[PATCH v2] eal: add notes to SMP memory barrier APIs

[Ruifeng Wang]

> -----Original Message-----
> From: Mattias Rönnblom <hofors at lysator.liu.se>
> Sent: Friday, June 30, 2023 3:44 AM
> To: Ruifeng Wang <Ruifeng.Wang at arm.com>; thomas at monjalon.net; david.marchand at redhat.com
> Cc: dev at dpdk.org; roretzla at linux.microsoft.com; konstantin.v.ananyev at yandex.ru; Honnappa
> Nagarahalli <Honnappa.Nagarahalli at arm.com>; nd <nd at arm.com>
> Subject: Re: [PATCH v2] eal: add notes to SMP memory barrier APIs
> 
> On 2023-06-26 09:12, 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>
> > ---
> > v2: Made the notes more specific.
> >
> >   lib/eal/include/generic/rte_atomic.h | 19 +++++++++++++++++++
> >   1 file changed, 19 insertions(+)
> >
> > diff --git a/lib/eal/include/generic/rte_atomic.h
> > b/lib/eal/include/generic/rte_atomic.h
> > index 58df843c54..35e0041ce6 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 provides fence synchronization
> > + *  primitive but doesn't take memory order parameter.
> > + *  rte_atomic_thread_fence() should be used instead.
> 
> I can't see why coding the memory model semantics into the name, rather than by
> specification-by-means-of-a-parameter, could be the real issue.
> Could you explain? Seems like just different syntax to me.

Yes, rte_smp_xx and rte_atomic_thread_fence have different syntaxes.

The compiler atomic builtins were accepted for memory ordering. It comprises atomic arithmetic,
atomic load/store, and atomic fence. It is simpler and clearer to do memory ordering by using
the atomic builtins whenever possible.
rte_smp_xx has functionality overlap with atomic fence builtins but with different memory model
semantics and different syntaxes. Because of the differences, it will make memory ordering a little
more complex if rte_smp_xx is kept aside atomic builtins suite.

> 
> The old <rte_atomic.h> atomic arithmetic and atomic load/store operations suffered from
> unspecified semantics in regards to any ordering they imposed on other memory accesses. I
> guess that shortcoming could be described as a "missing parameter", although that too
> would be misleading. Unclear semantics seems not be the case for the kernel-style barriers
> though.
> 
> >    */
> >   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 provides fence synchronization
> > + *  primitive but doesn't take memory order parameter.
> > + *  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 provides fence synchronization
> > + *  primitive but doesn't take memory order parameter.
> > + *  rte_atomic_thread_fence() should be used instead.
> >    */
> >   static inline void rte_smp_rmb(void);
> >   ///@}
> > @@ -122,6 +137,10 @@ static inline void rte_io_rmb(void);
> >
> >   /**
> >    * Synchronization fence between threads based on the specified memory order.
> > + *
> > + * @param memorder
> > + *   The memory order defined by compiler atomic builtin at:
> > + *   https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
> >    */
> >   static inline void rte_atomic_thread_fence(int memorder);
> >