Feature or enhancement

We have a lock that we use for thread-safety when working with the set of interpreters: _PyRuntimeState.interpreters.mutex. We use two macros in several core files to manage that lock: HEAD_LOCK() and HEAD_UNLOCK() (from pycore_pystate.h).

Recently we've begun using that runtime-global lock for thread-safety when working with a single interpreter's set of thread states (PyInterpreterState.threads). The problem is that this means we are potentially blocking interpreters unnecessarily.

Thus it makes sense to introduce an interpreter-specific "head" lock: PyInterpreterState.threads.mutex (plus corresponding macros).

Linked PRs

• gh-114940: Use fine-grained mutex protection for PyInterpreterState.threads #125561
• gh-114940: Add a Per-Interpreter Lock For the List of Thread States #127037
• gh-114940: Add _Py_FOR_EACH_TSTATE_UNLOCKED(), and Friends #127077
• gh-114940: Add a Per-Interpreter Lock For the List of Thread States #127115