rte_atomic_*_explicit

[Tyler Retzlaff]

On Fri, Jan 26, 2024 at 11:52:11AM +0100, Morten Brørup wrote:
> > From: Mattias Rönnblom [mailto:hofors at lysator.liu.se]
> > Sent: Friday, 26 January 2024 09.07
> > 
> > On 2024-01-25 23:10, Morten Brørup wrote:
> > >> From: Mattias Rönnblom [mailto:hofors at lysator.liu.se]
> > >> Sent: Thursday, 25 January 2024 19.54
> > >>
> > >> Why do rte_stdatomic.h functions have the suffix "_explicit"?
> > >> Especially
> > >> since there aren't any wrappers for the implicit variants.
> > >>
> > >> More to type, more to read.
> > >
> > > They have the "_explicit" suffix to make their names similar to those
> > in stdatomic.h.
> > >
> > 
> > OK, so to avoid a situation where someone accidentally misinterpret
> > rte_atomic_fetch_add(&counter, 1, rte_memory_order_relaxed);
> > as what, exactly?
> > 
> > If you have a wrapper, why not take the opportunity and reap some
> > benefits from this and fix/extend the standard API, making it a better
> > fit for your application. Like removing the redundant "_explicit",
> > "fetch"-less add/sub, maybe and a function for single-writer atomic add
> > (non-atomic read + non-atomic add + atomic store), etc.
> > 
> > Prohibiting implicit operations was done already, so it's already now
> > not a straight-off copy of the standard API.
> > 
> > > You might consider their existence somewhat temporary until C11
> > stdatomics can be fully phased in, so there's another argument for
> > similar names. (This probably does not happen as long as compilers
> > generate slower code for C11 stdatomics than with their atomic built-
> > ins.)
> > >
> > 
> > To me, it seems reasonable a wrapper API should stay as long as it
> > provide a benefit over the standard API. One could imagine that being a
> > long time.
> > 
> > I imagine some DPDK developers being tired of migrating from one
> > atomics
> > API to another. <rte_atomic.h> -> GCC built-ins (-> attempted C11
> > stdatomics?) -> <rte_stdatomic.h>. Now you are adding a future "-> CXY
> > atomics" move as well, and with that it also seems natural to add a
> > change back to a wrapper or complementary API, when CXY didn't turned
> > out good enough for some particular platform, or when some non-
> > complaint
> > compiler comes along.
> 
> Yes, more migrations seem to be on the roadmap.
> 
> We can take the opportunity to change direction now, and decide to keep the <rte_stdatomic.h> API long term.
> Then it would need more documentation (basically copying function descriptions from <stdatomic.h>), and the functions could have the "_explicit" suffix removed (macros with the suffix could be added for backwards compatibility), and more functions - like the ones you suggested above - could be added.
> 
> What do people think?
> 1. Keep considering <rte_stdatomic.h> a temporary wrapper for <stdatomic.h> until compilers reach some undefined level of maturity, or
> 2. Consider <rte_stdatomic.h> stable, clean it up (remove "_explicit" suffix), add documentation to the macros, and extend it.
> 
> Living in a DPDK-only environment, I would prefer option 2; but if mixing DPDK code with non-DPDK code (that uses <stdatomic.h>) it might be weird.

rte_stdatomic.h should be considered temporary, but how long temporary
is depends on when we can deprecate support for distributions and the
older toolchains they are tied to.

the macros were introduced to allow a path to gradually moving to
standard c11 atomics. gcc versions available on distributions we
promise support for is currently the biggest barrier to direct
adoption.

    * older versions of gcc generate sub-optimal code when using c11
      atomic generics in some instances.

    * gcc c++23 support is incomplete, in particular at the time we
      evaluated using the c11 atomics directly but gcc c++23 held us
      back because the stdatomic.h it shipped didn't interoperate.

    * the macros allow developers to easily evaluate/compare with and
      without standard C atomics with a single build-time option
      -Denable_stdatomic=true

    * eventually we will want to move directly to the names from the
      standard, arguably just search and replace of rte_atomic with
      atomic is the most mechanically trivial - hence there is some
      value in keeping _explicit suffix.

> > 
> > I suggested fixing the original <rte_atomic.h> API, or at least have a
> > wrapper API, already at the point DPDK moved to direct GCC built-in
> > calls. Then we wouldn't have had this atomics API ping-pong.
> 
> The decision back then might have been too hasty, and based on incomplete assumptions.
> Please shout louder next time you think a mistake is in the making.
> 
> > 
> > >>
> > >> When was this API introduced? Shouldn't it say "experimental"
> > >> somewhere?
> > >
> > > They were introduced as part of the migration to C11.
> > > I suppose they were not marked experimental because they replaced
> > something we didn't want anymore (the compiler built-ins for atomics,
> > e.g. __atomic_load_n()). I don't recall if we discussed experimental
> > marking or not.
> 
> In hindsight, they should probably have been marked "experimental".

i'm not sure i feel strongly that they need to be marked experimental
and by marked i assume we're only talking about placing a comment in a
file rather than __rte_experimental which has no application here.

typically we do that when we may want to change or remove the api without
going through the normal deprecation and removal process. for these
macros it is difficult to imagine why we would change them as that would
only cause them to deviate from the signature or behavior of the
standard C generics... why would we do that if our intention is to
eventually fully migrate to direct use of the standard C names?

ty