Lock contention inside `_PyType_LookupRef`

Feature or enhancement

Proposal:

As part of adding free-threading support to LibCST, we noticed there is a lot of lock contention on TYPE_LOCK inside the _PyType_LookupRef function. In the LibCST, the common "visitor" pattern is used. For example, in the _visitors.py module there is the code:

visit_func = getattr(self, f"visit_{type(node).__name__}", None)

The second argument to getattr() is a non-interned string and it causes the cached and lock-free path of _PyType_LookupRef() never to be taken. Instead, the TYPE_LOCK mutex is acquired on each lookup. This obviously scales very badly if there are multiple threads looking up class methods using this pattern.

Testing was done with Python 3.13 but I believe the same issue exists with 3.14.

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-132381
• gh-131174
• gh-132651
• gh-132652
• gh-133669
• gh-135112

先閱讀 CPython 中的 _PyType_LookupRef 和 TYPE_LOCK 路徑，接著檢查 LibCST 的 _visitors.py 中的 visitor 模式。比較 Python 3.13 和 3.14 上的行為，並檢視連結的 PR；完成時應包含一項經協議的變更，以處理鎖競爭問題，以及查找路徑擴展性更好的證據。