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

[Ruifeng Wang]

> -----Original Message-----
> From: Mattias Rönnblom <hofors at lysator.liu.se>
> Sent: Friday, June 23, 2023 2:20 AM
> To: Ruifeng Wang <Ruifeng.Wang at arm.com>; thomas at monjalon.net; david.marchand at redhat.com
> Cc: dev at dpdk.org; konstantin.v.ananyev at yandex.ru; Honnappa Nagarahalli
> <Honnappa.Nagarahalli at arm.com>; nd <nd at arm.com>
> Subject: Re: [PATCH] eal: add notes to SMP memory barrier APIs
> 
> 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 should say "memory order semantics". It will be more specific.
The wrapper function rte_atomic_thread_fence is a special case. It provides an optimized implementation
for __ATOMIC_SEQ_CST for x86:
https://www.dpdk.org/blog/2021/03/26/dpdk-adopts-the-c11-memory-model/

> 
> 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.

As compiler atomic built-ins are being used. The memory model parameters should be the ones listed in:
https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
We are not taking Linux kernel-style barriers. So no need to mention that.

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

Yes, warnings will come up. Some occurrences still remain in the project. 

> 
> >    */
> >   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);
> >   ///@}