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From: Thomas Monjalon <thomas@monjalon.net>
To: honnappa.nagarahalli@arm.com, ruifeng.wang@arm.com,
	Phil Yang <phil.yang@arm.com>
Cc: dev@dpdk.org, bruce.richardson@intel.com,
	konstantin.ananyev@intel.com, dev@dpdk.org,
	david.marchand@redhat.com,
	David Christensen <drc@linux.vnet.ibm.com>
Subject: Re: [dpdk-dev] atomic operations
Date: Sun, 04 Jul 2021 02:37:44 +0200	[thread overview]
Message-ID: <1806192.QIeOlYoxj9@thomas> (raw)
In-Reply-To: <1879045.6WiRxTbeAL@thomas>

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);



  parent reply	other threads:[~2021-07-04  0:37 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-03 11:29 Thomas Monjalon
2021-07-03 17:30 ` Stephen Hemminger
2021-07-04  0:40   ` Thomas Monjalon
2021-07-04  0:37 ` Thomas Monjalon [this message]
2021-07-05  7:00   ` Ruifeng Wang
2021-07-05  7:30     ` Thomas Monjalon
2021-07-05  8:33       ` Ruifeng Wang
2021-07-05 16:20         ` Stephen Hemminger
2021-07-07 19:04 ` Honnappa Nagarahalli
2021-07-07 19:28   ` Thomas Monjalon

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