diff --git a/Include/cpython/initconfig.h b/Include/cpython/initconfig.h
index 9c1783d272f1cd..efae2409b50069 100644
--- a/Include/cpython/initconfig.h
+++ b/Include/cpython/initconfig.h
@@ -252,6 +252,7 @@ typedef struct {
     int allow_threads;
     int allow_daemon_threads;
     int check_multi_interp_extensions;
+    int own_gil;
 } PyInterpreterConfig;
 
 #define _PyInterpreterConfig_INIT \
@@ -262,6 +263,7 @@ typedef struct {
         .allow_threads = 1, \
         .allow_daemon_threads = 0, \
         .check_multi_interp_extensions = 1, \
+        .own_gil = 1, \
     }
 
 #define _PyInterpreterConfig_LEGACY_INIT \
@@ -272,6 +274,7 @@ typedef struct {
         .allow_threads = 1, \
         .allow_daemon_threads = 1, \
         .check_multi_interp_extensions = 0, \
+        .own_gil = 0, \
     }
 
 /* --- Helper functions --------------------------------------- */
diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
index deda070a6dea79..3c8b368bd2af4e 100644
--- a/Include/internal/pycore_ceval.h
+++ b/Include/internal/pycore_ceval.h
@@ -21,8 +21,7 @@ struct _ceval_runtime_state;
 
 
 extern void _Py_FinishPendingCalls(PyThreadState *tstate);
-extern void _PyEval_InitRuntimeState(struct _ceval_runtime_state *);
-extern void _PyEval_InitState(struct _ceval_state *, PyThread_type_lock);
+extern void _PyEval_InitState(PyInterpreterState *, PyThread_type_lock);
 extern void _PyEval_FiniState(struct _ceval_state *ceval);
 PyAPI_FUNC(void) _PyEval_SignalReceived(PyInterpreterState *interp);
 PyAPI_FUNC(int) _PyEval_AddPendingCall(
@@ -96,11 +95,13 @@ _PyEval_Vector(PyThreadState *tstate,
             PyObject* const* args, size_t argcount,
             PyObject *kwnames);
 
-extern int _PyEval_ThreadsInitialized(struct pyruntimestate *runtime);
-extern PyStatus _PyEval_InitGIL(PyThreadState *tstate);
+extern int _PyEval_ThreadsInitialized(void);
+extern PyStatus _PyEval_InitGIL(PyThreadState *tstate, int own_gil);
 extern void _PyEval_FiniGIL(PyInterpreterState *interp);
 
+extern void _PyEval_AcquireLock(PyThreadState *tstate);
 extern void _PyEval_ReleaseLock(PyThreadState *tstate);
+extern PyThreadState * _PyThreadState_SwapNoGIL(PyThreadState *);
 
 extern void _PyEval_DeactivateOpCache(void);
 
diff --git a/Include/internal/pycore_ceval_state.h b/Include/internal/pycore_ceval_state.h
index 9ba42eb03b2676..b352801673c40a 100644
--- a/Include/internal/pycore_ceval_state.h
+++ b/Include/internal/pycore_ceval_state.h
@@ -49,7 +49,6 @@ struct _ceval_runtime_state {
        the main thread of the main interpreter can handle signals: see
        _Py_ThreadCanHandleSignals(). */
     _Py_atomic_int signals_pending;
-    struct _gil_runtime_state gil;
 };
 
 #ifdef PY_HAVE_PERF_TRAMPOLINE
@@ -83,6 +82,8 @@ struct _pending_calls {
 
 struct _ceval_state {
     int recursion_limit;
+    struct _gil_runtime_state *gil;
+    int own_gil;
     /* This single variable consolidates all requests to break out of
        the fast path in the eval loop. */
     _Py_atomic_int eval_breaker;
diff --git a/Include/internal/pycore_interp.h b/Include/internal/pycore_interp.h
index 7276ce35ba68f0..527b2121148f4c 100644
--- a/Include/internal/pycore_interp.h
+++ b/Include/internal/pycore_interp.h
@@ -178,6 +178,9 @@ struct _is {
        basis.  Also see _PyRuntimeState regarding the various mutex fields.
        */
 
+    /* The per-interpreter GIL, which might not be used. */
+    struct _gil_runtime_state _gil;
+
     /* the initial PyInterpreterState.threads.head */
     PyThreadState _initial_thread;
 };
diff --git a/Include/internal/pycore_runtime.h b/Include/internal/pycore_runtime.h
index d1b165d0ab9c38..6e06e874711bc2 100644
--- a/Include/internal/pycore_runtime.h
+++ b/Include/internal/pycore_runtime.h
@@ -32,8 +32,6 @@ struct _getargs_runtime_state {
     struct _PyArg_Parser *static_parsers;
 };
 
-/* ceval state */
-
 /* GIL state */
 
 struct _gilstate_runtime_state {
diff --git a/Include/moduleobject.h b/Include/moduleobject.h
index 555564ec73b4a2..b8bdfe29d80406 100644
--- a/Include/moduleobject.h
+++ b/Include/moduleobject.h
@@ -78,11 +78,17 @@ struct PyModuleDef_Slot {
 
 #define Py_mod_create 1
 #define Py_mod_exec 2
+#define Py_mod_multiple_interpreters 3
 
 #ifndef Py_LIMITED_API
-#define _Py_mod_LAST_SLOT 2
+#define _Py_mod_LAST_SLOT 3
 #endif
 
+/* for Py_mod_multiple_interpreters: */
+#define Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED ((void *)0)
+#define Py_MOD_MULTIPLE_INTERPRETERS_SUPPORTED ((void *)1)
+#define Py_MOD_PER_INTERPRETER_GIL_SUPPORTED ((void *)2)
+
 #endif /* New in 3.5 */
 
 struct PyModuleDef {
diff --git a/Lib/test/test__xxsubinterpreters.py b/Lib/test/test__xxsubinterpreters.py
index 1ee18774d17209..34f68e12278b0b 100644
--- a/Lib/test/test__xxsubinterpreters.py
+++ b/Lib/test/test__xxsubinterpreters.py
@@ -10,6 +10,7 @@
 import _testcapi
 from test import support
 from test.support import import_helper
+from test.support import os_helper
 from test.support import script_helper
 
 
@@ -917,7 +918,7 @@ def f():
         t = threading.Thread(target=f)
         t.start()
         """)
-        with support.temp_dir() as dirname:
+        with os_helper.temp_dir() as dirname:
             filename = script_helper.make_script(dirname, 'interp', script)
             with script_helper.spawn_python(filename) as proc:
                 retcode = proc.wait()
diff --git a/Lib/test/test_capi/test_misc.py b/Lib/test/test_capi/test_misc.py
index 1d426d0f8f825c..4c36bb84c21c92 100644
--- a/Lib/test/test_capi/test_misc.py
+++ b/Lib/test/test_capi/test_misc.py
@@ -1247,21 +1247,25 @@ def test_configured_settings(self):
         EXEC = 1<<16
 
         features = ['obmalloc', 'fork', 'exec', 'threads', 'daemon_threads',
-                    'extensions']
+                    'extensions', 'own_gil']
         kwlist = [f'allow_{n}' for n in features]
         kwlist[0] = 'use_main_obmalloc'
-        kwlist[-1] = 'check_multi_interp_extensions'
-
+        kwlist[-2] = 'check_multi_interp_extensions'
+        kwlist[-1] = 'own_gil'
         # expected to work
         for config, expected in {
-            (True, True, True, True, True, True):
-                OBMALLOC | FORK | EXEC | THREADS | DAEMON_THREADS | EXTENSIONS,
-            (True, False, False, False, False, False): OBMALLOC,
-            (False, False, False, True, False, True): THREADS | EXTENSIONS,
+            (True, True, True, True, True, True, True):
+                (OBMALLOC | FORK | EXEC | THREADS | DAEMON_THREADS | EXTENSIONS, True),
+            (True, False, False, False, False, False, False):
+                (OBMALLOC, False),
+            (False, False, False, True, False, True, False):
+                (THREADS | EXTENSIONS, False),
         }.items():
             kwargs = dict(zip(kwlist, config))
+            exp_flags, exp_gil = expected
             expected = {
-                'feature_flags': expected,
+                'feature_flags': exp_flags,
+                'own_gil': exp_gil,
             }
             with self.subTest(config):
                 r, w = os.pipe()
@@ -1281,7 +1285,7 @@ def test_configured_settings(self):
 
         # expected to fail
         for config in [
-            (False, False, False, False, False, False),
+            (False, False, False, False, False, False, False),
         ]:
             kwargs = dict(zip(kwlist, config))
             with self.subTest(config):
@@ -1317,6 +1321,7 @@ def test_overridden_setting_extensions_subinterp_check(self):
             'allow_exec': True,
             'allow_threads': True,
             'allow_daemon_threads': True,
+            'own_gil': False,
         }
 
         def check(enabled, override):
@@ -1327,6 +1332,7 @@ def check(enabled, override):
             flags = BASE_FLAGS | EXTENSIONS if enabled else BASE_FLAGS
             settings = {
                 'feature_flags': flags,
+                'own_gil': False,
             }
 
             expected = {
diff --git a/Lib/test/test_embed.py b/Lib/test/test_embed.py
index c9691bbf304915..582392ecddcb91 100644
--- a/Lib/test/test_embed.py
+++ b/Lib/test/test_embed.py
@@ -1666,6 +1666,7 @@ def test_init_main_interpreter_settings(self):
             # All optional features should be enabled.
             'feature_flags':
                 OBMALLOC | FORK | EXEC | THREADS | DAEMON_THREADS,
+            'own_gil': True,
         }
         out, err = self.run_embedded_interpreter(
             'test_init_main_interpreter_settings',
diff --git a/Lib/test/test_import/__init__.py b/Lib/test/test_import/__init__.py
index 41dfdaabe24664..8d6232f86b30fe 100644
--- a/Lib/test/test_import/__init__.py
+++ b/Lib/test/test_import/__init__.py
@@ -1639,6 +1639,7 @@ class SubinterpImportTests(unittest.TestCase):
         # Isolation-related config values aren't included here.
     )
     ISOLATED = dict(
+        own_gil=True,
         use_main_obmalloc=False,
     )
     NOT_ISOLATED = {k: not v for k, v in ISOLATED.items()}
diff --git a/Lib/test/test_threading.py b/Lib/test/test_threading.py
index fdd74c37e26235..97165264b34bbe 100644
--- a/Lib/test/test_threading.py
+++ b/Lib/test/test_threading.py
@@ -1349,6 +1349,7 @@ def func():
                 allow_threads={allowed},
                 allow_daemon_threads={daemon_allowed},
                 check_multi_interp_extensions=False,
+                own_gil=False,
             )
             """)
         with test.support.SuppressCrashReport():
diff --git a/Misc/NEWS.d/next/Core and Builtins/2023-05-03-17-46-47.gh-issue-104108.GOxAYt.rst b/Misc/NEWS.d/next/Core and Builtins/2023-05-03-17-46-47.gh-issue-104108.GOxAYt.rst
new file mode 100644
index 00000000000000..dad843636493ae
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2023-05-03-17-46-47.gh-issue-104108.GOxAYt.rst	
@@ -0,0 +1,6 @@
+Multi-phase init extension modules may now indicate whether or not they
+actually support multiple interpreters.  By default such modules are
+expected to support use in multiple interpreters.  In the uncommon case that
+one does not, it may use the new ``Py_mod_multiple_interpreters`` module def
+slot.  A value of ``0`` means the module does not support them. ``1`` means
+it does.  The default is ``1``.
diff --git a/Modules/_abc.c b/Modules/_abc.c
index 9694331339aa78..d3e405dadb664a 100644
--- a/Modules/_abc.c
+++ b/Modules/_abc.c
@@ -944,6 +944,7 @@ _abcmodule_free(void *module)
 
 static PyModuleDef_Slot _abcmodule_slots[] = {
     {Py_mod_exec, _abcmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_asynciomodule.c b/Modules/_asynciomodule.c
index 8b1a29b6d33e8b..b6dd8d47b63618 100644
--- a/Modules/_asynciomodule.c
+++ b/Modules/_asynciomodule.c
@@ -3805,6 +3805,8 @@ module_exec(PyObject *mod)
 
 static struct PyModuleDef_Slot module_slots[] = {
     {Py_mod_exec, module_exec},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_bisectmodule.c b/Modules/_bisectmodule.c
index 30801c2f87eee7..0773bbd191931d 100644
--- a/Modules/_bisectmodule.c
+++ b/Modules/_bisectmodule.c
@@ -457,6 +457,7 @@ bisect_modexec(PyObject *m)
 
 static PyModuleDef_Slot bisect_slots[] = {
     {Py_mod_exec, bisect_modexec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_blake2/blake2module.c b/Modules/_blake2/blake2module.c
index 44d783b40d0453..0d1d88c6603684 100644
--- a/Modules/_blake2/blake2module.c
+++ b/Modules/_blake2/blake2module.c
@@ -127,6 +127,7 @@ blake2_exec(PyObject *m)
 
 static PyModuleDef_Slot _blake2_slots[] = {
     {Py_mod_exec, blake2_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
@@ -146,4 +147,4 @@ PyMODINIT_FUNC
 PyInit__blake2(void)
 {
     return PyModuleDef_Init(&blake2_module);
-}
\ No newline at end of file
+}
diff --git a/Modules/_bz2module.c b/Modules/_bz2module.c
index 8e7b8e8078af4e..97bd44b4ac9694 100644
--- a/Modules/_bz2module.c
+++ b/Modules/_bz2module.c
@@ -799,6 +799,7 @@ _bz2_free(void *module)
 
 static struct PyModuleDef_Slot _bz2_slots[] = {
     {Py_mod_exec, _bz2_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_codecsmodule.c b/Modules/_codecsmodule.c
index d5035d20600ae2..777c753bd7c2a9 100644
--- a/Modules/_codecsmodule.c
+++ b/Modules/_codecsmodule.c
@@ -1049,6 +1049,7 @@ static PyMethodDef _codecs_functions[] = {
 };
 
 static PyModuleDef_Slot _codecs_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_collectionsmodule.c b/Modules/_collectionsmodule.c
index a9b1425177c3d7..9a81531bdffb16 100644
--- a/Modules/_collectionsmodule.c
+++ b/Modules/_collectionsmodule.c
@@ -2571,6 +2571,7 @@ collections_exec(PyObject *module) {
 
 static struct PyModuleDef_Slot collections_slots[] = {
     {Py_mod_exec, collections_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_contextvarsmodule.c b/Modules/_contextvarsmodule.c
index d13b5962c13c44..f621c1de6d42d6 100644
--- a/Modules/_contextvarsmodule.c
+++ b/Modules/_contextvarsmodule.c
@@ -44,6 +44,7 @@ _contextvars_exec(PyObject *m)
 
 static struct PyModuleDef_Slot _contextvars_slots[] = {
     {Py_mod_exec, _contextvars_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_cryptmodule.c b/Modules/_cryptmodule.c
index 72a4f44600d92c..75035084c9cd29 100644
--- a/Modules/_cryptmodule.c
+++ b/Modules/_cryptmodule.c
@@ -58,6 +58,7 @@ static PyMethodDef crypt_methods[] = {
 };
 
 static PyModuleDef_Slot _crypt_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_csv.c b/Modules/_csv.c
index 2217cc2ca7a775..0cde5c5a8bdc68 100644
--- a/Modules/_csv.c
+++ b/Modules/_csv.c
@@ -1798,6 +1798,7 @@ csv_exec(PyObject *module) {
 
 static PyModuleDef_Slot csv_slots[] = {
     {Py_mod_exec, csv_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_ctypes/_ctypes_test.c b/Modules/_ctypes/_ctypes_test.c
index a8811d03cc91a2..ce652b362d5bb3 100644
--- a/Modules/_ctypes/_ctypes_test.c
+++ b/Modules/_ctypes/_ctypes_test.c
@@ -1054,6 +1054,7 @@ _testfunc_pylist_append(PyObject *list, PyObject *item)
 }
 
 static struct PyModuleDef_Slot _ctypes_test_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_curses_panel.c b/Modules/_curses_panel.c
index 2144345de01ba3..58a98dea393757 100644
--- a/Modules/_curses_panel.c
+++ b/Modules/_curses_panel.c
@@ -690,6 +690,8 @@ _curses_panel_exec(PyObject *mod)
 
 static PyModuleDef_Slot _curses_slots[] = {
     {Py_mod_exec, _curses_panel_exec},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_dbmmodule.c b/Modules/_dbmmodule.c
index 54376022dcb182..9908174c94c450 100644
--- a/Modules/_dbmmodule.c
+++ b/Modules/_dbmmodule.c
@@ -583,6 +583,7 @@ _dbm_module_free(void *module)
 
 static PyModuleDef_Slot _dbmmodule_slots[] = {
     {Py_mod_exec, _dbm_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 97be89a167104f..c09954cc5fcd1e 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -4419,6 +4419,8 @@ module_exec(PyObject *m)
 
 static struct PyModuleDef_Slot elementtree_slots[] = {
     {Py_mod_exec, module_exec},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_functoolsmodule.c b/Modules/_functoolsmodule.c
index 4032ba79374fa4..a8001d71223fdc 100644
--- a/Modules/_functoolsmodule.c
+++ b/Modules/_functoolsmodule.c
@@ -1520,6 +1520,7 @@ _functools_free(void *module)
 
 static struct PyModuleDef_Slot _functools_slots[] = {
     {Py_mod_exec, _functools_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_gdbmmodule.c b/Modules/_gdbmmodule.c
index 4e8acdefc722b2..4dbb5741b2ede8 100644
--- a/Modules/_gdbmmodule.c
+++ b/Modules/_gdbmmodule.c
@@ -793,6 +793,7 @@ _gdbm_module_free(void *module)
 
 static PyModuleDef_Slot _gdbm_module_slots[] = {
     {Py_mod_exec, _gdbm_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_hashopenssl.c b/Modules/_hashopenssl.c
index 7476e5dc7dd61e..99d0b72819137e 100644
--- a/Modules/_hashopenssl.c
+++ b/Modules/_hashopenssl.c
@@ -2260,6 +2260,7 @@ static PyModuleDef_Slot hashlib_slots[] = {
     {Py_mod_exec, hashlib_md_meth_names},
     {Py_mod_exec, hashlib_init_constructors},
     {Py_mod_exec, hashlib_exception},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_heapqmodule.c b/Modules/_heapqmodule.c
index 07ddc7b0851241..00285ae01f8574 100644
--- a/Modules/_heapqmodule.c
+++ b/Modules/_heapqmodule.c
@@ -682,6 +682,7 @@ heapq_exec(PyObject *m)
 
 static struct PyModuleDef_Slot heapq_slots[] = {
     {Py_mod_exec, heapq_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_json.c b/Modules/_json.c
index fa8e2a936d2c33..c90de05b046b00 100644
--- a/Modules/_json.c
+++ b/Modules/_json.c
@@ -1801,6 +1801,7 @@ _json_exec(PyObject *module)
 
 static PyModuleDef_Slot _json_slots[] = {
     {Py_mod_exec, _json_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_localemodule.c b/Modules/_localemodule.c
index 96675cdfb661ad..1ada7305117bb7 100644
--- a/Modules/_localemodule.c
+++ b/Modules/_localemodule.c
@@ -874,6 +874,7 @@ _locale_exec(PyObject *module)
 
 static struct PyModuleDef_Slot _locale_slots[] = {
     {Py_mod_exec, _locale_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_lsprof.c b/Modules/_lsprof.c
index 83d034ae7eed78..cad82e24e9eca1 100644
--- a/Modules/_lsprof.c
+++ b/Modules/_lsprof.c
@@ -885,6 +885,8 @@ _lsprof_exec(PyObject *module)
 
 static PyModuleDef_Slot _lsprofslots[] = {
     {Py_mod_exec, _lsprof_exec},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_lzmamodule.c b/Modules/_lzmamodule.c
index bccab8639159e7..e34fbad230d51a 100644
--- a/Modules/_lzmamodule.c
+++ b/Modules/_lzmamodule.c
@@ -1611,6 +1611,7 @@ static PyMethodDef lzma_methods[] = {
 
 static PyModuleDef_Slot lzma_slots[] = {
     {Py_mod_exec, lzma_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_multiprocessing/multiprocessing.c b/Modules/_multiprocessing/multiprocessing.c
index 2463e1e1a8bf7e..8f9daa5c3de0cc 100644
--- a/Modules/_multiprocessing/multiprocessing.c
+++ b/Modules/_multiprocessing/multiprocessing.c
@@ -276,6 +276,7 @@ multiprocessing_exec(PyObject *module)
 
 static PyModuleDef_Slot multiprocessing_slots[] = {
     {Py_mod_exec, multiprocessing_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_multiprocessing/posixshmem.c b/Modules/_multiprocessing/posixshmem.c
index d64ded4168228f..88c93fe313785c 100644
--- a/Modules/_multiprocessing/posixshmem.c
+++ b/Modules/_multiprocessing/posixshmem.c
@@ -110,12 +110,19 @@ static PyMethodDef module_methods[ ] = {
 };
 
 
+static PyModuleDef_Slot module_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {0, NULL}
+};
+
+
 static struct PyModuleDef _posixshmemmodule = {
     PyModuleDef_HEAD_INIT,
     .m_name = "_posixshmem",
     .m_doc = "POSIX shared memory module",
     .m_size = 0,
     .m_methods = module_methods,
+    .m_slots = module_slots,
 };
 
 /* Module init function */
diff --git a/Modules/_opcode.c b/Modules/_opcode.c
index 99be977417743e..b70d426fa29bc0 100644
--- a/Modules/_opcode.c
+++ b/Modules/_opcode.c
@@ -94,12 +94,18 @@ opcode_functions[] =  {
     {NULL, NULL, 0, NULL}
 };
 
+static PyModuleDef_Slot module_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {0, NULL}
+};
+
 static struct PyModuleDef opcodemodule = {
     PyModuleDef_HEAD_INIT,
     .m_name = "_opcode",
     .m_doc = "Opcode support module.",
     .m_size = 0,
-    .m_methods = opcode_functions
+    .m_methods = opcode_functions,
+    .m_slots = module_slots,
 };
 
 PyMODINIT_FUNC
diff --git a/Modules/_operator.c b/Modules/_operator.c
index 38335b6995016c..68ccc90562d38d 100644
--- a/Modules/_operator.c
+++ b/Modules/_operator.c
@@ -1828,6 +1828,7 @@ operator_exec(PyObject *module)
 
 static struct PyModuleDef_Slot operator_slots[] = {
     {Py_mod_exec, operator_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_pickle.c b/Modules/_pickle.c
index 360c7910f67187..bf7ecae0cc0e50 100644
--- a/Modules/_pickle.c
+++ b/Modules/_pickle.c
@@ -7912,6 +7912,7 @@ _pickle_exec(PyObject *m)
 
 static PyModuleDef_Slot pickle_slots[] = {
     {Py_mod_exec, _pickle_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_posixsubprocess.c b/Modules/_posixsubprocess.c
index f5bce8cd7628ad..2bf83db0e228fb 100644
--- a/Modules/_posixsubprocess.c
+++ b/Modules/_posixsubprocess.c
@@ -1140,6 +1140,7 @@ static PyMethodDef module_methods[] = {
 };
 
 static PyModuleDef_Slot _posixsubprocess_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_queuemodule.c b/Modules/_queuemodule.c
index af19dd6c198b67..d36a911a57c02c 100644
--- a/Modules/_queuemodule.c
+++ b/Modules/_queuemodule.c
@@ -431,6 +431,7 @@ queuemodule_exec(PyObject *module)
 
 static PyModuleDef_Slot queuemodule_slots[] = {
     {Py_mod_exec, queuemodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_randommodule.c b/Modules/_randommodule.c
index 6e22053239305a..fda5ef267fb470 100644
--- a/Modules/_randommodule.c
+++ b/Modules/_randommodule.c
@@ -624,6 +624,7 @@ _random_exec(PyObject *module)
 
 static PyModuleDef_Slot _random_slots[] = {
     {Py_mod_exec, _random_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_scproxy.c b/Modules/_scproxy.c
index 344b66f9aad522..e66918016b8da6 100644
--- a/Modules/_scproxy.c
+++ b/Modules/_scproxy.c
@@ -232,6 +232,7 @@ static PyMethodDef mod_methods[] = {
 };
 
 static PyModuleDef_Slot _scproxy_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_sha3/sha3module.c b/Modules/_sha3/sha3module.c
index 633a0c0ea08d2a..93abc3b2710e55 100644
--- a/Modules/_sha3/sha3module.c
+++ b/Modules/_sha3/sha3module.c
@@ -641,6 +641,7 @@ _sha3_exec(PyObject *m)
 
 static PyModuleDef_Slot _sha3_slots[] = {
     {Py_mod_exec, _sha3_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_sqlite/module.c b/Modules/_sqlite/module.c
index 9c42faa232c70d..27bd42f4595e1c 100644
--- a/Modules/_sqlite/module.c
+++ b/Modules/_sqlite/module.c
@@ -785,6 +785,7 @@ module_exec(PyObject *module)
 
 static struct PyModuleDef_Slot module_slots[] = {
     {Py_mod_exec, module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_sre/sre.c b/Modules/_sre/sre.c
index 4b6290a5967932..f8a1a05a318889 100644
--- a/Modules/_sre/sre.c
+++ b/Modules/_sre/sre.c
@@ -3221,6 +3221,7 @@ sre_exec(PyObject *m)
 
 static PyModuleDef_Slot sre_slots[] = {
     {Py_mod_exec, sre_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index c9e2f24d66cc00..17043152cb23c0 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -6161,6 +6161,8 @@ static PyModuleDef_Slot sslmodule_slots[] = {
     {Py_mod_exec, sslmodule_init_constants},
     {Py_mod_exec, sslmodule_init_versioninfo},
     {Py_mod_exec, sslmodule_init_strings},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_stat.c b/Modules/_stat.c
index 546e6a5f94ca15..4218799103b59d 100644
--- a/Modules/_stat.c
+++ b/Modules/_stat.c
@@ -612,6 +612,7 @@ stat_exec(PyObject *module)
 
 static PyModuleDef_Slot stat_slots[] = {
     {Py_mod_exec, stat_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_statisticsmodule.c b/Modules/_statisticsmodule.c
index b9d1e4f1616036..1d5465fbe6d04e 100644
--- a/Modules/_statisticsmodule.c
+++ b/Modules/_statisticsmodule.c
@@ -129,6 +129,7 @@ PyDoc_STRVAR(statistics_doc,
 "Accelerators for the statistics module.\n");
 
 static struct PyModuleDef_Slot _statisticsmodule_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_struct.c b/Modules/_struct.c
index 3db7b991acd0a1..26434f714de5cc 100644
--- a/Modules/_struct.c
+++ b/Modules/_struct.c
@@ -2572,6 +2572,7 @@ _structmodule_exec(PyObject *m)
 
 static PyModuleDef_Slot _structmodule_slots[] = {
     {Py_mod_exec, _structmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index 47e0ed9be8e709..a1afaece4ac5c5 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -1488,6 +1488,7 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
     int allow_threads = -1;
     int allow_daemon_threads = -1;
     int check_multi_interp_extensions = -1;
+    int own_gil = -1;
     int r;
     PyThreadState *substate, *mainstate;
     /* only initialise 'cflags.cf_flags' to test backwards compatibility */
@@ -1500,13 +1501,19 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
                              "allow_threads",
                              "allow_daemon_threads",
                              "check_multi_interp_extensions",
+                             "own_gil",
                              NULL};
     if (!PyArg_ParseTupleAndKeywords(args, kwargs,
-                    "s$pppppp:run_in_subinterp_with_config", kwlist,
+                    "s$ppppppp:run_in_subinterp_with_config", kwlist,
                     &code, &use_main_obmalloc,
                     &allow_fork, &allow_exec,
                     &allow_threads, &allow_daemon_threads,
-                    &check_multi_interp_extensions)) {
+                    &check_multi_interp_extensions,
+                    &own_gil)) {
+        return NULL;
+    }
+    if (use_main_obmalloc < 0) {
+        PyErr_SetString(PyExc_ValueError, "missing use_main_obmalloc");
         return NULL;
     }
     if (use_main_obmalloc < 0) {
@@ -1525,6 +1532,10 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
         PyErr_SetString(PyExc_ValueError, "missing allow_threads");
         return NULL;
     }
+    if (own_gil < 0) {
+        PyErr_SetString(PyExc_ValueError, "missing own_gil");
+        return NULL;
+    }
     if (allow_daemon_threads < 0) {
         PyErr_SetString(PyExc_ValueError, "missing allow_daemon_threads");
         return NULL;
@@ -1545,6 +1556,7 @@ run_in_subinterp_with_config(PyObject *self, PyObject *args, PyObject *kwargs)
         .allow_threads = allow_threads,
         .allow_daemon_threads = allow_daemon_threads,
         .check_multi_interp_extensions = check_multi_interp_extensions,
+        .own_gil = own_gil,
     };
     PyStatus status = Py_NewInterpreterFromConfig(&substate, &config);
     if (PyStatus_Exception(status)) {
diff --git a/Modules/_testinternalcapi.c b/Modules/_testinternalcapi.c
index 1e38f1aa63499b..f35e3b48df9321 100644
--- a/Modules/_testinternalcapi.c
+++ b/Modules/_testinternalcapi.c
@@ -729,6 +729,13 @@ get_interp_settings(PyObject *self, PyObject *args)
         return NULL;
     }
 
+    /* "own GIL" */
+    PyObject *own_gil = interp->ceval.own_gil ? Py_True : Py_False;
+    if (PyDict_SetItemString(settings, "own_gil", own_gil) != 0) {
+        Py_DECREF(settings);
+        return NULL;
+    }
+
     return settings;
 }
 
@@ -789,6 +796,7 @@ module_exec(PyObject *module)
 
 static struct PyModuleDef_Slot module_slots[] = {
     {Py_mod_exec, module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_testmultiphase.c b/Modules/_testmultiphase.c
index cf8990a2df0a9b..6565500efeecb4 100644
--- a/Modules/_testmultiphase.c
+++ b/Modules/_testmultiphase.c
@@ -441,6 +441,7 @@ static int execfunc(PyObject *m)
 
 static PyModuleDef_Slot main_slots[] = {
     {Py_mod_exec, execfunc},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
@@ -745,6 +746,7 @@ PyInit__testmultiphase_create_unreported_exception(void)
 static PyModuleDef_Slot slots_nonmodule_with_exec_slots[] = {
     {Py_mod_create, createfunc_nonmodule},
     {Py_mod_exec, execfunc},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
@@ -765,6 +767,7 @@ execfunc_err(PyObject *mod)
 
 static PyModuleDef_Slot slots_exec_err[] = {
     {Py_mod_exec, execfunc_err},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
@@ -786,6 +789,7 @@ execfunc_raise(PyObject *spec)
 
 static PyModuleDef_Slot slots_exec_raise[] = {
     {Py_mod_exec, execfunc_raise},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
@@ -807,6 +811,7 @@ execfunc_unreported_exception(PyObject *mod)
 
 static PyModuleDef_Slot slots_exec_unreported_exception[] = {
     {Py_mod_exec, execfunc_unreported_exception},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
@@ -845,6 +850,7 @@ meth_state_access_exec(PyObject *m)
 
 static PyModuleDef_Slot meth_state_access_slots[] = {
     {Py_mod_exec, meth_state_access_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_threadmodule.c b/Modules/_threadmodule.c
index fd2fd9ab25f113..5d753b4a0ebc5e 100644
--- a/Modules/_threadmodule.c
+++ b/Modules/_threadmodule.c
@@ -1710,6 +1710,7 @@ The 'threading' module provides a more convenient interface.");
 
 static PyModuleDef_Slot thread_module_slots[] = {
     {Py_mod_exec, thread_module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_typingmodule.c b/Modules/_typingmodule.c
index 262dddb63fd5fe..64286375636aff 100644
--- a/Modules/_typingmodule.c
+++ b/Modules/_typingmodule.c
@@ -36,6 +36,7 @@ PyDoc_STRVAR(typing_doc,
 "Accelerators for the typing module.\n");
 
 static struct PyModuleDef_Slot _typingmodule_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_uuidmodule.c b/Modules/_uuidmodule.c
index eae38f5c98cc7f..ed3b2fedfd4d88 100644
--- a/Modules/_uuidmodule.c
+++ b/Modules/_uuidmodule.c
@@ -106,6 +106,7 @@ static PyMethodDef uuid_methods[] = {
 
 static PyModuleDef_Slot uuid_slots[] = {
     {Py_mod_exec, uuid_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_weakref.c b/Modules/_weakref.c
index 157a852ae9a378..387b8fa9d0a6f1 100644
--- a/Modules/_weakref.c
+++ b/Modules/_weakref.c
@@ -174,6 +174,7 @@ weakref_exec(PyObject *module)
 
 static struct PyModuleDef_Slot weakref_slots[] = {
     {Py_mod_exec, weakref_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_winapi.c b/Modules/_winapi.c
index fa380b8b798405..473bcb4736e925 100644
--- a/Modules/_winapi.c
+++ b/Modules/_winapi.c
@@ -2259,6 +2259,7 @@ static int winapi_exec(PyObject *m)
 
 static PyModuleDef_Slot winapi_slots[] = {
     {Py_mod_exec, winapi_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/_xxinterpchannelsmodule.c b/Modules/_xxinterpchannelsmodule.c
index 13b005eaef9866..616dd577688116 100644
--- a/Modules/_xxinterpchannelsmodule.c
+++ b/Modules/_xxinterpchannelsmodule.c
@@ -2418,6 +2418,7 @@ module_exec(PyObject *mod)
 
 static struct PyModuleDef_Slot module_slots[] = {
     {Py_mod_exec, module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 95273ab278d996..d7daae254638ec 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -513,6 +513,7 @@ interp_create(PyObject *self, PyObject *args, PyObject *kwds)
 
     // Create and initialize the new interpreter.
     PyThreadState *save_tstate = _PyThreadState_GET();
+    assert(save_tstate != NULL);
     const PyInterpreterConfig config = isolated
         ? (PyInterpreterConfig)_PyInterpreterConfig_INIT
         : (PyInterpreterConfig)_PyInterpreterConfig_LEGACY_INIT;
@@ -821,6 +822,7 @@ module_exec(PyObject *mod)
 
 static struct PyModuleDef_Slot module_slots[] = {
     {Py_mod_exec, module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/_zoneinfo.c b/Modules/_zoneinfo.c
index c215a75b804fdb..3b2d282d65cab9 100644
--- a/Modules/_zoneinfo.c
+++ b/Modules/_zoneinfo.c
@@ -2822,7 +2822,10 @@ zoneinfomodule_exec(PyObject *m)
 }
 
 static PyModuleDef_Slot zoneinfomodule_slots[] = {
-    {Py_mod_exec, zoneinfomodule_exec}, {0, NULL}};
+    {Py_mod_exec, zoneinfomodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {0, NULL},
+};
 
 static struct PyModuleDef zoneinfomodule = {
     .m_base = PyModuleDef_HEAD_INIT,
diff --git a/Modules/arraymodule.c b/Modules/arraymodule.c
index 798a7629257966..f94bbec8e0bb3c 100644
--- a/Modules/arraymodule.c
+++ b/Modules/arraymodule.c
@@ -3111,6 +3111,7 @@ array_modexec(PyObject *m)
 
 static PyModuleDef_Slot arrayslots[] = {
     {Py_mod_exec, array_modexec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/atexitmodule.c b/Modules/atexitmodule.c
index 47afd7f0751039..5882d405636400 100644
--- a/Modules/atexitmodule.c
+++ b/Modules/atexitmodule.c
@@ -314,12 +314,18 @@ upon normal program termination.\n\
 Two public functions, register and unregister, are defined.\n\
 ");
 
+static PyModuleDef_Slot atexitmodule_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {0, NULL}
+};
+
 static struct PyModuleDef atexitmodule = {
     PyModuleDef_HEAD_INIT,
     .m_name = "atexit",
     .m_doc = atexit__doc__,
     .m_size = 0,
     .m_methods = atexit_methods,
+    .m_slots = atexitmodule_slots,
 };
 
 PyMODINIT_FUNC
diff --git a/Modules/audioop.c b/Modules/audioop.c
index 9325f82f9a17e0..604306d449265c 100644
--- a/Modules/audioop.c
+++ b/Modules/audioop.c
@@ -1975,6 +1975,7 @@ audioop_exec(PyObject* module)
 
 static PyModuleDef_Slot audioop_slots[] = {
     {Py_mod_exec, audioop_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/binascii.c b/Modules/binascii.c
index 95ddb26988d6c9..4ecff4793be9a0 100644
--- a/Modules/binascii.c
+++ b/Modules/binascii.c
@@ -1291,6 +1291,7 @@ binascii_exec(PyObject *module) {
 
 static PyModuleDef_Slot binascii_slots[] = {
     {Py_mod_exec, binascii_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/cjkcodecs/cjkcodecs.h b/Modules/cjkcodecs/cjkcodecs.h
index e553ff3e17b898..36bc7024df9acc 100644
--- a/Modules/cjkcodecs/cjkcodecs.h
+++ b/Modules/cjkcodecs/cjkcodecs.h
@@ -502,6 +502,7 @@ static struct PyMethodDef _cjk_methods[] = {
 
 static PyModuleDef_Slot _cjk_slots[] = {
     {Py_mod_exec, _cjk_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/cjkcodecs/multibytecodec.c b/Modules/cjkcodecs/multibytecodec.c
index 233fc3020fd6a8..b501e4fb923232 100644
--- a/Modules/cjkcodecs/multibytecodec.c
+++ b/Modules/cjkcodecs/multibytecodec.c
@@ -2062,6 +2062,7 @@ static struct PyMethodDef _multibytecodec_methods[] = {
 
 static PyModuleDef_Slot _multibytecodec_slots[] = {
     {Py_mod_exec, _multibytecodec_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/cmathmodule.c b/Modules/cmathmodule.c
index b4f7e5424b4ccf..914a697f8e173b 100644
--- a/Modules/cmathmodule.c
+++ b/Modules/cmathmodule.c
@@ -1411,6 +1411,7 @@ cmath_exec(PyObject *mod)
 
 static PyModuleDef_Slot cmath_slots[] = {
     {Py_mod_exec, cmath_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/errnomodule.c b/Modules/errnomodule.c
index df4e494ba8a973..fddde960a5fe9a 100644
--- a/Modules/errnomodule.c
+++ b/Modules/errnomodule.c
@@ -940,6 +940,7 @@ errno_exec(PyObject *module)
 
 static PyModuleDef_Slot errno_slots[] = {
     {Py_mod_exec, errno_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/faulthandler.c b/Modules/faulthandler.c
index 9b4e4199cdc20a..428b090193f093 100644
--- a/Modules/faulthandler.c
+++ b/Modules/faulthandler.c
@@ -1274,6 +1274,8 @@ PyExec_faulthandler(PyObject *module) {
 
 static PyModuleDef_Slot faulthandler_slots[] = {
     {Py_mod_exec, PyExec_faulthandler},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/fcntlmodule.c b/Modules/fcntlmodule.c
index 9a8ec8dc9858d7..6ca0b62bc5dca8 100644
--- a/Modules/fcntlmodule.c
+++ b/Modules/fcntlmodule.c
@@ -686,6 +686,7 @@ fcntl_exec(PyObject *module)
 
 static PyModuleDef_Slot fcntl_slots[] = {
     {Py_mod_exec, fcntl_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index f4d5186ff1552e..26ddcdd538a4d4 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -2044,6 +2044,7 @@ gcmodule_exec(PyObject *module)
 
 static PyModuleDef_Slot gcmodule_slots[] = {
     {Py_mod_exec, gcmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/grpmodule.c b/Modules/grpmodule.c
index f6298ca0ee84c1..57cdde6064c24e 100644
--- a/Modules/grpmodule.c
+++ b/Modules/grpmodule.c
@@ -327,6 +327,7 @@ grpmodule_exec(PyObject *module)
 
 static PyModuleDef_Slot grpmodule_slots[] = {
     {Py_mod_exec, grpmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/itertoolsmodule.c b/Modules/itertoolsmodule.c
index c986e02867ca82..555eab09935e9e 100644
--- a/Modules/itertoolsmodule.c
+++ b/Modules/itertoolsmodule.c
@@ -4693,6 +4693,7 @@ itertoolsmodule_exec(PyObject *mod)
 
 static struct PyModuleDef_Slot itertoolsmodule_slots[] = {
     {Py_mod_exec, itertoolsmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/mathmodule.c b/Modules/mathmodule.c
index 4a2381d9611776..3737a9654575ab 100644
--- a/Modules/mathmodule.c
+++ b/Modules/mathmodule.c
@@ -4064,6 +4064,7 @@ static PyMethodDef math_methods[] = {
 
 static PyModuleDef_Slot math_slots[] = {
     {Py_mod_exec, math_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/md5module.c b/Modules/md5module.c
index 4f7bc77a8836a3..86605771d9643f 100644
--- a/Modules/md5module.c
+++ b/Modules/md5module.c
@@ -340,6 +340,7 @@ md5_exec(PyObject *m)
 
 static PyModuleDef_Slot _md5_slots[] = {
     {Py_mod_exec, md5_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/mmapmodule.c b/Modules/mmapmodule.c
index fe76ca6eafaa88..a470dd3c2f3bba 100644
--- a/Modules/mmapmodule.c
+++ b/Modules/mmapmodule.c
@@ -1713,6 +1713,7 @@ mmap_exec(PyObject *module)
 
 static PyModuleDef_Slot mmap_slots[] = {
     {Py_mod_exec, mmap_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/nismodule.c b/Modules/nismodule.c
index ec7f6d8031e84b..9b708d33a81dc5 100644
--- a/Modules/nismodule.c
+++ b/Modules/nismodule.c
@@ -502,6 +502,8 @@ nis_exec(PyObject *module)
 
 static PyModuleDef_Slot nis_slots[] = {
     {Py_mod_exec, nis_exec},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/overlapped.c b/Modules/overlapped.c
index 02c0f401be4c9e..ac637316583d2d 100644
--- a/Modules/overlapped.c
+++ b/Modules/overlapped.c
@@ -2050,6 +2050,7 @@ overlapped_exec(PyObject *module)
 
 static PyModuleDef_Slot overlapped_slots[] = {
     {Py_mod_exec, overlapped_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/posixmodule.c b/Modules/posixmodule.c
index dcb5e7a0e0408c..359f40dd375d8e 100644
--- a/Modules/posixmodule.c
+++ b/Modules/posixmodule.c
@@ -16785,6 +16785,7 @@ posixmodule_exec(PyObject *m)
 
 static PyModuleDef_Slot posixmodile_slots[] = {
     {Py_mod_exec, posixmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/pwdmodule.c b/Modules/pwdmodule.c
index a757380bd09f70..cc2e2a43893971 100644
--- a/Modules/pwdmodule.c
+++ b/Modules/pwdmodule.c
@@ -336,6 +336,7 @@ pwdmodule_exec(PyObject *module)
 
 static PyModuleDef_Slot pwdmodule_slots[] = {
     {Py_mod_exec, pwdmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/pyexpat.c b/Modules/pyexpat.c
index 0a744998b6c514..b52273fbe92953 100644
--- a/Modules/pyexpat.c
+++ b/Modules/pyexpat.c
@@ -2056,6 +2056,8 @@ pyexpat_free(void *module)
 
 static PyModuleDef_Slot pyexpat_slots[] = {
     {Py_mod_exec, pyexpat_exec},
+    // XXX gh-103092: fix isolation.
+    //{Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/resource.c b/Modules/resource.c
index a97fb870062b82..2a8158c9be5359 100644
--- a/Modules/resource.c
+++ b/Modules/resource.c
@@ -514,6 +514,7 @@ resource_exec(PyObject *module)
 
 static struct PyModuleDef_Slot resource_slots[] = {
     {Py_mod_exec, resource_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/selectmodule.c b/Modules/selectmodule.c
index 5a1e40d0b4a482..79bd5b59ab68f9 100644
--- a/Modules/selectmodule.c
+++ b/Modules/selectmodule.c
@@ -2651,6 +2651,7 @@ _select_exec(PyObject *m)
 
 static PyModuleDef_Slot _select_slots[] = {
     {Py_mod_exec, _select_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/sha1module.c b/Modules/sha1module.c
index f8d4056fd34b65..bdb76c56f1a6e8 100644
--- a/Modules/sha1module.c
+++ b/Modules/sha1module.c
@@ -344,6 +344,7 @@ _sha1_exec(PyObject *module)
 
 static PyModuleDef_Slot _sha1_slots[] = {
     {Py_mod_exec, _sha1_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/sha2module.c b/Modules/sha2module.c
index 72de20b44762d7..37d9b5c538fd0b 100644
--- a/Modules/sha2module.c
+++ b/Modules/sha2module.c
@@ -785,6 +785,7 @@ static int sha2_exec(PyObject *module)
 
 static PyModuleDef_Slot _sha2_slots[] = {
     {Py_mod_exec, sha2_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/signalmodule.c b/Modules/signalmodule.c
index fdd1450050fa1b..2350236ad46b25 100644
--- a/Modules/signalmodule.c
+++ b/Modules/signalmodule.c
@@ -1695,6 +1695,7 @@ _signal_module_free(void *module)
 
 static PyModuleDef_Slot signal_slots[] = {
     {Py_mod_exec, signal_module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/socketmodule.c b/Modules/socketmodule.c
index f11d4b1a6e0591..86d61c923bfaba 100644
--- a/Modules/socketmodule.c
+++ b/Modules/socketmodule.c
@@ -8857,6 +8857,7 @@ socket_exec(PyObject *m)
 
 static struct PyModuleDef_Slot socket_slots[] = {
     {Py_mod_exec, socket_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/spwdmodule.c b/Modules/spwdmodule.c
index 42123c93b59365..13f1115feefa86 100644
--- a/Modules/spwdmodule.c
+++ b/Modules/spwdmodule.c
@@ -224,6 +224,7 @@ spwdmodule_exec(PyObject *module)
 
 static PyModuleDef_Slot spwdmodule_slots[] = {
     {Py_mod_exec, spwdmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/symtablemodule.c b/Modules/symtablemodule.c
index 91538b4fb15cbd..1cc319cc3410d8 100644
--- a/Modules/symtablemodule.c
+++ b/Modules/symtablemodule.c
@@ -100,6 +100,7 @@ symtable_init_constants(PyObject *m)
 
 static PyModuleDef_Slot symtable_slots[] = {
     {Py_mod_exec, symtable_init_constants},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/syslogmodule.c b/Modules/syslogmodule.c
index f45aa5227f1cbf..6db8de9c491dd9 100644
--- a/Modules/syslogmodule.c
+++ b/Modules/syslogmodule.c
@@ -406,6 +406,7 @@ syslog_exec(PyObject *module)
 
 static PyModuleDef_Slot syslog_slots[] = {
     {Py_mod_exec, syslog_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/termios.c b/Modules/termios.c
index fcc8f042679870..169a36fc6477d8 100644
--- a/Modules/termios.c
+++ b/Modules/termios.c
@@ -1253,6 +1253,7 @@ termios_exec(PyObject *mod)
 
 static PyModuleDef_Slot termios_slots[] = {
     {Py_mod_exec, termios_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/timemodule.c b/Modules/timemodule.c
index c50e689bb6986c..3607855dbd8f27 100644
--- a/Modules/timemodule.c
+++ b/Modules/timemodule.c
@@ -2107,6 +2107,7 @@ time_module_free(void *module)
 
 static struct PyModuleDef_Slot time_slots[] = {
     {Py_mod_exec, time_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/unicodedata.c b/Modules/unicodedata.c
index c108f14871f946..41dcd5f8f883f2 100644
--- a/Modules/unicodedata.c
+++ b/Modules/unicodedata.c
@@ -1516,6 +1516,7 @@ unicodedata_exec(PyObject *module)
 
 static PyModuleDef_Slot unicodedata_slots[] = {
     {Py_mod_exec, unicodedata_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/xxlimited.c b/Modules/xxlimited.c
index 5f5297ba6337af..3935c00fc26530 100644
--- a/Modules/xxlimited.c
+++ b/Modules/xxlimited.c
@@ -390,6 +390,7 @@ xx_modexec(PyObject *m)
 
 static PyModuleDef_Slot xx_slots[] = {
     {Py_mod_exec, xx_modexec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/xxlimited_35.c b/Modules/xxlimited_35.c
index 361c7e76d77f50..1ff3ef1cb6f296 100644
--- a/Modules/xxlimited_35.c
+++ b/Modules/xxlimited_35.c
@@ -293,6 +293,7 @@ xx_modexec(PyObject *m)
 
 static PyModuleDef_Slot xx_slots[] = {
     {Py_mod_exec, xx_modexec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Modules/xxmodule.c b/Modules/xxmodule.c
index a676fdb4ec773a..1e4e0ea3743ce3 100644
--- a/Modules/xxmodule.c
+++ b/Modules/xxmodule.c
@@ -383,6 +383,7 @@ xx_exec(PyObject *m)
 
 static struct PyModuleDef_Slot xx_slots[] = {
     {Py_mod_exec, xx_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/xxsubtype.c b/Modules/xxsubtype.c
index 8512baf7cd0a2d..744ba7bf5d28b6 100644
--- a/Modules/xxsubtype.c
+++ b/Modules/xxsubtype.c
@@ -286,6 +286,7 @@ xxsubtype_exec(PyObject* m)
 
 static struct PyModuleDef_Slot xxsubtype_slots[] = {
     {Py_mod_exec, xxsubtype_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/Modules/zlibmodule.c b/Modules/zlibmodule.c
index e2f7dbaca87a9f..b67844a67c315c 100644
--- a/Modules/zlibmodule.c
+++ b/Modules/zlibmodule.c
@@ -2109,6 +2109,7 @@ zlib_exec(PyObject *mod)
 
 static PyModuleDef_Slot zlib_slots[] = {
     {Py_mod_exec, zlib_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Objects/moduleobject.c b/Objects/moduleobject.c
index a0be19a3ca8ac8..ed6b035a658372 100644
--- a/Objects/moduleobject.c
+++ b/Objects/moduleobject.c
@@ -245,9 +245,12 @@ PyModule_FromDefAndSpec2(PyModuleDef* def, PyObject *spec, int module_api_versio
     PyObject *(*create)(PyObject *, PyModuleDef*) = NULL;
     PyObject *nameobj;
     PyObject *m = NULL;
+    int has_multiple_interpreters_slot = 0;
+    void *multiple_interpreters;
     int has_execution_slots = 0;
     const char *name;
     int ret;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
 
     PyModuleDef_Init(def);
 
@@ -287,6 +290,17 @@ PyModule_FromDefAndSpec2(PyModuleDef* def, PyObject *spec, int module_api_versio
             case Py_mod_exec:
                 has_execution_slots = 1;
                 break;
+            case Py_mod_multiple_interpreters:
+                if (has_multiple_interpreters_slot) {
+                    PyErr_Format(
+                        PyExc_SystemError,
+                        "module %s has more than one 'multiple interpreters' slots",
+                        name);
+                    goto error;
+                }
+                multiple_interpreters = cur_slot->value;
+                has_multiple_interpreters_slot = 1;
+                break;
             default:
                 assert(cur_slot->slot < 0 || cur_slot->slot > _Py_mod_LAST_SLOT);
                 PyErr_Format(
@@ -297,6 +311,26 @@ PyModule_FromDefAndSpec2(PyModuleDef* def, PyObject *spec, int module_api_versio
         }
     }
 
+    /* By default, multi-phase init modules are expected
+       to work under multiple interpreters. */
+    if (!has_multiple_interpreters_slot) {
+        multiple_interpreters = Py_MOD_MULTIPLE_INTERPRETERS_SUPPORTED;
+    }
+    if (multiple_interpreters == Py_MOD_MULTIPLE_INTERPRETERS_NOT_SUPPORTED
+        && !_Py_IsMainInterpreter(interp)
+        && _PyImport_CheckSubinterpIncompatibleExtensionAllowed(name) < 0)
+    {
+        goto error;
+    }
+    // XXX This case needs a test.
+    else if (multiple_interpreters != Py_MOD_PER_INTERPRETER_GIL_SUPPORTED
+             && interp->ceval.own_gil
+             && !_Py_IsMainInterpreter(interp)
+             && _PyImport_CheckSubinterpIncompatibleExtensionAllowed(name) < 0)
+    {
+        goto error;
+    }
+
     if (create) {
         m = create(spec, def);
         if (m == NULL) {
@@ -421,6 +455,9 @@ PyModule_ExecDef(PyObject *module, PyModuleDef *def)
                     return -1;
                 }
                 break;
+            case Py_mod_multiple_interpreters:
+                /* handled in PyModule_FromDefAndSpec2 */
+                break;
             default:
                 PyErr_Format(
                     PyExc_SystemError,
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 6ae68cc20f7dee..1585a582f00aad 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -15192,12 +15192,18 @@ static PyMethodDef _string_methods[] = {
     {NULL, NULL}
 };
 
+static PyModuleDef_Slot module_slots[] = {
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
+    {0, NULL}
+};
+
 static struct PyModuleDef _string_module = {
     PyModuleDef_HEAD_INIT,
     .m_name = "_string",
     .m_doc = PyDoc_STR("string helper module"),
     .m_size = 0,
     .m_methods = _string_methods,
+    .m_slots = module_slots,
 };
 
 PyMODINIT_FUNC
diff --git a/PC/_testconsole.c b/PC/_testconsole.c
index f14a2d45b1be26..3221b985d01ba0 100644
--- a/PC/_testconsole.c
+++ b/PC/_testconsole.c
@@ -31,6 +31,7 @@ static int execfunc(PyObject *m)
 
 PyModuleDef_Slot testconsole_slots[] = {
     {Py_mod_exec, execfunc},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL},
 };
 
diff --git a/PC/msvcrtmodule.c b/PC/msvcrtmodule.c
index 090254befc934d..53ef26b732f615 100644
--- a/PC/msvcrtmodule.c
+++ b/PC/msvcrtmodule.c
@@ -661,6 +661,7 @@ exec_module(PyObject* m)
 
 static PyModuleDef_Slot msvcrt_slots[] = {
     {Py_mod_exec, exec_module},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/PC/winreg.c b/PC/winreg.c
index 4884125c3609ad..e2d5322f458c2a 100644
--- a/PC/winreg.c
+++ b/PC/winreg.c
@@ -2184,6 +2184,7 @@ exec_module(PyObject *m)
 
 static PyModuleDef_Slot winreg_slots[] = {
     {Py_mod_exec, exec_module},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/PC/winsound.c b/PC/winsound.c
index 17ce2ef423b1f9..68a917810f884d 100644
--- a/PC/winsound.c
+++ b/PC/winsound.c
@@ -235,6 +235,7 @@ exec_module(PyObject *module)
 
 static PyModuleDef_Slot sound_slots[] = {
     {Py_mod_exec, exec_module},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index b44e303ac2594b..5d5a05a70ca7ec 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -1206,6 +1206,7 @@ def visitModule(self, mod):
         self.emit("""
 static PyModuleDef_Slot astmodule_slots[] = {
     {Py_mod_exec, astmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Python/Python-ast.c b/Python/Python-ast.c
index 6c878474afb192..81ab71c0fc3b29 100644
--- a/Python/Python-ast.c
+++ b/Python/Python-ast.c
@@ -12193,6 +12193,7 @@ astmodule_exec(PyObject *m)
 
 static PyModuleDef_Slot astmodule_slots[] = {
     {Py_mod_exec, astmodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Python/Python-tokenize.c b/Python/Python-tokenize.c
index 416dc5971bca3d..3394a5108cb535 100644
--- a/Python/Python-tokenize.c
+++ b/Python/Python-tokenize.c
@@ -151,6 +151,7 @@ static PyMethodDef tokenize_methods[] = {
 
 static PyModuleDef_Slot tokenizemodule_slots[] = {
     {Py_mod_exec, tokenizemodule_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Python/_warnings.c b/Python/_warnings.c
index d510381c365b66..5644db9a3770cb 100644
--- a/Python/_warnings.c
+++ b/Python/_warnings.c
@@ -1449,6 +1449,7 @@ warnings_module_exec(PyObject *module)
 
 static PyModuleDef_Slot warnings_slots[] = {
     {Py_mod_exec, warnings_module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Python/ceval_gil.c b/Python/ceval_gil.c
index 29796be4b80e93..257a0281f7402e 100644
--- a/Python/ceval_gil.c
+++ b/Python/ceval_gil.c
@@ -229,6 +229,9 @@ static void _gil_initialize(struct _gil_runtime_state *gil)
 
 static int gil_created(struct _gil_runtime_state *gil)
 {
+    if (gil == NULL) {
+        return 0;
+    }
     return (_Py_atomic_load_explicit(&gil->locked, _Py_memory_order_acquire) >= 0);
 }
 
@@ -273,10 +276,9 @@ static void recreate_gil(struct _gil_runtime_state *gil)
 #endif
 
 static void
-drop_gil(struct _ceval_runtime_state *ceval, struct _ceval_state *ceval2,
-         PyThreadState *tstate)
+drop_gil(struct _ceval_state *ceval, PyThreadState *tstate)
 {
-    struct _gil_runtime_state *gil = &ceval->gil;
+    struct _gil_runtime_state *gil = ceval->gil;
     if (!_Py_atomic_load_relaxed(&gil->locked)) {
         Py_FatalError("drop_gil: GIL is not locked");
     }
@@ -296,7 +298,7 @@ drop_gil(struct _ceval_runtime_state *ceval, struct _ceval_state *ceval2,
     MUTEX_UNLOCK(gil->mutex);
 
 #ifdef FORCE_SWITCHING
-    if (_Py_atomic_load_relaxed(&ceval2->gil_drop_request) && tstate != NULL) {
+    if (_Py_atomic_load_relaxed(&ceval->gil_drop_request) && tstate != NULL) {
         MUTEX_LOCK(gil->switch_mutex);
         /* Not switched yet => wait */
         if (((PyThreadState*)_Py_atomic_load_relaxed(&gil->last_holder)) == tstate)
@@ -358,9 +360,8 @@ take_gil(PyThreadState *tstate)
 
     assert(is_tstate_valid(tstate));
     PyInterpreterState *interp = tstate->interp;
-    struct _ceval_runtime_state *ceval = &interp->runtime->ceval;
-    struct _ceval_state *ceval2 = &interp->ceval;
-    struct _gil_runtime_state *gil = &ceval->gil;
+    struct _ceval_state *ceval = &interp->ceval;
+    struct _gil_runtime_state *gil = ceval->gil;
 
     /* Check that _PyEval_InitThreads() was called to create the lock */
     assert(gil_created(gil));
@@ -434,12 +435,12 @@ take_gil(PyThreadState *tstate)
            in take_gil() while the main thread called
            wait_for_thread_shutdown() from Py_Finalize(). */
         MUTEX_UNLOCK(gil->mutex);
-        drop_gil(ceval, ceval2, tstate);
+        drop_gil(ceval, tstate);
         PyThread_exit_thread();
     }
     assert(is_tstate_valid(tstate));
 
-    if (_Py_atomic_load_relaxed(&ceval2->gil_drop_request)) {
+    if (_Py_atomic_load_relaxed(&ceval->gil_drop_request)) {
         RESET_GIL_DROP_REQUEST(interp);
     }
     else {
@@ -448,7 +449,7 @@ take_gil(PyThreadState *tstate)
            handle signals.
 
            Note: RESET_GIL_DROP_REQUEST() calls COMPUTE_EVAL_BREAKER(). */
-        COMPUTE_EVAL_BREAKER(interp, ceval, ceval2);
+        COMPUTE_EVAL_BREAKER(interp, &_PyRuntime.ceval, ceval);
     }
 
     /* Don't access tstate if the thread must exit */
@@ -463,63 +464,99 @@ take_gil(PyThreadState *tstate)
 
 void _PyEval_SetSwitchInterval(unsigned long microseconds)
 {
-    struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;
+    PyInterpreterState *interp = _PyInterpreterState_Get();
+    struct _gil_runtime_state *gil = interp->ceval.gil;
+    assert(gil != NULL);
     gil->interval = microseconds;
 }
 
 unsigned long _PyEval_GetSwitchInterval(void)
 {
-    struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;
+    PyInterpreterState *interp = _PyInterpreterState_Get();
+    struct _gil_runtime_state *gil = interp->ceval.gil;
+    assert(gil != NULL);
     return gil->interval;
 }
 
 
 int
-_PyEval_ThreadsInitialized(_PyRuntimeState *runtime)
+_PyEval_ThreadsInitialized(void)
 {
-    return gil_created(&runtime->ceval.gil);
+    /* XXX This is only needed for an assert in PyGILState_Ensure(),
+     * which currently does not work with subinterpreters.
+     * Thus we only use the main interpreter. */
+    PyInterpreterState *interp = _PyInterpreterState_Main();
+    if (interp == NULL) {
+        return 0;
+    }
+    struct _gil_runtime_state *gil = interp->ceval.gil;
+    return gil_created(gil);
 }
 
 int
 PyEval_ThreadsInitialized(void)
 {
-    _PyRuntimeState *runtime = &_PyRuntime;
-    return _PyEval_ThreadsInitialized(runtime);
+    return _PyEval_ThreadsInitialized();
 }
 
-PyStatus
-_PyEval_InitGIL(PyThreadState *tstate)
+static void
+init_shared_gil(PyInterpreterState *interp, struct _gil_runtime_state *gil)
 {
-    if (!_Py_IsMainInterpreter(tstate->interp)) {
-        /* Currently, the GIL is shared by all interpreters,
-           and only the main interpreter is responsible to create
-           and destroy it. */
-        return _PyStatus_OK();
-    }
+    assert(gil_created(gil));
+    interp->ceval.gil = gil;
+    interp->ceval.own_gil = 0;
+}
 
-    struct _gil_runtime_state *gil = &tstate->interp->runtime->ceval.gil;
+static void
+init_own_gil(PyInterpreterState *interp, struct _gil_runtime_state *gil)
+{
     assert(!gil_created(gil));
-
-    PyThread_init_thread();
     create_gil(gil);
-
-    take_gil(tstate);
-
     assert(gil_created(gil));
+    interp->ceval.gil = gil;
+    interp->ceval.own_gil = 1;
+}
+
+PyStatus
+_PyEval_InitGIL(PyThreadState *tstate, int own_gil)
+{
+    assert(tstate->interp->ceval.gil == NULL);
+    int locked;
+    if (!own_gil) {
+        /* The interpreter will share the main interpreter's instead. */
+        PyInterpreterState *main_interp = _PyInterpreterState_Main();
+        assert(tstate->interp != main_interp);
+        init_shared_gil(tstate->interp, main_interp->ceval.gil);
+        locked = _Py_atomic_load_relaxed(&main_interp->ceval.gil->locked);
+    }
+    else {
+        PyThread_init_thread();
+        init_own_gil(tstate->interp, &tstate->interp->_gil);
+        locked = 0;
+    }
+    if (!locked) {
+        take_gil(tstate);
+    }
     return _PyStatus_OK();
 }
 
 void
 _PyEval_FiniGIL(PyInterpreterState *interp)
 {
-    if (!_Py_IsMainInterpreter(interp)) {
-        /* Currently, the GIL is shared by all interpreters,
-           and only the main interpreter is responsible to create
-           and destroy it. */
+    struct _gil_runtime_state *gil = interp->ceval.gil;
+    if (gil == NULL) {
+        /* It was already finalized (or hasn't been initialized yet). */
+        assert(!interp->ceval.own_gil);
+        return;
+    }
+    else if (!interp->ceval.own_gil) {
+        PyInterpreterState *main_interp = _PyInterpreterState_Main();
+        assert(main_interp != NULL && interp != main_interp);
+        assert(interp->ceval.gil == main_interp->ceval.gil);
+        interp->ceval.gil = NULL;
         return;
     }
 
-    struct _gil_runtime_state *gil = &interp->runtime->ceval.gil;
     if (!gil_created(gil)) {
         /* First Py_InitializeFromConfig() call: the GIL doesn't exist
            yet: do nothing. */
@@ -528,6 +565,7 @@ _PyEval_FiniGIL(PyInterpreterState *interp)
 
     destroy_gil(gil);
     assert(!gil_created(gil));
+    interp->ceval.gil = NULL;
 }
 
 void
@@ -561,17 +599,23 @@ PyEval_ReleaseLock(void)
     /* This function must succeed when the current thread state is NULL.
        We therefore avoid PyThreadState_Get() which dumps a fatal error
        in debug mode. */
-    struct _ceval_runtime_state *ceval = &runtime->ceval;
-    struct _ceval_state *ceval2 = &tstate->interp->ceval;
-    drop_gil(ceval, ceval2, tstate);
+    struct _ceval_state *ceval = &tstate->interp->ceval;
+    drop_gil(ceval, tstate);
+}
+
+void
+_PyEval_AcquireLock(PyThreadState *tstate)
+{
+    _Py_EnsureTstateNotNULL(tstate);
+    take_gil(tstate);
 }
 
 void
 _PyEval_ReleaseLock(PyThreadState *tstate)
 {
-    struct _ceval_runtime_state *ceval = &tstate->interp->runtime->ceval;
-    struct _ceval_state *ceval2 = &tstate->interp->ceval;
-    drop_gil(ceval, ceval2, tstate);
+    _Py_EnsureTstateNotNULL(tstate);
+    struct _ceval_state *ceval = &tstate->interp->ceval;
+    drop_gil(ceval, tstate);
 }
 
 void
@@ -581,7 +625,7 @@ PyEval_AcquireThread(PyThreadState *tstate)
 
     take_gil(tstate);
 
-    if (_PyThreadState_Swap(tstate->interp->runtime, tstate) != NULL) {
+    if (_PyThreadState_SwapNoGIL(tstate) != NULL) {
         Py_FatalError("non-NULL old thread state");
     }
 }
@@ -591,14 +635,12 @@ PyEval_ReleaseThread(PyThreadState *tstate)
 {
     assert(is_tstate_valid(tstate));
 
-    _PyRuntimeState *runtime = tstate->interp->runtime;
-    PyThreadState *new_tstate = _PyThreadState_Swap(runtime, NULL);
+    PyThreadState *new_tstate = _PyThreadState_SwapNoGIL(NULL);
     if (new_tstate != tstate) {
         Py_FatalError("wrong thread state");
     }
-    struct _ceval_runtime_state *ceval = &runtime->ceval;
-    struct _ceval_state *ceval2 = &tstate->interp->ceval;
-    drop_gil(ceval, ceval2, tstate);
+    struct _ceval_state *ceval = &tstate->interp->ceval;
+    drop_gil(ceval, tstate);
 }
 
 #ifdef HAVE_FORK
@@ -608,9 +650,9 @@ PyEval_ReleaseThread(PyThreadState *tstate)
 PyStatus
 _PyEval_ReInitThreads(PyThreadState *tstate)
 {
-    _PyRuntimeState *runtime = tstate->interp->runtime;
+    assert(tstate->interp == _PyInterpreterState_Main());
 
-    struct _gil_runtime_state *gil = &runtime->ceval.gil;
+    struct _gil_runtime_state *gil = tstate->interp->ceval.gil;
     if (!gil_created(gil)) {
         return _PyStatus_OK();
     }
@@ -641,14 +683,12 @@ _PyEval_SignalAsyncExc(PyInterpreterState *interp)
 PyThreadState *
 PyEval_SaveThread(void)
 {
-    _PyRuntimeState *runtime = &_PyRuntime;
-    PyThreadState *tstate = _PyThreadState_Swap(runtime, NULL);
+    PyThreadState *tstate = _PyThreadState_SwapNoGIL(NULL);
     _Py_EnsureTstateNotNULL(tstate);
 
-    struct _ceval_runtime_state *ceval = &runtime->ceval;
-    struct _ceval_state *ceval2 = &tstate->interp->ceval;
-    assert(gil_created(&ceval->gil));
-    drop_gil(ceval, ceval2, tstate);
+    struct _ceval_state *ceval = &tstate->interp->ceval;
+    assert(gil_created(ceval->gil));
+    drop_gil(ceval, tstate);
     return tstate;
 }
 
@@ -659,7 +699,7 @@ PyEval_RestoreThread(PyThreadState *tstate)
 
     take_gil(tstate);
 
-    _PyThreadState_Swap(tstate->interp->runtime, tstate);
+    _PyThreadState_SwapNoGIL(tstate);
 }
 
 
@@ -902,20 +942,13 @@ Py_MakePendingCalls(void)
     return 0;
 }
 
-/* The interpreter's recursion limit */
-
 void
-_PyEval_InitRuntimeState(struct _ceval_runtime_state *ceval)
+_PyEval_InitState(PyInterpreterState *interp, PyThread_type_lock pending_lock)
 {
-    _gil_initialize(&ceval->gil);
-}
+    _gil_initialize(&interp->_gil);
 
-void
-_PyEval_InitState(struct _ceval_state *ceval, PyThread_type_lock pending_lock)
-{
-    struct _pending_calls *pending = &ceval->pending;
+    struct _pending_calls *pending = &interp->ceval.pending;
     assert(pending->lock == NULL);
-
     pending->lock = pending_lock;
 }
 
@@ -962,16 +995,16 @@ _Py_HandlePending(PyThreadState *tstate)
     /* GIL drop request */
     if (_Py_atomic_load_relaxed_int32(&interp_ceval_state->gil_drop_request)) {
         /* Give another thread a chance */
-        if (_PyThreadState_Swap(runtime, NULL) != tstate) {
+        if (_PyThreadState_SwapNoGIL(NULL) != tstate) {
             Py_FatalError("tstate mix-up");
         }
-        drop_gil(ceval, interp_ceval_state, tstate);
+        drop_gil(interp_ceval_state, tstate);
 
         /* Other threads may run now */
 
         take_gil(tstate);
 
-        if (_PyThreadState_Swap(runtime, tstate) != NULL) {
+        if (_PyThreadState_SwapNoGIL(tstate) != NULL) {
             Py_FatalError("orphan tstate");
         }
     }
diff --git a/Python/import.c b/Python/import.c
index 0bf107b28d3990..9e1857d5f3e4e6 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -3840,6 +3840,7 @@ imp_module_exec(PyObject *module)
 
 static PyModuleDef_Slot imp_slots[] = {
     {Py_mod_exec, imp_module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Python/marshal.c b/Python/marshal.c
index 2966139cec9ae9..208996b05fc484 100644
--- a/Python/marshal.c
+++ b/Python/marshal.c
@@ -1870,6 +1870,7 @@ marshal_module_exec(PyObject *mod)
 
 static PyModuleDef_Slot marshalmodule_slots[] = {
     {Py_mod_exec, marshal_module_exec},
+    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
     {0, NULL}
 };
 
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index b9add89b9c6c52..fdc1ecba0383c2 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -585,12 +585,13 @@ init_interp_settings(PyInterpreterState *interp,
 
 
 static PyStatus
-init_interp_create_gil(PyThreadState *tstate)
+init_interp_create_gil(PyThreadState *tstate, int own_gil)
 {
     PyStatus status;
 
     /* finalize_interp_delete() comment explains why _PyEval_FiniGIL() is
        only called here. */
+    // XXX This is broken with a per-interpreter GIL.
     _PyEval_FiniGIL(tstate->interp);
 
     /* Auto-thread-state API */
@@ -600,7 +601,7 @@ init_interp_create_gil(PyThreadState *tstate)
     }
 
     /* Create the GIL and take it */
-    status = _PyEval_InitGIL(tstate);
+    status = _PyEval_InitGIL(tstate, own_gil);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
     }
@@ -632,7 +633,9 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
         return status;
     }
 
-    const PyInterpreterConfig config = _PyInterpreterConfig_LEGACY_INIT;
+    PyInterpreterConfig config = _PyInterpreterConfig_LEGACY_INIT;
+    // The main interpreter always has its own GIL.
+    config.own_gil = 1;
     status = init_interp_settings(interp, &config);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
@@ -643,9 +646,10 @@ pycore_create_interpreter(_PyRuntimeState *runtime,
         return _PyStatus_ERR("can't make first thread");
     }
     _PyThreadState_Bind(tstate);
-    (void) PyThreadState_Swap(tstate);
+    // XXX For now we do this before the GIL is created.
+    (void) _PyThreadState_SwapNoGIL(tstate);
 
-    status = init_interp_create_gil(tstate);
+    status = init_interp_create_gil(tstate, config.own_gil);
     if (_PyStatus_EXCEPTION(status)) {
         return status;
     }
@@ -2024,11 +2028,14 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
     }
     _PyThreadState_Bind(tstate);
 
-    PyThreadState *save_tstate = PyThreadState_Swap(tstate);
+    // XXX For now we do this before the GIL is created.
+    PyThreadState *save_tstate = _PyThreadState_SwapNoGIL(tstate);
+    int has_gil = 0;
 
     /* Copy the current interpreter config into the new interpreter */
     const PyConfig *src_config;
     if (save_tstate != NULL) {
+        _PyEval_ReleaseLock(save_tstate);
         src_config = _PyInterpreterState_GetConfig(save_tstate->interp);
     }
     else
@@ -2048,10 +2055,11 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
         goto error;
     }
 
-    status = init_interp_create_gil(tstate);
+    status = init_interp_create_gil(tstate, config->own_gil);
     if (_PyStatus_EXCEPTION(status)) {
         goto error;
     }
+    has_gil = 1;
 
     status = pycore_interp_init(tstate);
     if (_PyStatus_EXCEPTION(status)) {
@@ -2071,7 +2079,12 @@ new_interpreter(PyThreadState **tstate_p, const PyInterpreterConfig *config)
 
     /* Oops, it didn't work.  Undo it all. */
     PyErr_PrintEx(0);
-    PyThreadState_Swap(save_tstate);
+    if (has_gil) {
+        PyThreadState_Swap(save_tstate);
+    }
+    else {
+        _PyThreadState_SwapNoGIL(save_tstate);
+    }
     PyThreadState_Clear(tstate);
     PyThreadState_Delete(tstate);
     PyInterpreterState_Delete(interp);
diff --git a/Python/pystate.c b/Python/pystate.c
index f103a059f0f369..907c9fbec4f3a6 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -425,8 +425,6 @@ init_runtime(_PyRuntimeState *runtime,
     runtime->open_code_userdata = open_code_userdata;
     runtime->audit_hook_head = audit_hook_head;
 
-    _PyEval_InitRuntimeState(&runtime->ceval);
-
     PyPreConfig_InitPythonConfig(&runtime->preconfig);
 
     PyThread_type_lock *lockptrs[NUMLOCKS] = {
@@ -682,7 +680,7 @@ init_interpreter(PyInterpreterState *interp,
         memcpy(&interp->obmalloc.pools.used, temp, sizeof(temp));
     }
 
-    _PyEval_InitState(&interp->ceval, pending_lock);
+    _PyEval_InitState(interp, pending_lock);
     _PyGC_InitState(&interp->gc);
     PyConfig_InitPythonConfig(&interp->config);
     _PyType_InitCache(interp);
@@ -1863,17 +1861,11 @@ PyThreadState_Get(void)
 }
 
 
-PyThreadState *
-_PyThreadState_Swap(_PyRuntimeState *runtime, PyThreadState *newts)
+static void
+_swap_thread_states(_PyRuntimeState *runtime,
+                    PyThreadState *oldts, PyThreadState *newts)
 {
-#if defined(Py_DEBUG)
-    /* This can be called from PyEval_RestoreThread(). Similar
-       to it, we need to ensure errno doesn't change.
-    */
-    int err = errno;
-#endif
-    PyThreadState *oldts = current_fast_get(runtime);
-
+    // XXX Do this only if oldts != NULL?
     current_fast_clear(runtime);
 
     if (oldts != NULL) {
@@ -1887,6 +1879,20 @@ _PyThreadState_Swap(_PyRuntimeState *runtime, PyThreadState *newts)
         current_fast_set(runtime, newts);
         tstate_activate(newts);
     }
+}
+
+PyThreadState *
+_PyThreadState_SwapNoGIL(PyThreadState *newts)
+{
+#if defined(Py_DEBUG)
+    /* This can be called from PyEval_RestoreThread(). Similar
+       to it, we need to ensure errno doesn't change.
+    */
+    int err = errno;
+#endif
+
+    PyThreadState *oldts = current_fast_get(&_PyRuntime);
+    _swap_thread_states(&_PyRuntime, oldts, newts);
 
 #if defined(Py_DEBUG)
     errno = err;
@@ -1894,6 +1900,20 @@ _PyThreadState_Swap(_PyRuntimeState *runtime, PyThreadState *newts)
     return oldts;
 }
 
+PyThreadState *
+_PyThreadState_Swap(_PyRuntimeState *runtime, PyThreadState *newts)
+{
+    PyThreadState *oldts = current_fast_get(runtime);
+    if (oldts != NULL) {
+        _PyEval_ReleaseLock(oldts);
+    }
+    _swap_thread_states(runtime, oldts, newts);
+    if (newts != NULL) {
+        _PyEval_AcquireLock(newts);
+    }
+    return oldts;
+}
+
 PyThreadState *
 PyThreadState_Swap(PyThreadState *newts)
 {
@@ -2186,7 +2206,7 @@ PyGILState_Ensure(void)
 
     /* Ensure that _PyEval_InitThreads() and _PyGILState_Init() have been
        called by Py_Initialize() */
-    assert(_PyEval_ThreadsInitialized(runtime));
+    assert(_PyEval_ThreadsInitialized());
     assert(gilstate_tss_initialized(runtime));
     assert(runtime->gilstate.autoInterpreterState != NULL);
 
@@ -2761,6 +2781,10 @@ _register_builtins_for_crossinterpreter_data(struct _xidregistry *xidregistry)
 }
 
 
+/*************/
+/* other API */
+/*************/
+
 _PyFrameEvalFunction
 _PyInterpreterState_GetEvalFrameFunc(PyInterpreterState *interp)
 {