rte_deprecated? Too many warnings?

>    */
>   static inline void rte_smp_mb(void);
>   
> @@ -64,6 +69,11 @@ static inline void rte_smp_mb(void);
>    * 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.
> + *
> + * @note
> + *  This function is deprecated. It adds complexity to the memory model
> + *  used by this project. C11 memory model should always be used.
> + *  rte_atomic_thread_fence() should be used instead.
>    */
>   static inline void rte_smp_wmb(void);
>   
> @@ -73,6 +83,11 @@ static inline void rte_smp_wmb(void);
>    * 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.
> + *
> + * @note
> + *  This function is deprecated. It adds complexity to the memory model
> + *  used by this project. C11 memory model should always be used.
> + *  rte_atomic_thread_fence() should be used instead.
>    */
>   static inline void rte_smp_rmb(void);
>   ///@}