05/07/2021 09:00, Ruifeng Wang: > From: Thomas Monjalon <tho...@monjalon.net> > > 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? > > Yes. The decision was to use GCC atomic built-ins directly. > And rte_atomic_thread_fence() is an exception. It is a wrapper of > __atomic_thread_fence(), because mem order __ATOMIC_SEQ_CST has an optimized > implementation for x86.
Then above deprecation is wrong. > > We are missing clear recommendations. > > > > > Should we keep these notifications forever? > > Targeting to obsolete APIs rte_atomicNN_xxx and rte_smp_*mb. > Arm is working on replace occurrences with equivalent atomic built-ins. > There is still a lot work to do in drivers. This is an ongoing work. In the meantime we need clear recommendation what to use. > > > 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#loc > > > ks-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? > > Agree that the documentation needs improve. > Add link to list of atomic built-ins and the above mentioned blog posts can > be part of the improvement. It should be more than a link. We need to know when to use what. First thing, please fix the deprecation notice. > > One simple example: increment a counter atomically. > > __atomic_fetch_add(&counter, 1, __ATOMIC_RELAXED); or > > __atomic_add_fetch(&counter, 1, __ATOMIC_RELAXED); I really hate how atomics are "documented" in GCC doc. For instance, it doesn't say what is returned (old or new value) in above functions.
