[PATCH] eal: add notes to SMP memory barrier APIs

[Mattias Rönnblom]

On 2023-06-25 10:17, Ruifeng Wang wrote:
>> -----Original Message-----
>> From: Mattias Rönnblom <hofors at lysator.liu.se>
>> Sent: Friday, June 23, 2023 2:20 AM
>> To: Ruifeng Wang <Ruifeng.Wang at arm.com>; thomas at monjalon.net; david.marchand at redhat.com
>> Cc: dev at dpdk.org; konstantin.v.ananyev at yandex.ru; Honnappa Nagarahalli
>> <Honnappa.Nagarahalli at arm.com>; nd <nd at arm.com>
>> Subject: Re: [PATCH] eal: add notes to SMP memory barrier APIs
>>
>> On 2023-06-21 08:44, Ruifeng Wang wrote:
>>> The rte_smp_xx() APIs are deprecated. But it is not mentioned in the
>>> function header.
>>> Added notes in function header for clarification.
>>>
>>> Signed-off-by: Ruifeng Wang <ruifeng.wang at arm.com>
>>> ---
>>>    lib/eal/include/generic/rte_atomic.h | 15 +++++++++++++++
>>>    1 file changed, 15 insertions(+)
>>>
>>> diff --git a/lib/eal/include/generic/rte_atomic.h
>>> b/lib/eal/include/generic/rte_atomic.h
>>> index 58df843c54..542a2c16ff 100644
>>> --- a/lib/eal/include/generic/rte_atomic.h
>>> +++ b/lib/eal/include/generic/rte_atomic.h
>>> @@ -55,6 +55,11 @@ static inline void rte_rmb(void);
>>>     * Guarantees that the LOAD and STORE operations that precede the
>>>     * rte_smp_mb() call are globally visible across the lcores
>>>     * before the LOAD and STORE operations that follows it.
>>> + *
>>> + * @note
>>> + *  This function is deprecated. It adds complexity to the memory
>>> + model
>>> + *  used by this project. C11 memory model should always be used.
>>> + *  rte_atomic_thread_fence() should be used instead.
>>
>> It's somewhat confusing to learn I should use the C11 memory model, and then in the next
>> sentence that I should call a function which is not in C11.
> 
> I should say "memory order semantics". It will be more specific.
> The wrapper function rte_atomic_thread_fence is a special case. It provides an optimized implementation
> for __ATOMIC_SEQ_CST for x86:
> https://www.dpdk.org/blog/2021/03/26/dpdk-adopts-the-c11-memory-model/
> 
>>
>> I think it would be helpful to say which memory_model parameters should be used to replace
>> the rte_smp_*mb() calls, and if there are any difference in semantics between the Linux
>> kernel-style barriers and their C11 (near-)equivalents.
> 
> As compiler atomic built-ins are being used. The memory model parameters should be the ones listed in:
> https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html
> We are not taking Linux kernel-style barriers. So no need to mention that.
> 

Yeah, sure. But which one of the C11 memory models, for respective 
legacy barrier?

What you are moving from is Linux kernel-style barriers, so if you are 
to recommend a migration path, their semantics will matter.

>>
>> Is there some particular reason these functions aren't marked __rte_deprecated? Too many
>> warnings?
> 
> Yes, warnings will come up. Some occurrences still remain in the project.
> 
>>
>>>     */
>>>    static inline void rte_smp_mb(void);
>>>
>>> @@ -64,6 +69,11 @@ static inline void rte_smp_mb(void);
>>>     * Guarantees that the STORE operations that precede the
>>>     * rte_smp_wmb() call are globally visible across the lcores
>>>     * before the STORE operations that follows it.
>>> + *
>>> + * @note
>>> + *  This function is deprecated. It adds complexity to the memory
>>> + model
>>> + *  used by this project. C11 memory model should always be used.
>>> + *  rte_atomic_thread_fence() should be used instead.
>>>     */
>>>    static inline void rte_smp_wmb(void);
>>>
>>> @@ -73,6 +83,11 @@ static inline void rte_smp_wmb(void);
>>>     * Guarantees that the LOAD operations that precede the
>>>     * rte_smp_rmb() call are globally visible across the lcores
>>>     * before the LOAD operations that follows it.
>>> + *
>>> + * @note
>>> + *  This function is deprecated. It adds complexity to the memory
>>> + model
>>> + *  used by this project. C11 memory model should always be used.
>>> + *  rte_atomic_thread_fence() should be used instead.
>>>     */
>>>    static inline void rte_smp_rmb(void);
>>>    ///@}