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

先阅读 CPython 中的 _PyType_LookupRef 和 TYPE_LOCK 路径，然后检查 LibCST 的 _visitors.py 中的 visitor 模式。比较 Python 3.13 和 3.14 上的行为，并审阅链接的 PR；完成时应包括一项经协商一致的更改，以解决锁竞争问题，以及查找路径扩展性更好的证据。