[3.11] bpo-40514: Drop EXPERIMENTAL_ISOLATED_SUBINTERPRETERS (gh-93185) (GH-93306)

(cherry picked from commit caa279d6fd5f151e57f891cd4f6ba51b532501c6)

This was added for bpo-40514 (gh-84694) to test out a per-interpreter GIL.  However, it has since proven unnecessary to keep the experiment in the repo.  (It can be done as a branch in a fork like normal.)  So here we are removing:

* the configure option
* the macro
* the code enabled by the macro

Automerge-Triggered-By: GH:ericsnowcurrently

18 files changed, 10 insertions, 234 deletions

diff --git a/Doc/whatsnew/3.11.rst b/Doc/whatsnew/3.11.rst
index 24d1343689..4f9bafefe2 100644
--- a/Doc/whatsnew/3.11.rst
+++ b/Doc/whatsnew/3.11.rst
@@ -1425,6 +1425,11 @@ Removed
   Python's test suite."
   (Contributed by Victor Stinner in :issue:`46852`.)
 
+* The ``--experimental-isolated-subinterpreters`` configure flag
+  (and corresponding ``EXPERIMENTAL_ISOLATED_SUBINTERPRETERS``)
+  have been removed.
+
+
 Porting to Python 3.11
 ======================
 
diff --git a/Include/internal/pycore_ceval.h b/Include/internal/pycore_ceval.h
index 3efd6beb03..d969a5e6eb 100644
--- a/Include/internal/pycore_ceval.h
+++ b/Include/internal/pycore_ceval.h
@@ -80,11 +80,7 @@ _PyEval_Vector(PyThreadState *tstate,
             PyObject* const* args, size_t argcount,
             PyObject *kwnames);
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-extern int _PyEval_ThreadsInitialized(PyInterpreterState *interp);
-#else
 extern int _PyEval_ThreadsInitialized(struct pyruntimestate *runtime);
-#endif
 extern PyStatus _PyEval_InitGIL(PyThreadState *tstate);
 extern void _PyEval_FiniGIL(PyInterpreterState *interp);
 
diff --git a/Include/internal/pycore_interp.h b/Include/internal/pycore_interp.h
index d55627908a..bcfcd88d84 100644
--- a/Include/internal/pycore_interp.h
+++ b/Include/internal/pycore_interp.h
@@ -51,9 +51,6 @@ struct _ceval_state {
     /* Request for dropping the GIL */
     _Py_atomic_int gil_drop_request;
     struct _pending_calls pending;
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    struct _gil_runtime_state gil;
-#endif
 };
 
 
diff --git a/Include/internal/pycore_pystate.h b/Include/internal/pycore_pystate.h
index c4bc53c707..e2d7c5bee2 100644
--- a/Include/internal/pycore_pystate.h
+++ b/Include/internal/pycore_pystate.h
@@ -64,18 +64,10 @@ _Py_ThreadCanHandlePendingCalls(void)
 /* Variable and macro for in-line access to current thread
    and interpreter state */
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-PyAPI_FUNC(PyThreadState*) _PyThreadState_GetTSS(void);
-#endif
-
 static inline PyThreadState*
 _PyRuntimeState_GetThreadState(_PyRuntimeState *runtime)
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    return _PyThreadState_GetTSS();
-#else
     return (PyThreadState*)_Py_atomic_load_relaxed(&runtime->gilstate.tstate_current);
-#endif
 }
 
 /* Get the current Python thread state.
@@ -90,11 +82,7 @@ _PyRuntimeState_GetThreadState(_PyRuntimeState *runtime)
 static inline PyThreadState*
 _PyThreadState_GET(void)
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    return _PyThreadState_GetTSS();
-#else
     return _PyRuntimeState_GetThreadState(&_PyRuntime);
-#endif
 }
 
 PyAPI_FUNC(void) _Py_NO_RETURN _Py_FatalError_TstateNULL(const char *func);
diff --git a/Include/internal/pycore_runtime.h b/Include/internal/pycore_runtime.h
index 18191c3771..ae63ae74af 100644
--- a/Include/internal/pycore_runtime.h
+++ b/Include/internal/pycore_runtime.h
@@ -23,9 +23,7 @@ struct _ceval_runtime_state {
        the main thread of the main interpreter can handle signals: see
        _Py_ThreadCanHandleSignals(). */
     _Py_atomic_int signals_pending;
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     struct _gil_runtime_state gil;
-#endif
 };
 
 /* GIL state */
diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-05-25-12-30-12.gh-issue-84694.5sjy2w.rst b/Misc/NEWS.d/next/Core and Builtins/2022-05-25-12-30-12.gh-issue-84694.5sjy2w.rst
new file mode 100644
index 0000000000..c062d28b29
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-05-25-12-30-12.gh-issue-84694.5sjy2w.rst
@@ -0,0 +1,2 @@
+The ``--experimental-isolated-subinterpreters`` configure option and
+``EXPERIMENTAL_ISOLATED_SUBINTERPRETERS`` macro have been removed.
diff --git a/Modules/_xxsubinterpretersmodule.c b/Modules/_xxsubinterpretersmodule.c
index 9b1f186c5b..d1df00111c 100644
--- a/Modules/_xxsubinterpretersmodule.c
+++ b/Modules/_xxsubinterpretersmodule.c
@@ -1932,20 +1932,6 @@ _run_script_in_interpreter(PyInterpreterState *interp, const char *codestr,
         return -1;
     }
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    // Switch to interpreter.
-    PyThreadState *new_tstate = PyInterpreterState_ThreadHead(interp);
-    PyThreadState *save1 = PyEval_SaveThread();
-
-    (void)PyThreadState_Swap(new_tstate);
-
-    // Run the script.
-    _sharedexception *exc = NULL;
-    int result = _run_script(interp, codestr, shared, &exc);
-
-    // Switch back.
-    PyEval_RestoreThread(save1);
-#else
     // Switch to interpreter.
     PyThreadState *save_tstate = NULL;
     if (interp != PyInterpreterState_Get()) {
@@ -1963,7 +1949,6 @@ _run_script_in_interpreter(PyInterpreterState *interp, const char *codestr,
     if (save_tstate != NULL) {
         PyThreadState_Swap(save_tstate);
     }
-#endif
 
     // Propagate any exception out to the caller.
     if (exc != NULL) {
diff --git a/Modules/gcmodule.c b/Modules/gcmodule.c
index 45dad8f082..3bda6e4bb8 100644
--- a/Modules/gcmodule.c
+++ b/Modules/gcmodule.c
@@ -1195,14 +1195,6 @@ gc_collect_main(PyThreadState *tstate, int generation,
     assert(gcstate->garbage != NULL);
     assert(!_PyErr_Occurred(tstate));
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    if (tstate->interp->config._isolated_interpreter) {
-        // bpo-40533: The garbage collector must not be run on parallel on
-        // Python objects shared by multiple interpreters.
-        return 0;
-    }
-#endif
-
     if (gcstate->debug & DEBUG_STATS) {
         PySys_WriteStderr("gc: collecting generation %d...\n", generation);
         show_stats_each_generations(gcstate);
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 1bcfd9a9c5..c1bae0b9a6 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -54,11 +54,6 @@ typedef struct PySlot_Offset {
 } PySlot_Offset;
 
 
-/* bpo-40521: Interned strings are shared by all subinterpreters */
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-#  define INTERN_NAME_STRINGS
-#endif
-
 static PyObject *
 slot_tp_new(PyTypeObject *type, PyObject *args, PyObject *kwds);
 
@@ -4009,7 +4004,7 @@ type_setattro(PyTypeObject *type, PyObject *name, PyObject *value)
             if (name == NULL)
                 return -1;
         }
-#ifdef INTERN_NAME_STRINGS
+        /* bpo-40521: Interned strings are shared by all subinterpreters */
         if (!PyUnicode_CHECK_INTERNED(name)) {
             PyUnicode_InternInPlace(&name);
             if (!PyUnicode_CHECK_INTERNED(name)) {
@@ -4019,7 +4014,6 @@ type_setattro(PyTypeObject *type, PyObject *name, PyObject *value)
                 return -1;
             }
         }
-#endif
     }
     else {
         /* Will fail in _PyObject_GenericSetAttrWithDict. */
@@ -8456,17 +8450,11 @@ _PyTypes_InitSlotDefs(void)
     for (slotdef *p = slotdefs; p->name; p++) {
         /* Slots must be ordered by their offset in the PyHeapTypeObject. */
         assert(!p[1].name || p->offset <= p[1].offset);
-#ifdef INTERN_NAME_STRINGS
+        /* bpo-40521: Interned strings are shared by all subinterpreters */
         p->name_strobj = PyUnicode_InternFromString(p->name);
         if (!p->name_strobj || !PyUnicode_CHECK_INTERNED(p->name_strobj)) {
             return _PyStatus_NO_MEMORY();
         }
-#else
-        p->name_strobj = PyUnicode_FromString(p->name);
-        if (!p->name_strobj) {
-            return _PyStatus_NO_MEMORY();
-        }
-#endif
     }
     slotdefs_initialized = 1;
     return _PyStatus_OK();
@@ -8491,24 +8479,17 @@ update_slot(PyTypeObject *type, PyObject *name)
     int offset;
 
     assert(PyUnicode_CheckExact(name));
-#ifdef INTERN_NAME_STRINGS
     assert(PyUnicode_CHECK_INTERNED(name));
-#endif
 
     assert(slotdefs_initialized);
     pp = ptrs;
     for (p = slotdefs; p->name; p++) {
         assert(PyUnicode_CheckExact(p->name_strobj));
         assert(PyUnicode_CheckExact(name));
-#ifdef INTERN_NAME_STRINGS
+        /* bpo-40521: Using interned strings. */
         if (p->name_strobj == name) {
             *pp++ = p;
         }
-#else
-        if (p->name_strobj == name || _PyUnicode_EQ(p->name_strobj, name)) {
-            *pp++ = p;
-        }
-#endif
     }
     *pp = NULL;
     for (pp = ptrs; *pp; pp++) {
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 656c7ccc8e..a7bd961c64 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -219,11 +219,6 @@ extern "C" {
 #  define OVERALLOCATE_FACTOR 4
 #endif
 
-/* bpo-40521: Interned strings are shared by all interpreters. */
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-#  define INTERNED_STRINGS
-#endif
-
 /* This dictionary holds all interned unicode strings.  Note that references
    to strings in this dictionary are *not* counted in the string's ob_refcnt.
    When the interned string reaches a refcnt of 0 the string deallocation
@@ -232,9 +227,7 @@ extern "C" {
    Another way to look at this is that to say that the actual reference
    count of a string is:  s->ob_refcnt + (s->state ? 2 : 0)
 */
-#ifdef INTERNED_STRINGS
 static PyObject *interned = NULL;
-#endif
 
 /* Forward declaration */
 static inline int
@@ -1924,10 +1917,8 @@ unicode_dealloc(PyObject *unicode)
     switch (PyUnicode_CHECK_INTERNED(unicode)) {
     case SSTATE_NOT_INTERNED:
         break;
-
     case SSTATE_INTERNED_MORTAL:
     {
-#ifdef INTERNED_STRINGS
         /* Revive the dead object temporarily. PyDict_DelItem() removes two
            references (key and value) which were ignored by
            PyUnicode_InternInPlace(). Use refcnt=3 rather than refcnt=2
@@ -1941,7 +1932,6 @@ unicode_dealloc(PyObject *unicode)
         }
         assert(Py_REFCNT(unicode) == 1);
         Py_SET_REFCNT(unicode, 0);
-#endif
         break;
     }
 
@@ -11314,13 +11304,11 @@ _PyUnicode_EqualToASCIIId(PyObject *left, _Py_Identifier *right)
     if (PyUnicode_CHECK_INTERNED(left))
         return 0;
 
-#ifdef INTERNED_STRINGS
     assert(_PyUnicode_HASH(right_uni) != -1);
     Py_hash_t hash = _PyUnicode_HASH(left);
     if (hash != -1 && hash != _PyUnicode_HASH(right_uni)) {
         return 0;
     }
-#endif
 
     return unicode_compare_eq(left, right_uni);
 }
@@ -15562,7 +15550,6 @@ PyUnicode_InternInPlace(PyObject **p)
         return;
     }
 
-#ifdef INTERNED_STRINGS
     if (PyUnicode_READY(s) == -1) {
         PyErr_Clear();
         return;
@@ -15593,11 +15580,6 @@ PyUnicode_InternInPlace(PyObject **p)
        this. */
     Py_SET_REFCNT(s, Py_REFCNT(s) - 2);
     _PyUnicode_STATE(s).interned = SSTATE_INTERNED_MORTAL;
-#else
-    // PyDict expects that interned strings have their hash
-    // (PyASCIIObject.hash) already computed.
-    (void)unicode_hash(s);
-#endif
 }
 
 void
diff --git a/Python/ceval.c b/Python/ceval.c
index 62379b4a85..b8b27a815f 100644
--- a/Python/ceval.c
+++ b/Python/ceval.c
@@ -348,21 +348,6 @@ _Py_FatalError_TstateNULL(const char *func)
                        "(the current Python thread state is NULL)");
 }
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-int
-_PyEval_ThreadsInitialized(PyInterpreterState *interp)
-{
-    return gil_created(&interp->ceval.gil);
-}
-
-int
-PyEval_ThreadsInitialized(void)
-{
-    // Fatal error if there is no current interpreter
-    PyInterpreterState *interp = PyInterpreterState_Get();
-    return _PyEval_ThreadsInitialized(interp);
-}
-#else
 int
 _PyEval_ThreadsInitialized(_PyRuntimeState *runtime)
 {
@@ -375,25 +360,18 @@ PyEval_ThreadsInitialized(void)
     _PyRuntimeState *runtime = &_PyRuntime;
     return _PyEval_ThreadsInitialized(runtime);
 }
-#endif
 
 PyStatus
 _PyEval_InitGIL(PyThreadState *tstate)
 {
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     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();
     }
-#endif
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    struct _gil_runtime_state *gil = &tstate->interp->ceval.gil;
-#else
     struct _gil_runtime_state *gil = &tstate->interp->runtime->ceval.gil;
-#endif
     assert(!gil_created(gil));
 
     PyThread_init_thread();
@@ -408,20 +386,14 @@ _PyEval_InitGIL(PyThreadState *tstate)
 void
 _PyEval_FiniGIL(PyInterpreterState *interp)
 {
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     if (!_Py_IsMainInterpreter(interp)) {
         /* Currently, the GIL is shared by all interpreters,
            and only the main interpreter is responsible to create
            and destroy it. */
         return;
     }
-#endif
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    struct _gil_runtime_state *gil = &interp->ceval.gil;
-#else
     struct _gil_runtime_state *gil = &interp->runtime->ceval.gil;
-#endif
     if (!gil_created(gil)) {
         /* First Py_InitializeFromConfig() call: the GIL doesn't exist
            yet: do nothing. */
@@ -485,13 +457,9 @@ PyEval_AcquireThread(PyThreadState *tstate)
     take_gil(tstate);
 
     struct _gilstate_runtime_state *gilstate = &tstate->interp->runtime->gilstate;
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    (void)_PyThreadState_Swap(gilstate, tstate);
-#else
     if (_PyThreadState_Swap(gilstate, tstate) != NULL) {
         Py_FatalError("non-NULL old thread state");
     }
-#endif
 }
 
 void
@@ -518,11 +486,7 @@ _PyEval_ReInitThreads(PyThreadState *tstate)
 {
     _PyRuntimeState *runtime = tstate->interp->runtime;
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    struct _gil_runtime_state *gil = &tstate->interp->ceval.gil;
-#else
     struct _gil_runtime_state *gil = &runtime->ceval.gil;
-#endif
     if (!gil_created(gil)) {
         return _PyStatus_OK();
     }
@@ -554,21 +518,12 @@ PyThreadState *
 PyEval_SaveThread(void)
 {
     _PyRuntimeState *runtime = &_PyRuntime;
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    PyThreadState *old_tstate = _PyThreadState_GET();
-    PyThreadState *tstate = _PyThreadState_Swap(&runtime->gilstate, old_tstate);
-#else
     PyThreadState *tstate = _PyThreadState_Swap(&runtime->gilstate, NULL);
-#endif
     _Py_EnsureTstateNotNULL(tstate);
 
     struct _ceval_runtime_state *ceval = &runtime->ceval;
     struct _ceval_state *ceval2 = &tstate->interp->ceval;
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    assert(gil_created(&ceval2->gil));
-#else
     assert(gil_created(&ceval->gil));
-#endif
     drop_gil(ceval, ceval2, tstate);
     return tstate;
 }
@@ -832,9 +787,7 @@ Py_MakePendingCalls(void)
 void
 _PyEval_InitRuntimeState(struct _ceval_runtime_state *ceval)
 {
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     _gil_initialize(&ceval->gil);
-#endif
 }
 
 void
@@ -844,10 +797,6 @@ _PyEval_InitState(struct _ceval_state *ceval, PyThread_type_lock pending_lock)
     assert(pending->lock == NULL);
 
     pending->lock = pending_lock;
-
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    _gil_initialize(&ceval->gil);
-#endif
 }
 
 void
@@ -1262,13 +1211,9 @@ eval_frame_handle_pending(PyThreadState *tstate)
 
         take_gil(tstate);
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-        (void)_PyThreadState_Swap(&runtime->gilstate, tstate);
-#else
         if (_PyThreadState_Swap(&runtime->gilstate, tstate) != NULL) {
             Py_FatalError("orphan tstate");
         }
-#endif
     }
 
     /* Check for asynchronous exception. */
diff --git a/Python/ceval_gil.h b/Python/ceval_gil.h
index 9b8b43253f..1b2dc7f8e1 100644
--- a/Python/ceval_gil.h
+++ b/Python/ceval_gil.h
@@ -144,11 +144,7 @@ static void
 drop_gil(struct _ceval_runtime_state *ceval, struct _ceval_state *ceval2,
          PyThreadState *tstate)
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    struct _gil_runtime_state *gil = &ceval2->gil;
-#else
     struct _gil_runtime_state *gil = &ceval->gil;
-#endif
     if (!_Py_atomic_load_relaxed(&gil->locked)) {
         Py_FatalError("drop_gil: GIL is not locked");
     }
@@ -232,11 +228,7 @@ take_gil(PyThreadState *tstate)
     PyInterpreterState *interp = tstate->interp;
     struct _ceval_runtime_state *ceval = &interp->runtime->ceval;
     struct _ceval_state *ceval2 = &interp->ceval;
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    struct _gil_runtime_state *gil = &ceval2->gil;
-#else
     struct _gil_runtime_state *gil = &ceval->gil;
-#endif
 
     /* Check that _PyEval_InitThreads() was called to create the lock */
     assert(gil_created(gil));
@@ -328,22 +320,12 @@ _ready:
 
 void _PyEval_SetSwitchInterval(unsigned long microseconds)
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    PyInterpreterState *interp = PyInterpreterState_Get();
-    struct _gil_runtime_state *gil = &interp->ceval.gil;
-#else
     struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;
-#endif
     gil->interval = microseconds;
 }
 
 unsigned long _PyEval_GetSwitchInterval()
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    PyInterpreterState *interp = PyInterpreterState_Get();
-    struct _gil_runtime_state *gil = &interp->ceval.gil;
-#else
     struct _gil_runtime_state *gil = &_PyRuntime.ceval.gil;
-#endif
     return gil->interval;
 }
diff --git a/Python/preconfig.c b/Python/preconfig.c
index afa16cccf3..0deb07a893 100644
--- a/Python/preconfig.c
+++ b/Python/preconfig.c
@@ -294,17 +294,7 @@ _PyPreConfig_InitCompatConfig(PyPreConfig *config)
     config->coerce_c_locale_warn = 0;
 
     config->dev_mode = -1;
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    /* bpo-40512: pymalloc is not compatible with subinterpreters,
-       force usage of libc malloc() which is thread-safe. */
-#ifdef Py_DEBUG
-    config->allocator = PYMEM_ALLOCATOR_MALLOC_DEBUG;
-#else
-    config->allocator = PYMEM_ALLOCATOR_MALLOC;
-#endif
-#else
     config->allocator = PYMEM_ALLOCATOR_NOT_SET;
-#endif
 #ifdef MS_WINDOWS
     config->legacy_windows_fs_encoding = -1;
 #endif
diff --git a/Python/pylifecycle.c b/Python/pylifecycle.c
index 273f6d62b2..960a38aebe 100644
--- a/Python/pylifecycle.c
+++ b/Python/pylifecycle.c
@@ -1986,12 +1986,10 @@ new_interpreter(PyThreadState **tstate_p, int isolated_subinterpreter)
 
     /* Copy the current interpreter config into the new interpreter */
     const PyConfig *config;
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     if (save_tstate != NULL) {
         config = _PyInterpreterState_GetConfig(save_tstate->interp);
     }
     else
-#endif
     {
         /* No current thread state, copy from the main interpreter */
         PyInterpreterState *main_interp = _PyInterpreterState_Main();
diff --git a/Python/pystate.c b/Python/pystate.c
index 3e28a6ab69..df56c0530f 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1165,14 +1165,6 @@ _PyThreadState_DeleteExcept(_PyRuntimeState *runtime, PyThreadState *tstate)
 }
 
 
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-PyThreadState*
-_PyThreadState_GetTSS(void) {
-    return PyThread_tss_get(&_PyRuntime.gilstate.autoTSSkey);
-}
-#endif
-
-
 PyThreadState *
 _PyThreadState_UncheckedGet(void)
 {
@@ -1192,11 +1184,7 @@ PyThreadState_Get(void)
 PyThreadState *
 _PyThreadState_Swap(struct _gilstate_runtime_state *gilstate, PyThreadState *newts)
 {
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    PyThreadState *oldts = _PyThreadState_GetTSS();
-#else
     PyThreadState *oldts = _PyRuntimeGILState_GetThreadState(gilstate);
-#endif
 
     _PyRuntimeGILState_SetThreadState(gilstate, newts);
     /* It should not be possible for more than one thread state
@@ -1215,9 +1203,6 @@ _PyThreadState_Swap(struct _gilstate_runtime_state *gilstate, PyThreadState *new
         errno = err;
     }
 #endif
-#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-    PyThread_tss_set(&gilstate->autoTSSkey, newts);
-#endif
     return oldts;
 }
 
@@ -1665,9 +1650,7 @@ PyGILState_Ensure(void)
 
     /* Ensure that _PyEval_InitThreads() and _PyGILState_Init() have been
        called by Py_Initialize() */
-#ifndef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
     assert(_PyEval_ThreadsInitialized(runtime));
-#endif
     assert(gilstate->autoInterpreterState);
 
     PyThreadState *tcur = (PyThreadState *)PyThread_tss_get(&gilstate->autoTSSkey);
diff --git a/configure b/configure
index b3267025b7..5ee45f320d 100755
--- a/configure
+++ b/configure
@@ -1061,7 +1061,6 @@ with_openssl
 with_openssl_rpath
 with_ssl_default_suites
 with_builtin_hashlib_hashes
-with_experimental_isolated_subinterpreters
 enable_test_modules
 '
       ac_precious_vars='build_alias
@@ -1861,9 +1860,6 @@ Optional Packages:
   --with-builtin-hashlib-hashes=md5,sha1,sha256,sha512,sha3,blake2
                           builtin hash modules, md5, sha1, sha256, sha512,
                           sha3 (with shake), blake2
-  --with-experimental-isolated-subinterpreters
-                          better isolate subinterpreters, experimental build
-                          mode (default is no)
 
 Some influential environment variables:
   PKG_CONFIG  path to pkg-config utility
@@ -22475,30 +22471,6 @@ fi
 
 fi
 
-# --with-experimental-isolated-subinterpreters
-
-{ $as_echo "$as_me:${as_lineno-$LINENO}: checking for --with-experimental-isolated-subinterpreters" >&5
-$as_echo_n "checking for --with-experimental-isolated-subinterpreters... " >&6; }
-
-# Check whether --with-experimental-isolated-subinterpreters was given.
-if test "${with_experimental_isolated_subinterpreters+set}" = set; then :
-  withval=$with_experimental_isolated_subinterpreters;
-if test "$withval" != no
-then
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: yes" >&5
-$as_echo "yes" >&6; };
-  $as_echo "#define EXPERIMENTAL_ISOLATED_SUBINTERPRETERS 1" >>confdefs.h
-
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; };
-fi
-else
-  { $as_echo "$as_me:${as_lineno-$LINENO}: result: no" >&5
-$as_echo "no" >&6; }
-fi
-
-
 # Check whether to disable test modules. Once set, setup.py will not build
 # test extension modules and "make install" will not install test suites.
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for --disable-test-modules" >&5
diff --git a/configure.ac b/configure.ac
index 65ec1393e5..91f124aebb 100644
--- a/configure.ac
+++ b/configure.ac
@@ -6607,23 +6607,6 @@ AS_VAR_IF([with_builtin_blake2], [yes], [
   ], [have_libb2=no])
 ])
 
-# --with-experimental-isolated-subinterpreters
-AH_TEMPLATE(EXPERIMENTAL_ISOLATED_SUBINTERPRETERS,
-            [Better isolate subinterpreters, experimental build mode.])
-AC_MSG_CHECKING(for --with-experimental-isolated-subinterpreters)
-AC_ARG_WITH(experimental-isolated-subinterpreters,
-  AS_HELP_STRING([--with-experimental-isolated-subinterpreters],
-                 [better isolate subinterpreters, experimental build mode (default is no)]),
-[
-if test "$withval" != no
-then
-  AC_MSG_RESULT(yes);
-  AC_DEFINE(EXPERIMENTAL_ISOLATED_SUBINTERPRETERS)
-else
-  AC_MSG_RESULT(no);
-fi],
-[AC_MSG_RESULT(no)])
-
 # Check whether to disable test modules. Once set, setup.py will not build
 # test extension modules and "make install" will not install test suites.
 AC_MSG_CHECKING([for --disable-test-modules])
diff --git a/pyconfig.h.in b/pyconfig.h.in
index 383fd47dd4..b325ef259c 100644
--- a/pyconfig.h.in
+++ b/pyconfig.h.in
@@ -44,9 +44,6 @@
 /* Define if --enable-ipv6 is specified */
 #undef ENABLE_IPV6
 
-/* Better isolate subinterpreters, experimental build mode. */
-#undef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
-
 /* Define to 1 if your system stores words within floats with the most
    significant word first */
 #undef FLOAT_WORDS_BIGENDIAN