diff --git a/Include/internal/pycore_import.h b/Include/internal/pycore_import.h
index c048ae88d9000c..eb2265d370ffbd 100644
--- a/Include/internal/pycore_import.h
+++ b/Include/internal/pycore_import.h
@@ -28,6 +28,13 @@ PyAPI_FUNC(PyObject *) _PyImport_GetModuleAttr(PyObject *, PyObject *);
 PyAPI_FUNC(PyObject *) _PyImport_GetModuleAttrString(const char *, const char *);
 
 
+struct _cached_module_def {
+    char *filename;
+    char *name;
+    PyModuleDef *def;
+    struct _cached_module_def *next;
+};
+
 struct _import_runtime_state {
     /* The builtin modules (defined in config.c). */
     struct _inittab *inittab;
@@ -37,19 +44,15 @@ struct _import_runtime_state {
        See PyInterpreterState.modules_by_index for more info. */
     Py_ssize_t last_module_index;
     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 lock to guard the dict. */
+        /* A lock to guard the cache. */
         PyThread_type_lock mutex;
-        /* A dict mapping (filename, name) to PyModuleDef for modules.
+        /* The actual cache of (filename, name, 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().
+           This is added to in _PyImport_FixupExtensionObject().
            Modules are added there and looked up in _imp.find_extension(). */
-        PyObject *dict;
+        struct _cached_module_def *head;
     } 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 0659084194d293..2dee4dfec72325 100644
--- a/Include/internal/pycore_pystate.h
+++ b/Include/internal/pycore_pystate.h
@@ -125,11 +125,6 @@ PyAPI_FUNC(PyThreadState *) _PyThreadState_New(PyInterpreterState *interp);
 PyAPI_FUNC(void) _PyThreadState_Bind(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 *);
-
 PyAPI_FUNC(PyObject*) _PyThreadState_GetDict(PyThreadState *tstate);
 
 /* The implementation of sys._current_frames()  Returns a dict mapping
diff --git a/Include/internal/pycore_runtime_init.h b/Include/internal/pycore_runtime_init.h
index e72e7422c7207e..4ab358839dbc61 100644
--- a/Include/internal/pycore_runtime_init.h
+++ b/Include/internal/pycore_runtime_init.h
@@ -97,11 +97,6 @@ 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/Misc/NEWS.d/next/Core and Builtins/2023-07-19-09-58-57.gh-issue-105699.tRrKUL.rst b/Misc/NEWS.d/next/Core and Builtins/2023-07-19-09-58-57.gh-issue-105699.tRrKUL.rst
new file mode 100644
index 00000000000000..ed4638a13c7f61
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2023-07-19-09-58-57.gh-issue-105699.tRrKUL.rst	
@@ -0,0 +1,3 @@
+Python no longer crashes due to an infrequent race when simultaneously
+initializing the sys/builtins modules in multiple subinterpreters that have
+their own GILs.
diff --git a/Python/import.c b/Python/import.c
index 3e52a4e4eb1450..8cc05f0329fc14 100644
--- a/Python/import.c
+++ b/Python/import.c
@@ -919,167 +919,134 @@ extensions_lock_release(void)
 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());
+    /* The lock is initialized directly with the general runtime state. */
+    assert(EXTENSIONS.mutex != NULL);
+    //assert(EXTENSIONS.head == NULL);
 }
 
-static PyModuleDef *
-_extensions_cache_get(PyObject *filename, PyObject *name)
+static struct _cached_module_def *
+_extensions_cache_lookup(PyObject *filename, PyObject *name,
+                         struct _cached_module_def **prev_p)
 {
-    PyModuleDef *def = NULL;
-    extensions_lock_acquire();
-
-    PyObject *key = PyTuple_Pack(2, filename, name);
-    if (key == NULL) {
-        goto finally;
+    struct _cached_module_def *prev = NULL;
+    struct _cached_module_def *cached = EXTENSIONS.head;
+    while (cached != NULL) {
+        if (PyUnicode_CompareWithASCIIString(filename, cached->filename) == 0) {
+            if (PyUnicode_CompareWithASCIIString(name, cached->name) == 0) {
+                break;
+            }
+        }
+        prev = cached;
+        cached = cached->next;
     }
-
-    PyObject *extensions = EXTENSIONS.dict;
-    if (extensions == NULL) {
-        goto finally;
+    if (prev_p != NULL) {
+        *prev_p = prev;
     }
-    def = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
+    return cached;
+}
 
-finally:
-    Py_XDECREF(key);
+static PyModuleDef *
+_extensions_cache_get(PyObject *filename, PyObject *name)
+{
+    extensions_lock_acquire();
+    struct _cached_module_def *found =
+                _extensions_cache_lookup(filename, name, NULL);
     extensions_lock_release();
-    return def;
+    return (found != NULL) ? found->def : NULL;
 }
 
 static int
 _extensions_cache_set(PyObject *filename, PyObject *name, PyModuleDef *def)
 {
     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) {
+    struct _cached_module_def *found =
+                _extensions_cache_lookup(filename, name, NULL);
+    if (found != NULL) {
+        assert(found->def == def);
+        res = 0;
         goto finally;
     }
 
-    PyObject *extensions = EXTENSIONS.dict;
-    if (extensions == NULL) {
-        extensions = PyDict_New();
-        if (extensions == NULL) {
-            goto finally;
-        }
-        EXTENSIONS.dict = extensions;
-    }
-
-    PyModuleDef *actual = (PyModuleDef *)PyDict_GetItemWithError(extensions, key);
-    if (PyErr_Occurred()) {
+    /* Create and populate the new entry. */
+    struct _cached_module_def *cached = PyMem_RawMalloc(
+                                            sizeof(struct _cached_module_def));
+    if (cached == NULL) {
+        PyErr_NoMemory();
         goto finally;
     }
-    else if (actual != NULL) {
-        /* We expect it to be static, so it must be the same pointer. */
-        assert(def == actual);
-        res = 0;
+    cached->filename = PyMem_RawMalloc(strlen(PyUnicode_AsUTF8(filename)) + 1);
+    if (cached->filename == NULL) {
+        PyMem_RawFree(cached);
+        PyErr_NoMemory();
         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) {
-        res = -1;
+    cached->name = PyMem_RawMalloc(strlen(PyUnicode_AsUTF8(name)) + 1);
+    if (cached->name == NULL) {
+        PyMem_RawFree(cached->filename);
+        PyMem_RawFree(cached);
+        PyErr_NoMemory();
         goto finally;
     }
+    strcpy(cached->filename, PyUnicode_AsUTF8(filename));
+    strcpy(cached->name, PyUnicode_AsUTF8(name));
+    cached->def = (PyModuleDef *)Py_NewRef(def);
+
+    /* Add it to the list. */
+    cached->next = EXTENSIONS.head;
+    EXTENSIONS.head = cached;
+
     res = 0;
 
 finally:
-    Py_XDECREF(key);
-    if (oldts != NULL) {
-        _PyThreadState_Swap(interp->runtime, oldts);
-        _PyThreadState_UnbindDetached(main_tstate);
-        Py_DECREF(name);
-        Py_DECREF(filename);
-    }
     extensions_lock_release();
     return res;
 }
 
-static int
+static void
 _extensions_cache_delete(PyObject *filename, PyObject *name)
 {
-    int res = -1;
-    PyThreadState *oldts = NULL;
     extensions_lock_acquire();
 
-    PyObject *key = PyTuple_Pack(2, filename, name);
-    if (key == NULL) {
-        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) {
+    struct _cached_module_def *prev = NULL;
+    struct _cached_module_def *found =
+                _extensions_cache_lookup(filename, name, &prev);
+    if (found == 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));
+    /* Drop it from the list. */
+    if (prev == NULL) {
+        assert(EXTENSIONS.head == found);
+        EXTENSIONS.head = found->next;
     }
-
-    if (PyDict_DelItem(extensions, key) < 0) {
-        goto finally;
+    else {
+        prev->next = found->next;
     }
-    res = 0;
+
+    /* Destroy the cache data. */
+    PyMem_RawFree(found->filename);
+    PyMem_RawFree(found->name);
+    PyMem_RawFree(found);
 
 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)
 {
-    /* 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);
+    struct _cached_module_def *cached = EXTENSIONS.head;
+    while (cached != NULL) {
+        struct _cached_module_def *last = cached;
+        cached = cached->next;
+        PyMem_RawFree(last->filename);
+        PyMem_RawFree(last->name);
+        Py_DECREF(last->def);
+        PyMem_RawFree(last);
+    }
 }
 
 
@@ -1233,6 +1200,8 @@ import_find_extension(PyThreadState *tstate, PyObject *name,
         PyObject *m_copy = def->m_base.m_copy;
         /* Module does not support repeated initialization */
         if (m_copy == NULL) {
+            /* It might be a core module (e.g. sys & builtins),
+               for which we don't set m_copy. */
             m_copy = get_core_module_dict(tstate->interp, name, filename);
             if (m_copy == NULL) {
                 return NULL;
@@ -1302,9 +1271,7 @@ clear_singlephase_extension(PyInterpreterState *interp,
     }
 
     /* Clear the cached module def. */
-    if (_extensions_cache_delete(filename, name) < 0) {
-        return -1;
-    }
+    _extensions_cache_delete(filename, name);
 
     return 0;
 }
@@ -3053,6 +3020,8 @@ void
 _PyImport_Fini(void)
 {
     /* Destroy the database used by _PyImport_{Fixup,Find}Extension */
+    // XXX Should we actually leave them (mostly) intact, since we don't
+    // ever dlclose() the module files?
     _extensions_cache_clear_all();
 
     /* Use the same memory allocator as _PyImport_Init(). */
diff --git a/Python/pystate.c b/Python/pystate.c
index a9b404bd5c93e3..c48a2e99e4d878 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -1640,75 +1640,6 @@ _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
 //----------