Removed invalid mail address. > -----Original Message----- > From: Thomas Monjalon <tho...@monjalon.net> > Sent: Sunday, July 4, 2021 8:38 AM > To: Honnappa Nagarahalli <honnappa.nagaraha...@arm.com>; Ruifeng Wang > <ruifeng.w...@arm.com>; Phil Yang <phil.y...@arm.com> > Cc: dev@dpdk.org; bruce.richard...@intel.com; > konstantin.anan...@intel.com; dev@dpdk.org; > david.march...@redhat.com; David Christensen <d...@linux.vnet.ibm.com> > Subject: Re: [dpdk-dev] atomic operations > > 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.
> 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. > > > > 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. > > One simple example: increment a counter atomically. > __atomic_fetch_add(&counter, 1, __ATOMIC_RELAXED); or > __atomic_add_fetch(&counter, 1, __ATOMIC_RELAXED); >
