diff --git a/Include/internal/pycore_import.h b/Include/internal/pycore_import.h
index 69ed6273b7e..7a78a91aa61 100644
--- a/Include/internal/pycore_import.h
+++ b/Include/internal/pycore_import.h
@@ -14,13 +14,19 @@ struct _import_runtime_state {
        which is just about every time an extension module is imported.
        See PyInterpreterState.modules_by_index for more info. */
     Py_ssize_t last_module_index;
-    /* A dict mapping (filename, name) to PyModuleDef for modules.
-       Only legacy (single-phase init) extension modules are added
-       and only if they support multiple initialization (m_size >- 0)
-       or are imported in the main interpreter.
-       This is initialized lazily in _PyImport_FixupExtensionObject().
-       Modules are added there and looked up in _imp.find_extension(). */
-    PyObject *extensions;
+    struct {
+        /* A thread state tied to the main interpreter,
+           used exclusively for when the extensions dict is access/modified
+           from an arbitrary thread. */
+        PyThreadState main_tstate;
+        /* A dict mapping (filename, name) to PyModuleDef for modules.
+           Only legacy (single-phase init) extension modules are added
+           and only if they support multiple initialization (m_size >- 0)
+           or are imported in the main interpreter.
+           This is initialized lazily in _PyImport_FixupExtensionObject().
+           Modules are added there and looked up in _imp.find_extension(). */
+        PyObject *dict;
+    } extensions;
     /* Package context -- the full module name for package imports */
     const char * pkgcontext;
 };
diff --git a/Include/internal/pycore_pystate.h b/Include/internal/pycore_pystate.h
index 7046ec8d9ad..b5408622d9d 100644
--- a/Include/internal/pycore_pystate.h
+++ b/Include/internal/pycore_pystate.h
@@ -127,6 +127,11 @@ PyAPI_FUNC(void) _PyThreadState_Init(
     PyThreadState *tstate);
 PyAPI_FUNC(void) _PyThreadState_DeleteExcept(PyThreadState *tstate);
 
+extern void _PyThreadState_InitDetached(PyThreadState *, PyInterpreterState *);
+extern void _PyThreadState_ClearDetached(PyThreadState *);
+extern void _PyThreadState_BindDetached(PyThreadState *);
+extern void _PyThreadState_UnbindDetached(PyThreadState *);
+
 
 static inline void
 _PyThreadState_UpdateTracingState(PyThreadState *tstate)
diff --git a/Include/internal/pycore_runtime_init.h b/Include/internal/pycore_runtime_init.h
index 7cfa7c0c024..5b09a45e41c 100644
--- a/Include/internal/pycore_runtime_init.h
+++ b/Include/internal/pycore_runtime_init.h
@@ -41,6 +41,11 @@ extern PyTypeObject _PyExc_MemoryError;
            in accordance with the specification. */ \
         .autoTSSkey = Py_tss_NEEDS_INIT, \
         .parser = _parser_runtime_state_INIT, \
+        .imports = { \
+            .extensions = { \
+                .main_tstate = _PyThreadState_INIT, \
+            }, \
+        }, \
         .ceval = { \
             .perf = _PyEval_RUNTIME_PERF_INIT, \
         }, \
diff --git a/Python/import.c b/Python/import.c
index c68bd1f7db4..a45b3bfaacb 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -862,6 +862,18 @@ Generally, when multiple interpreters are involved, some of the above
 gets even messier.
 */
 
+static inline void
+extensions_lock_acquire(void)
+{
+    // XXX For now the GIL is sufficient.
+}
+
+static inline void
+extensions_lock_release(void)
+{
+    // XXX For now the GIL is sufficient.
+}
+
 /* Magic for extension modules (built-in as well as dynamically
    loaded).  To prevent initializing an extension module more than
    once, we keep a static dictionary 'extensions' keyed by the tuple
@@ -878,71 +890,170 @@ gets even messier.
    dictionary, to avoid loading shared libraries twice.
 */
 
+static void
+_extensions_cache_init(void)
+{
+    /* The runtime (i.e. main interpreter) must be initializing,
+       so we don't need to worry about the lock. */
+    _PyThreadState_InitDetached(&EXTENSIONS.main_tstate,
+                                _PyInterpreterState_Main());
+}
+
 static PyModuleDef *
 _extensions_cache_get(PyObject *filename, PyObject *name)
 {
-    PyObject *extensions = EXTENSIONS;
-    if (extensions == NULL) {
-        return NULL;
-    }
+    PyModuleDef *def = NULL;
+    extensions_lock_acquire();
+
     PyObject *key = PyTuple_Pack(2, filename, name);
     if (key == NULL) {
-        return NULL;
+        goto finally;
     }
-    PyModuleDef *def = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
-    Py_DECREF(key);
+
+    PyObject *extensions = EXTENSIONS.dict;
+    if (extensions == NULL) {
+        goto finally;
+    }
+    def = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
+
+finally:
+    Py_XDECREF(key);
+    extensions_lock_release();
     return def;
 }
 
 static int
 _extensions_cache_set(PyObject *filename, PyObject *name, PyModuleDef *def)
 {
-    PyObject *extensions = EXTENSIONS;
+    int res = -1;
+    PyThreadState *oldts = NULL;
+    extensions_lock_acquire();
+
+    /* Swap to the main interpreter, if necessary.  This matters if
+       the dict hasn't been created yet or if the item isn't in the
+       dict yet.  In both cases we must ensure the relevant objects
+       are created using the main interpreter. */
+    PyThreadState *main_tstate = &EXTENSIONS.main_tstate;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (!_Py_IsMainInterpreter(interp)) {
+        _PyThreadState_BindDetached(main_tstate);
+        oldts = _PyThreadState_Swap(interp->runtime, main_tstate);
+        assert(!_Py_IsMainInterpreter(oldts->interp));
+
+        /* Make sure the name and filename objects are owned
+           by the main interpreter. */
+        name = PyUnicode_InternFromString(PyUnicode_AsUTF8(name));
+        assert(name != NULL);
+        filename = PyUnicode_InternFromString(PyUnicode_AsUTF8(filename));
+        assert(filename != NULL);
+    }
+
+    PyObject *key = PyTuple_Pack(2, filename, name);
+    if (key == NULL) {
+        goto finally;
+    }
+
+    PyObject *extensions = EXTENSIONS.dict;
     if (extensions == NULL) {
         extensions = PyDict_New();
         if (extensions == NULL) {
-            return -1;
+            goto finally;
         }
-        EXTENSIONS = extensions;
+        EXTENSIONS.dict = extensions;
     }
-    PyObject *key = PyTuple_Pack(2, filename, name);
-    if (key == NULL) {
-        return -1;
+
+    PyModuleDef *actual = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
+    if (PyErr_Occurred()) {
+        goto finally;
     }
-    int res = PyDict_SetItem(extensions, key, (PyObject *)def);
-    Py_DECREF(key);
+    else if (actual != NULL) {
+        /* We expect it to be static, so it must be the same pointer. */
+        assert(def == actual);
+        res = 0;
+        goto finally;
+    }
+
+    /* This might trigger a resize, which is why we must switch
+       to the main interpreter. */
+    res = PyDict_SetItem(extensions, key, (PyObject *)def);
     if (res < 0) {
-        return -1;
+        res = -1;
+        goto finally;
     }
-    return 0;
+    res = 0;
+
+finally:
+    if (oldts != NULL) {
+        _PyThreadState_Swap(interp->runtime, oldts);
+        _PyThreadState_UnbindDetached(main_tstate);
+        Py_DECREF(name);
+        Py_DECREF(filename);
+    }
+    Py_XDECREF(key);
+    extensions_lock_release();
+    return res;
 }
 
 static int
 _extensions_cache_delete(PyObject *filename, PyObject *name)
 {
-    PyObject *extensions = EXTENSIONS;
-    if (extensions == NULL) {
-        return 0;
-    }
+    int res = -1;
+    PyThreadState *oldts = NULL;
+    extensions_lock_acquire();
+
     PyObject *key = PyTuple_Pack(2, filename, name);
     if (key == NULL) {
-        return -1;
+        goto finally;
     }
+
+    PyObject *extensions = EXTENSIONS.dict;
+    if (extensions == NULL) {
+        res = 0;
+        goto finally;
+    }
+
+    PyModuleDef *actual = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
+    if (PyErr_Occurred()) {
+        goto finally;
+    }
+    else if (actual == NULL) {
+        /* It was already removed or never added. */
+        res = 0;
+        goto finally;
+    }
+
+    /* Swap to the main interpreter, if necessary. */
+    PyThreadState *main_tstate = &EXTENSIONS.main_tstate;
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    if (!_Py_IsMainInterpreter(interp)) {
+        _PyThreadState_BindDetached(main_tstate);
+        oldts = _PyThreadState_Swap(interp->runtime, main_tstate);
+        assert(!_Py_IsMainInterpreter(oldts->interp));
+    }
+
     if (PyDict_DelItem(extensions, key) < 0) {
-        if (!PyErr_ExceptionMatches(PyExc_KeyError)) {
-            Py_DECREF(key);
-            return -1;
-        }
-        PyErr_Clear();
+        goto finally;
     }
-    Py_DECREF(key);
-    return 0;
+    res = 0;
+
+finally:
+    if (oldts != NULL) {
+        _PyThreadState_Swap(interp->runtime, oldts);
+        _PyThreadState_UnbindDetached(main_tstate);
+    }
+    Py_XDECREF(key);
+    extensions_lock_release();
+    return res;
 }
 
 static void
 _extensions_cache_clear_all(void)
 {
-    Py_CLEAR(EXTENSIONS);
+    /* The runtime (i.e. main interpreter) must be finalizing,
+       so we don't need to worry about the lock. */
+    // XXX assert(_Py_IsMainInterpreter(_PyInterpreterState_GET()));
+    Py_CLEAR(EXTENSIONS.dict);
+    _PyThreadState_ClearDetached(&EXTENSIONS.main_tstate);
 }
 
 
@@ -2941,6 +3052,10 @@ _PyImport_Fini2(void)
 PyStatus
 _PyImport_InitCore(PyThreadState *tstate, PyObject *sysmod, int importlib)
 {
+    if (_Py_IsMainInterpreter(tstate->interp)) {
+        _extensions_cache_init();
+    }
+
     // XXX Initialize here: interp->modules and interp->import_func.
     // XXX Initialize here: sys.modules and sys.meta_path.
 
diff --git a/Python/pystate.c b/Python/pystate.c
index b17efdbefd1..1e59a8c5f89 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1217,8 +1217,7 @@ free_threadstate(PyThreadState *tstate)
 
 static void
 init_threadstate(PyThreadState *tstate,
-                 PyInterpreterState *interp, uint64_t id,
-                 PyThreadState *next)
+                 PyInterpreterState *interp, uint64_t id)
 {
     if (tstate->_status.initialized) {
         Py_FatalError("thread state already initialized");
@@ -1227,18 +1226,13 @@ init_threadstate(PyThreadState *tstate,
     assert(interp != NULL);
     tstate->interp = interp;
 
+    // next/prev are set in add_threadstate().
+    assert(tstate->next == NULL);
+    assert(tstate->prev == NULL);
+
     assert(id > 0);
     tstate->id = id;
 
-    assert(interp->threads.head == tstate);
-    assert((next != NULL && id != 1) || (next == NULL && id == 1));
-    if (next != NULL) {
-        assert(next->prev == NULL || next->prev == tstate);
-        next->prev = tstate;
-    }
-    tstate->next = next;
-    assert(tstate->prev == NULL);
-
     // thread_id and native_thread_id are set in bind_tstate().
 
     tstate->py_recursion_limit = interp->ceval.recursion_limit,
@@ -1259,6 +1253,22 @@ init_threadstate(PyThreadState *tstate,
     tstate->_status.initialized = 1;
 }
 
+static void
+add_threadstate(PyInterpreterState *interp, PyThreadState *tstate,
+                PyThreadState *next)
+{
+    assert(interp->threads.head != tstate);
+    assert((next != NULL && tstate->id != 1) ||
+           (next == NULL && tstate->id == 1));
+    if (next != NULL) {
+        assert(next->prev == NULL || next->prev == tstate);
+        next->prev = tstate;
+    }
+    tstate->next = next;
+    assert(tstate->prev == NULL);
+    interp->threads.head = tstate;
+}
+
 static PyThreadState *
 new_threadstate(PyInterpreterState *interp)
 {
@@ -1298,9 +1308,9 @@ new_threadstate(PyInterpreterState *interp)
                &initial._main_interpreter._initial_thread,
                sizeof(*tstate));
     }
-    interp->threads.head = tstate;
 
-    init_threadstate(tstate, interp, id, old_head);
+    init_threadstate(tstate, interp, id);
+    add_threadstate(interp, tstate, old_head);
 
     HEAD_UNLOCK(runtime);
     if (!used_newtstate) {
@@ -1347,6 +1357,19 @@ _PyThreadState_Init(PyThreadState *tstate)
     Py_FatalError("_PyThreadState_Init() is for internal use only");
 }
 
+
+static void
+clear_datastack(PyThreadState *tstate)
+{
+    _PyStackChunk *chunk = tstate->datastack_chunk;
+    tstate->datastack_chunk = NULL;
+    while (chunk != NULL) {
+        _PyStackChunk *prev = chunk->previous;
+        _PyObject_VirtualFree(chunk, chunk->size);
+        chunk = prev;
+    }
+}
+
 void
 PyThreadState_Clear(PyThreadState *tstate)
 {
@@ -1421,7 +1444,6 @@ PyThreadState_Clear(PyThreadState *tstate)
     // XXX Do it as early in the function as possible.
 }
 
-
 /* Common code for PyThreadState_Delete() and PyThreadState_DeleteCurrent() */
 static void
 tstate_delete_common(PyThreadState *tstate)
@@ -1454,18 +1476,11 @@ tstate_delete_common(PyThreadState *tstate)
     unbind_tstate(tstate);
 
     // XXX Move to PyThreadState_Clear()?
-    _PyStackChunk *chunk = tstate->datastack_chunk;
-    tstate->datastack_chunk = NULL;
-    while (chunk != NULL) {
-        _PyStackChunk *prev = chunk->previous;
-        _PyObject_VirtualFree(chunk, chunk->size);
-        chunk = prev;
-    }
+    clear_datastack(tstate);
 
     tstate->_status.finalized = 1;
 }
 
-
 static void
 zapthreads(PyInterpreterState *interp)
 {
@@ -1552,6 +1567,75 @@ _PyThreadState_DeleteExcept(PyThreadState *tstate)
 }
 
 
+//-------------------------
+// "detached" thread states
+//-------------------------
+
+void
+_PyThreadState_InitDetached(PyThreadState *tstate, PyInterpreterState *interp)
+{
+    _PyRuntimeState *runtime = interp->runtime;
+
+    HEAD_LOCK(runtime);
+    interp->threads.next_unique_id += 1;
+    uint64_t id = interp->threads.next_unique_id;
+    HEAD_UNLOCK(runtime);
+
+    init_threadstate(tstate, interp, id);
+    // We do not call add_threadstate().
+}
+
+void
+_PyThreadState_ClearDetached(PyThreadState *tstate)
+{
+    assert(!tstate->_status.bound);
+    assert(!tstate->_status.bound_gilstate);
+    assert(tstate->datastack_chunk == NULL);
+    assert(tstate->thread_id == 0);
+    assert(tstate->native_thread_id == 0);
+    assert(tstate->next == NULL);
+    assert(tstate->prev == NULL);
+
+    PyThreadState_Clear(tstate);
+    clear_datastack(tstate);
+}
+
+void
+_PyThreadState_BindDetached(PyThreadState *tstate)
+{
+    assert(!_Py_IsMainInterpreter(
+        current_fast_get(tstate->interp->runtime)->interp));
+    assert(_Py_IsMainInterpreter(tstate->interp));
+    bind_tstate(tstate);
+    /* Unlike _PyThreadState_Bind(), we do not modify gilstate TSS. */
+}
+
+void
+_PyThreadState_UnbindDetached(PyThreadState *tstate)
+{
+    assert(!_Py_IsMainInterpreter(
+        current_fast_get(tstate->interp->runtime)->interp));
+    assert(_Py_IsMainInterpreter(tstate->interp));
+    assert(tstate_is_alive(tstate));
+    assert(!tstate->_status.active);
+    assert(gilstate_tss_get(tstate->interp->runtime) != tstate);
+
+    unbind_tstate(tstate);
+
+    /* This thread state may be bound/unbound repeatedly,
+       so we must erase evidence that it was ever bound (or unbound). */
+    tstate->_status.bound = 0;
+    tstate->_status.unbound = 0;
+
+    /* We must fully unlink the thread state from any OS thread,
+       to allow it to be bound more than once. */
+    tstate->thread_id = 0;
+#ifdef PY_HAVE_THREAD_NATIVE_ID
+    tstate->native_thread_id = 0;
+#endif
+}
+
+
 //----------
 // accessors
 //----------