Relax memory ordering of shared refcount atomics

Feature or enhancement

Proposal:

In the free-threaded build, all operations on ob_ref_shared use sequentially consistent atomics. Full ordering is stronger than the biased reference counting protocol requires, and on ARM64 the ordered instructions are measurably slower under contention. Instead, we could (and I think should) match what C++ shared_ptr implementations do which is relaxed increfs and acquire/release decrefs ^1 ^2 ^3 ^4.

Has this already been discussed elsewhere?

No response given

Links to previous discussion of this feature:

No response

Linked PRs

• gh-156135

首先定位 ob_ref_shared 的 free-threaded 实现，并审查链接的 gh-156135 工作。将当前的 atomic ordering 与提案及其引用的 shared_ptr 实现进行比较；当 ordering 更改正确且已验证其对 contention 性能的影响时，即视为完成。