Re: [dpdk-dev] atomic operations

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

<snip>

> 
> In the deprecation notices of DPDK 21.05, we can still read this:
> "
> * rte_atomicNN_xxx: These APIs do not take memory order parameter. This
> does
>   not allow for writing optimized code for all the CPU architectures supported
>   in DPDK. DPDK will adopt C11 atomic operations semantics and provide
> wrappers
>   using C11 atomic built-ins. These wrappers must be used for patches that
>   need to be merged in 20.08 onwards. This change will not introduce any
>   performance degradation.
> 
> * rte_smp_*mb: These APIs provide full barrier functionality. However, many
>   use cases do not require full barriers. To support such use cases, DPDK will
>   adopt C11 barrier semantics and provide wrappers using C11 atomic built-ins.
>   These wrappers must be used for patches that need to be merged in 20.08
>   onwards. This change will not introduce any performance degradation.
> "
> 
> Should we keep these notifications forever?
I do not think we need to keep them forever (unless the precedence is to keep all the older deprecations).

> 
> It is very difficult to find which wrapper to use.
Actually, the deprecations are incorrect on the 'wrappers'.
When the deprecations were added, the understanding was we will develop wrappers, the discussion was not concluded. When we made the decision, we decided to use the C++11 atomic built-ins. Only wrapper developed was rte_atomic_thread_fence. This is documented in the following blog.

> 
> This is the guide we have:
> https://doc.dpdk.org/guides/prog_guide/writing_efficient_code.html#locks-
> and-atomic-operations
> There are 2 blog posts:
> https://www.dpdk.org/blog/2021/03/26/dpdk-adopts-the-c11-memory-
> model/
> https://www.dpdk.org/blog/2021/06/09/reader-writer-concurrency/
> 
> Basically it says we should use "__atomic builtins" but there is example for
> simple situations like counters, memory barriers, etc.
> Please who could work on improving the documentation?
There is good amount of information on how to use the __atomic builtins for counters, memory barriers etc.
It would make sense to document something in DPDK if we implement our own wrappers.

>