Free-threaded builds can experience heavy contention on PyType_IsSubtype

[DinoV]

Bug report

Bug description:

PyType_IsSubtype takes the type lock to ensure that MRO is being accessed in a thread-safe manner as it needs to read and incref it. This can result in heavy contention in some pretty common scenarios (e.g. when sys.setprofile is used we experience heavy contention across threads doing PyCFunction_Check, profiling simple workloads can be nearly 50% slower in some cases). We can avoid this by using Py_TryIncref and only fallback to the lock when the MRO isn't yet shared.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-116868: Avoid locking in PyType_IsSubtype #116829