[RFC v7 3/6] eal: add exactly-once bit access functions

[Mattias Rönnblom]

On 2024-05-07 21:17, Morten Brørup wrote:
>> From: Mattias Rönnblom [mailto:mattias.ronnblom at ericsson.com]
>> Sent: Sunday, 5 May 2024 10.38
>>
>> Add test/set/clear/assign/flip functions which prevents certain
>> compiler optimizations and guarantees that program-level memory loads
>> and/or stores will actually occur.
>>
>> These functions are useful when interacting with memory-mapped
>> hardware devices.
>>
>> The "once" family of functions does not promise atomicity and provides
>> no memory ordering guarantees beyond the C11 relaxed memory model.
> 
> In another thread, Stephen referred to the extended discussion on memory models in Linux kernel documentation:
> https://www.kernel.org/doc/html/latest/core-api/wrappers/memory-barriers.html
> 
> Unlike the "once" family of functions in this RFC, the "once" family of functions in the kernel also guarantee memory ordering, specifically for memory-mapped hardware devices. The document describes the rationale with examples.
> 

What more specifically did you have in mind? READ_ONCE() and 
WRITE_ONCE()? They give almost no guarantees. Very much relaxed.

I've read that document.

What you should keep in mind if you read that document, is that DPDK 
doesn't use the kernel's memory model, and doesn't have the kernel's 
barrier and atomics APIs. What we have are an obsolete, miniature 
look-alike in <rte_atomic.h> and something C11-like in <rte_stdatomic.h>.

My general impression is that DPDK was moving in the C11 direction 
memory model-wise, which is not the model the kernel uses.

> It makes me think that DPDK "once" family of functions should behave similarly.

I think they do already.

Also, rte_bit_once_set() works as the kernel's __set_bit().

> Alternatively, if the "once" family of functions cannot be generically implemented with a memory ordering that is optimal for all use cases, drop this family of functions, and instead rely on the "atomic" family of functions for interacting with memory-mapped hardware devices.
> 
>>
>> RFC v7:
>>   * Fix various minor issues in documentation.
>>
>> RFC v6:
>>   * Have rte_bit_once_test() accept const-marked bitsets.
>>
>> RFC v3:
>>   * Work around lack of C++ support for _Generic (Tyler Retzlaff).
>>
>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom at ericsson.com>
>> Acked-by: Morten Brørup <mb at smartsharesystems.com>
>> Acked-by: Tyler Retzlaff <roretzla at linux.microsoft.com>
>> ---
>