[dpdk-dev] atomic operations

[Thomas Monjalon]

03/07/2021 13:29, Thomas Monjalon:
> 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.
> "

The only new wrapper is rte_atomic_thread_fence(). What else?
We are missing clear recommendations.

> Should we keep these notifications forever?
> 
> It is very difficult to find which wrapper to use.

We should make function names explicit instead of "These".

> 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.

Precision: I meant "there is *no* example".

> Please who could work on improving the documentation?

One simple example: increment a counter atomically.
__atomic_fetch_add(&counter, 1, __ATOMIC_RELAXED);
or
__atomic_add_fetch(&counter, 1, __ATOMIC_RELAXED);