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 實作，並檢視連結的 gh-156135 工作。將目前的 atomic ordering 與提案及其引用的 shared_ptr 實作進行比較；當 ordering 變更正確，且已驗證其對 contention 效能的影響時，即視為完成。