pythongh-124370: Add "howto" for free-threaded Python

This is a guide aimed at people writing Python code, as oppposed to the
existing guide for C API extension authors.

Doc/howto/free-threading-python.rst

@@ -0,0 +1,119 @@
+.. _freethreading-python-howto:
+
+*********************************
+Python Support for Free Threading
+*********************************
+
+Starting with the 3.13 release, CPython has experimental support for running
+with the :term:`global interpreter lock` (GIL) disabled in a configuration
+called :term:`free threading`.  This document describes the implications of
+free threading for Python code.  See :ref:`freethreading-extensions-howto` for
+information on how to write C extensions that support the free-threaded build.
+
+
+Installation
+============
+
+Starting with Python 3.13.0b2, the offical macOS and Windows installers
+optionally support installing free-threaded Python binaries.  The installers
+are available at https://www.python.org/downloads/.
+
+.. seealso::
+
+   `Installing a Free-Threaded Python
+   <https://py-free-threading.github.io/installing_cpython/>`_:
+   A community-maintained installation guide for installing free-threaded
+   Python.
+
+
+Identifying Free-Threaded Python
+================================
+
+The free-threaded build of CPython can optionally run with the global
+interpreter lock enabled, such as when :envvar:`PYTHON_GIL` is set to ``1``,
+or when importing an extension module that requires the GIL.
+
+The :func:`sys._is_gil_enabled` function will return ``False`` if the global
+interpreter lock is currently disabled.  This is the recommended mechanism for
+decisions like whether to use multithreading or multiprocessing.
+
+The ``sysconfig.get_config_var("Py_GIL_DISABLED")`` configuration variable can
+be used to determine whether the build supports free threading.  If the variable
+is set to ``1``, then the build supports free threading.  This is the recommended
+mechanism for decisions related to the build configuration.
+
+
+Thread Safety
+=============
+
+The free-threaded build of CPython aims to provide similar thread-safety
+behavior at the Python level to the GIL-enabled build.  Built-in
+types like :class:`dict`, :class:`list`, and :class:`set` use internal locks
+to protect against concurrent modifications in ways that behave similarly to
+the GIL.  However, Python has not historically guaranteed specific behavior for
+concurrent modifications to these built-in types, so this should be treated
+as a description of the current implementation, not a guarantee of future
+behavior.
+
+.. note::
+
+   It's recommended to use the :class:`threading.Lock` or other synchronization
+   primitives instead of relying on the internal locks of built-in types, when
+   possible.
+
+
+
+Known Limitations
+=================
+
+This section describes known limitations of the free-threaded CPython build.
+
+Immortalization
+---------------
+
+The free-threaded build of the 3.13 release makes some objects :term:`immortal`
+in order to avoid reference count contention that would prevent efficient
+multi-threaded scaling.  This means that these objects are never deallocated.
+This expected to be addressed in the upcoming 3.14 release with
+`deferred reference counting <https://peps.python.org/pep-0703/#deferred-reference-counting>`_.
+
+The objects that are immortalized are:
+
+* :ref:`function <user-defined-funcs>` objects declared at the module level
+* :ref:`method <instance-methods>` descriptors
+* :ref:`code <code-objects>` objects
+* :term:`module` objects and their dictionaries
+* :ref:`classes <classes>` (type objects)
+
+The immortalization of these objects happens the first time a thread is started
+after the main thread.
+
+Additionally, numeric and string literals in the code as well as strings
+returned by :func:`sys.intern` are also interned.  This behavior is expected to
+remainin the 3.14 free-threaded build.
+
+
+Frame Objects
+-------------
+
+It is not safe to access :ref:`frame <frame-objects>` objects from other
+threads.  This means that :func:`sys._current_frames` is generally not safe to
+use in a free-threaded build.
+
+Iterators
+---------
+
+Sharing the same iterator object between multiple threads is generally not
+safe and threads may see duplicate or missing elements when iterating or crash
+the interpreter.
+
+
+Single-Threaded Performance
+---------------------------
+
+The free-threaded build has additional overhead when executing Python code
+compared to the default GIL-enabled build.  In 3.13, this overhead is about
+40% on the `pyperformance <https://pyperformance.readthedocs.io/>`_ suite.
+Programs that spend most of the their time in C extensions or I/O will see
+less of an impact.  This overhead is expected to be reduced in the upcoming
+3.14 release.

Doc/howto/index.rst

@@ -32,6 +32,7 @@ Python Library Reference.
    isolating-extensions.rst
    timerfd.rst
    mro.rst
+   free-threading-python.rst
    free-threading-extensions.rst
 
 General:
@@ -52,6 +53,7 @@ General:
 Advanced development:
 
 * :ref:`curses-howto`
+* :ref:`freethreading-python-howto`
 * :ref:`freethreading-extensions-howto`
 * :ref:`isolating-extensions-howto`
 * :ref:`python_2.3_mro`

Doc/reference/datamodel.rst

@@ -846,6 +846,7 @@ this case, the special read-only attribute :attr:`!__self__` is set to the objec
 denoted by *alist*. (The attribute has the same semantics as it does with
 :attr:`other instance methods <method.__self__>`.)
 
+.. _classes:
 
 Classes
 ^^^^^^^