gh-111956: Add thread-safe one-time initialization. (gh-111960)

Include/internal/pycore_ast_state.h

@@ -2,6 +2,9 @@
 
 #ifndef Py_INTERNAL_AST_STATE_H
 #define Py_INTERNAL_AST_STATE_H
+
+#include "pycore_lock.h"    // _PyOnceFlag
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -11,7 +14,8 @@ extern "C" {
 #endif
 
 struct ast_state {
-    int initialized;
+    _PyOnceFlag once;
+    int finalized;
     int recursion_depth;
     int recursion_limit;
     PyObject *AST_type;

Include/internal/pycore_lock.h

@@ -46,6 +46,7 @@ typedef struct _PyMutex PyMutex;
 #define _Py_UNLOCKED    0
 #define _Py_LOCKED      1
 #define _Py_HAS_PARKED  2
+#define _Py_ONCE_INITIALIZED 4
 
 // (private) slow path for locking the mutex
 PyAPI_FUNC(void) _PyMutex_LockSlow(PyMutex *m);
@@ -166,6 +167,35 @@ _PyRawMutex_Unlock(_PyRawMutex *m)
     _PyRawMutex_UnlockSlow(m);
 }
 
+// A data structure that can be used to run initialization code once in a
+// thread-safe manner. The C++11 equivalent is std::call_once.
+typedef struct {
+    uint8_t v;
+} _PyOnceFlag;
+
+// Type signature for one-time initialization functions. The function should
+// return 0 on success and -1 on failure.
+typedef int _Py_once_fn_t(void *arg);
+
+// (private) slow path for one time initialization
+PyAPI_FUNC(int)
+_PyOnceFlag_CallOnceSlow(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg);
+
+// Calls `fn` once using `flag`. The `arg` is passed to the call to `fn`.
+//
+// Returns 0 on success and -1 on failure.
+//
+// If `fn` returns 0 (success), then subsequent calls immediately return 0.
+// If `fn` returns -1 (failure), then subsequent calls will retry the call.
+static inline int
+_PyOnceFlag_CallOnce(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg)
+{
+    if (_Py_atomic_load_uint8(&flag->v) == _Py_ONCE_INITIALIZED) {
+        return 0;
+    }
+    return _PyOnceFlag_CallOnceSlow(flag, fn, arg);
+}
+
 #ifdef __cplusplus
 }
 #endif

Include/internal/pycore_modsupport.h

@@ -1,5 +1,8 @@
 #ifndef Py_INTERNAL_MODSUPPORT_H
 #define Py_INTERNAL_MODSUPPORT_H
+
+#include "pycore_lock.h"    // _PyOnceFlag
+
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -65,15 +68,16 @@ PyAPI_FUNC(void) _PyArg_BadArgument(
 // --- _PyArg_Parser API ---------------------------------------------------
 
 typedef struct _PyArg_Parser {
-    int initialized;
     const char *format;
     const char * const *keywords;
     const char *fname;
     const char *custom_msg;
-    int pos;            /* number of positional-only arguments */
+    _PyOnceFlag once;       /* atomic one-time initialization flag */
-    int min;            /* minimal number of arguments */
+    int is_kwtuple_owned;   /* does this parser own the kwtuple object? */
-    int max;            /* maximal number of positional arguments */
+    int pos;                /* number of positional-only arguments */
-    PyObject *kwtuple;  /* tuple of keyword parameter names */
+    int min;                /* minimal number of arguments */
+    int max;                /* maximal number of positional arguments */
+    PyObject *kwtuple;      /* tuple of keyword parameter names */
     struct _PyArg_Parser *next;
 } _PyArg_Parser;
 

Include/internal/pycore_runtime.h

@@ -27,7 +27,6 @@ extern "C" {
 #include "pycore_unicodeobject.h"   // struct _Py_unicode_runtime_state
 
 struct _getargs_runtime_state {
-    PyThread_type_lock mutex;
     struct _PyArg_Parser *static_parsers;
 };
 

Misc/NEWS.d/next/C API/2023-11-10-10-24-28.gh-issue-111956.ImE6Cx.rst

@@ -0,0 +1,2 @@
+Add internal-only one-time initialization API: ``_PyOnceFlag`` and
+``_PyOnceFlag_CallOnce``.

Modules/_testinternalcapi/test_lock.c

@@ -341,6 +341,37 @@ test_lock_benchmark(PyObject *module, PyObject *obj)
     Py_RETURN_NONE;
 }
 
+static int
+init_maybe_fail(void *arg)
+{
+    int *counter = (int *)arg;
+    (*counter)++;
+    if (*counter < 5) {
+        // failure
+        return -1;
+    }
+    assert(*counter == 5);
+    return 0;
+}
+
+static PyObject *
+test_lock_once(PyObject *self, PyObject *obj)
+{
+    _PyOnceFlag once = {0};
+    int counter = 0;
+    for (int i = 0; i < 10; i++) {
+        int res = _PyOnceFlag_CallOnce(&once, init_maybe_fail, &counter);
+        if (i < 4) {
+            assert(res == -1);
+        }
+        else {
+            assert(res == 0);
+            assert(counter == 5);
+        }
+    }
+    Py_RETURN_NONE;
+}
+
 static PyMethodDef test_methods[] = {
     {"test_lock_basic", test_lock_basic, METH_NOARGS},
     {"test_lock_two_threads", test_lock_two_threads, METH_NOARGS},
@@ -348,6 +379,7 @@ static PyMethodDef test_methods[] = {
     {"test_lock_counter_slow", test_lock_counter_slow, METH_NOARGS},
     _TESTINTERNALCAPI_BENCHMARK_LOCKS_METHODDEF
     {"test_lock_benchmark", test_lock_benchmark, METH_NOARGS},
+    {"test_lock_once", test_lock_once, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };
 

Parser/asdl_c.py

@@ -518,7 +518,7 @@ class Obj2ModVisitor(PickleVisitor):
         if add_label:
             self.emit("failed:", 1)
             self.emit("Py_XDECREF(tmp);", 1)
-        self.emit("return 1;", 1)
+        self.emit("return -1;", 1)
         self.emit("}", 0)
         self.emit("", 0)
 
@@ -529,7 +529,7 @@ class Obj2ModVisitor(PickleVisitor):
                     "state->%s_type);")
             self.emit(line % (t.name,), 1)
             self.emit("if (isinstance == -1) {", 1)
-            self.emit("return 1;", 2)
+            self.emit("return -1;", 2)
             self.emit("}", 1)
             self.emit("if (isinstance) {", 1)
             self.emit("*out = %s;" % t.name, 2)
@@ -558,7 +558,7 @@ class Obj2ModVisitor(PickleVisitor):
             self.emit("tp = state->%s_type;" % (t.name,), 1)
             self.emit("isinstance = PyObject_IsInstance(obj, tp);", 1)
             self.emit("if (isinstance == -1) {", 1)
-            self.emit("return 1;", 2)
+            self.emit("return -1;", 2)
             self.emit("}", 1)
             self.emit("if (isinstance) {", 1)
             for f in t.fields:
@@ -605,7 +605,7 @@ class Obj2ModVisitor(PickleVisitor):
         self.emit("return 0;", 1)
         self.emit("failed:", 0)
         self.emit("Py_XDECREF(tmp);", 1)
-        self.emit("return 1;", 1)
+        self.emit("return -1;", 1)
         self.emit("}", 0)
         self.emit("", 0)
 
@@ -631,13 +631,13 @@ class Obj2ModVisitor(PickleVisitor):
         ctype = get_c_type(field.type)
         line = "if (PyObject_GetOptionalAttr(obj, state->%s, &tmp) < 0) {"
         self.emit(line % field.name, depth)
-        self.emit("return 1;", depth+1)
+        self.emit("return -1;", depth+1)
         self.emit("}", depth)
         if field.seq:
             self.emit("if (tmp == NULL) {", depth)
             self.emit("tmp = PyList_New(0);", depth+1)
             self.emit("if (tmp == NULL) {", depth+1)
-            self.emit("return 1;", depth+2)
+            self.emit("return -1;", depth+2)
             self.emit("}", depth+1)
             self.emit("}", depth)
             self.emit("{", depth)
@@ -647,7 +647,7 @@ class Obj2ModVisitor(PickleVisitor):
                 message = "required field \\\"%s\\\" missing from %s" % (field.name, name)
                 format = "PyErr_SetString(PyExc_TypeError, \"%s\");"
                 self.emit(format % message, depth+1, reflow=False)
-                self.emit("return 1;", depth+1)
+                self.emit("return -1;", depth+1)
             else:
                 self.emit("if (tmp == NULL || tmp == Py_None) {", depth)
                 self.emit("Py_CLEAR(tmp);", depth+1)
@@ -968,16 +968,16 @@ add_attributes(struct ast_state *state, PyObject *type, const char * const *attr
     int i, result;
     PyObject *s, *l = PyTuple_New(num_fields);
     if (!l)
-        return 0;
+        return -1;
     for (i = 0; i < num_fields; i++) {
         s = PyUnicode_InternFromString(attrs[i]);
         if (!s) {
             Py_DECREF(l);
-            return 0;
+            return -1;
         }
         PyTuple_SET_ITEM(l, i, s);
     }
-    result = PyObject_SetAttr(type, state->_attributes, l) >= 0;
+    result = PyObject_SetAttr(type, state->_attributes, l);
     Py_DECREF(l);
     return result;
 }
@@ -1052,7 +1052,7 @@ static int obj2ast_identifier(struct ast_state *state, PyObject* obj, PyObject**
 {
     if (!PyUnicode_CheckExact(obj) && obj != Py_None) {
         PyErr_SetString(PyExc_TypeError, "AST identifier must be of type str");
-        return 1;
+        return -1;
     }
     return obj2ast_object(state, obj, out, arena);
 }
@@ -1061,7 +1061,7 @@ static int obj2ast_string(struct ast_state *state, PyObject* obj, PyObject** out
 {
     if (!PyUnicode_CheckExact(obj) && !PyBytes_CheckExact(obj)) {
         PyErr_SetString(PyExc_TypeError, "AST string must be of type str");
-        return 1;
+        return -1;
     }
     return obj2ast_object(state, obj, out, arena);
 }
@@ -1071,12 +1071,12 @@ static int obj2ast_int(struct ast_state* Py_UNUSED(state), PyObject* obj, int* o
     int i;
     if (!PyLong_Check(obj)) {
         PyErr_Format(PyExc_ValueError, "invalid integer value: %R", obj);
-        return 1;
+        return -1;
     }
 
     i = PyLong_AsInt(obj);
     if (i == -1 && PyErr_Occurred())
-        return 1;
+        return -1;
     *out = i;
     return 0;
 }
@@ -1102,22 +1102,15 @@ static int add_ast_fields(struct ast_state *state)
             static int
             init_types(struct ast_state *state)
             {
-                // init_types() must not be called after _PyAST_Fini()
-                // has been called
-                assert(state->initialized >= 0);
-
-                if (state->initialized) {
-                    return 1;
-                }
                 if (init_identifiers(state) < 0) {
-                    return 0;
+                    return -1;
                 }
                 state->AST_type = PyType_FromSpec(&AST_type_spec);
                 if (!state->AST_type) {
-                    return 0;
+                    return -1;
                 }
                 if (add_ast_fields(state) < 0) {
-                    return 0;
+                    return -1;
                 }
         '''))
         for dfn in mod.dfns:
@@ -1125,8 +1118,7 @@ static int add_ast_fields(struct ast_state *state)
         self.file.write(textwrap.dedent('''
                 state->recursion_depth = 0;
                 state->recursion_limit = 0;
-                state->initialized = 1;
+                return 0;
-                return 1;
             }
         '''))
 
@@ -1138,12 +1130,12 @@ static int add_ast_fields(struct ast_state *state)
         self.emit('state->%s_type = make_type(state, "%s", state->AST_type, %s, %d,' %
                         (name, name, fields, len(prod.fields)), 1)
         self.emit('%s);' % reflow_c_string(asdl_of(name, prod), 2), 2, reflow=False)
-        self.emit("if (!state->%s_type) return 0;" % name, 1)
+        self.emit("if (!state->%s_type) return -1;" % name, 1)
         if prod.attributes:
-            self.emit("if (!add_attributes(state, state->%s_type, %s_attributes, %d)) return 0;" %
+            self.emit("if (add_attributes(state, state->%s_type, %s_attributes, %d) < 0) return -1;" %
                             (name, name, len(prod.attributes)), 1)
         else:
-            self.emit("if (!add_attributes(state, state->%s_type, NULL, 0)) return 0;" % name, 1)
+            self.emit("if (add_attributes(state, state->%s_type, NULL, 0) < 0) return -1;" % name, 1)
         self.emit_defaults(name, prod.fields, 1)
         self.emit_defaults(name, prod.attributes, 1)
 
@@ -1151,12 +1143,12 @@ static int add_ast_fields(struct ast_state *state)
         self.emit('state->%s_type = make_type(state, "%s", state->AST_type, NULL, 0,' %
                   (name, name), 1)
         self.emit('%s);' % reflow_c_string(asdl_of(name, sum), 2), 2, reflow=False)
-        self.emit("if (!state->%s_type) return 0;" % name, 1)
+        self.emit("if (!state->%s_type) return -1;" % name, 1)
         if sum.attributes:
-            self.emit("if (!add_attributes(state, state->%s_type, %s_attributes, %d)) return 0;" %
+            self.emit("if (add_attributes(state, state->%s_type, %s_attributes, %d) < 0) return -1;" %
                             (name, name, len(sum.attributes)), 1)
         else:
-            self.emit("if (!add_attributes(state, state->%s_type, NULL, 0)) return 0;" % name, 1)
+            self.emit("if (add_attributes(state, state->%s_type, NULL, 0) < 0) return -1;" % name, 1)
         self.emit_defaults(name, sum.attributes, 1)
         simple = is_simple(sum)
         for t in sum.types:
@@ -1170,20 +1162,20 @@ static int add_ast_fields(struct ast_state *state)
         self.emit('state->%s_type = make_type(state, "%s", state->%s_type, %s, %d,' %
                             (cons.name, cons.name, name, fields, len(cons.fields)), 1)
         self.emit('%s);' % reflow_c_string(asdl_of(cons.name, cons), 2), 2, reflow=False)
-        self.emit("if (!state->%s_type) return 0;" % cons.name, 1)
+        self.emit("if (!state->%s_type) return -1;" % cons.name, 1)
         self.emit_defaults(cons.name, cons.fields, 1)
         if simple:
             self.emit("state->%s_singleton = PyType_GenericNew((PyTypeObject *)"
                       "state->%s_type, NULL, NULL);" %
                              (cons.name, cons.name), 1)
-            self.emit("if (!state->%s_singleton) return 0;" % cons.name, 1)
+            self.emit("if (!state->%s_singleton) return -1;" % cons.name, 1)
 
     def emit_defaults(self, name, fields, depth):
         for field in fields:
             if field.opt:
                 self.emit('if (PyObject_SetAttr(state->%s_type, state->%s, Py_None) == -1)' %
                             (name, field.name), depth)
-                self.emit("return 0;", depth+1)
+                self.emit("return -1;", depth+1)
 
 
 class ASTModuleVisitor(PickleVisitor):
@@ -1279,7 +1271,7 @@ class ObjVisitor(PickleVisitor):
         self.emit("if (++state->recursion_depth > state->recursion_limit) {", 1)
         self.emit("PyErr_SetString(PyExc_RecursionError,", 2)
         self.emit('"maximum recursion depth exceeded during ast construction");', 3)
-        self.emit("return 0;", 2)
+        self.emit("return NULL;", 2)
         self.emit("}", 1)
 
     def func_end(self):
@@ -1400,7 +1392,7 @@ PyObject* PyAST_mod2obj(mod_ty t)
     int COMPILER_STACK_FRAME_SCALE = 2;
     PyThreadState *tstate = _PyThreadState_GET();
     if (!tstate) {
-        return 0;
+        return NULL;
     }
     state->recursion_limit = Py_C_RECURSION_LIMIT * COMPILER_STACK_FRAME_SCALE;
     int recursion_depth = Py_C_RECURSION_LIMIT - tstate->c_recursion_remaining;
@@ -1414,7 +1406,7 @@ PyObject* PyAST_mod2obj(mod_ty t)
         PyErr_Format(PyExc_SystemError,
             "AST constructor recursion depth mismatch (before=%d, after=%d)",
             starting_recursion_depth, state->recursion_depth);
-        return 0;
+        return NULL;
     }
     return result;
 }
@@ -1481,7 +1473,8 @@ class ChainOfVisitors:
 
 def generate_ast_state(module_state, f):
     f.write('struct ast_state {\n')
-    f.write('    int initialized;\n')
+    f.write('    _PyOnceFlag once;\n')
+    f.write('    int finalized;\n')
     f.write('    int recursion_depth;\n')
     f.write('    int recursion_limit;\n')
     for s in module_state:
@@ -1501,11 +1494,8 @@ def generate_ast_fini(module_state, f):
     f.write(textwrap.dedent("""
                 Py_CLEAR(_Py_INTERP_CACHED_OBJECT(interp, str_replace_inf));
 
-            #if !defined(NDEBUG)
+                state->finalized = 1;
-                state->initialized = -1;
+                state->once = (_PyOnceFlag){0};
-            #else
-                state->initialized = 0;
-            #endif
             }
 
     """))
@@ -1544,6 +1534,7 @@ def generate_module_def(mod, metadata, f, internal_h):
         #include "pycore_ast.h"
         #include "pycore_ast_state.h"     // struct ast_state
         #include "pycore_ceval.h"         // _Py_EnterRecursiveCall
+        #include "pycore_lock.h"          // _PyOnceFlag
         #include "pycore_interp.h"        // _PyInterpreterState.ast
         #include "pycore_pystate.h"       // _PyInterpreterState_GET()
         #include <stddef.h>
@@ -1556,7 +1547,8 @@ def generate_module_def(mod, metadata, f, internal_h):
         {
             PyInterpreterState *interp = _PyInterpreterState_GET();
             struct ast_state *state = &interp->ast;
-            if (!init_types(state)) {
+            assert(!state->finalized);
+            if (_PyOnceFlag_CallOnce(&state->once, (_Py_once_fn_t *)&init_types, state) < 0) {
                 return NULL;
             }
             return state;
@@ -1570,8 +1562,8 @@ def generate_module_def(mod, metadata, f, internal_h):
     for identifier in state_strings:
         f.write('    if ((state->' + identifier)
         f.write(' = PyUnicode_InternFromString("')
-        f.write(identifier + '")) == NULL) return 0;\n')
+        f.write(identifier + '")) == NULL) return -1;\n')
-    f.write('    return 1;\n')
+    f.write('    return 0;\n')
     f.write('};\n\n')
 
 def write_header(mod, metadata, f):
@@ -1629,6 +1621,9 @@ def write_internal_h_header(mod, f):
     print(textwrap.dedent("""
         #ifndef Py_INTERNAL_AST_STATE_H
         #define Py_INTERNAL_AST_STATE_H
+
+        #include "pycore_lock.h"    // _PyOnceFlag
+
         #ifdef __cplusplus
         extern "C" {
         #endif

Python/getargs.c

@@ -1877,8 +1877,9 @@ new_kwtuple(const char * const *keywords, int total, int pos)
 }
 
 static int
-_parser_init(struct _PyArg_Parser *parser)
+_parser_init(void *arg)
 {
+    struct _PyArg_Parser *parser = (struct _PyArg_Parser *)arg;
     const char * const *keywords = parser->keywords;
     assert(keywords != NULL);
     assert(parser->pos == 0 &&
@@ -1889,7 +1890,7 @@ _parser_init(struct _PyArg_Parser *parser)
 
     int len, pos;
     if (scan_keywords(keywords, &len, &pos) < 0) {
-        return 0;
+        return -1;
     }
 
     const char *fname, *custommsg = NULL;
@@ -1898,7 +1899,7 @@ _parser_init(struct _PyArg_Parser *parser)
         assert(parser->fname == NULL);
         if (parse_format(parser->format, len, pos,
                          &fname, &custommsg, &min, &max) < 0) {
-            return 0;
+            return -1;
         }
     }
     else {
@@ -1911,7 +1912,7 @@ _parser_init(struct _PyArg_Parser *parser)
     if (kwtuple == NULL) {
         kwtuple = new_kwtuple(keywords, len, pos);
         if (kwtuple == NULL) {
-            return 0;
+            return -1;
         }
         owned = 1;
     }
@@ -1925,40 +1926,27 @@ _parser_init(struct _PyArg_Parser *parser)
     parser->min = min;
     parser->max = max;
     parser->kwtuple = kwtuple;
-    parser->initialized = owned ? 1 : -1;
+    parser->is_kwtuple_owned = owned;
 
     assert(parser->next == NULL);
-    parser->next = _PyRuntime.getargs.static_parsers;
+    parser->next = _Py_atomic_load_ptr(&_PyRuntime.getargs.static_parsers);
-    _PyRuntime.getargs.static_parsers = parser;
+    do {
-    return 1;
+        // compare-exchange updates parser->next on failure
+    } while (_Py_atomic_compare_exchange_ptr(&_PyRuntime.getargs.static_parsers,
+                                             &parser->next, parser));
+    return 0;
 }
 
 static int
 parser_init(struct _PyArg_Parser *parser)
 {
-    // volatile as it can be modified by other threads
+    return _PyOnceFlag_CallOnce(&parser->once, &_parser_init, parser);
-    // and should not be optimized or reordered by compiler
-    if (*((volatile int *)&parser->initialized)) {
-        assert(parser->kwtuple != NULL);
-        return 1;
-    }
-    PyThread_acquire_lock(_PyRuntime.getargs.mutex, WAIT_LOCK);
-    // Check again if another thread initialized the parser
-    // while we were waiting for the lock.
-    if (*((volatile int *)&parser->initialized)) {
-        assert(parser->kwtuple != NULL);
-        PyThread_release_lock(_PyRuntime.getargs.mutex);
-        return 1;
-    }
-    int ret = _parser_init(parser);
-    PyThread_release_lock(_PyRuntime.getargs.mutex);
-    return ret;
 }
 
 static void
 parser_clear(struct _PyArg_Parser *parser)
 {
-    if (parser->initialized == 1) {
+    if (parser->is_kwtuple_owned) {
         Py_CLEAR(parser->kwtuple);
     }
 }
@@ -2025,7 +2013,7 @@ vgetargskeywordsfast_impl(PyObject *const *args, Py_ssize_t nargs,
         return 0;
     }
 
-    if (!parser_init(parser)) {
+    if (parser_init(parser) < 0) {
         return 0;
     }
 
@@ -2258,7 +2246,7 @@ _PyArg_UnpackKeywords(PyObject *const *args, Py_ssize_t nargs,
         args = buf;
     }
 
-    if (!parser_init(parser)) {
+    if (parser_init(parser) < 0) {
         return NULL;
     }
 
@@ -2435,7 +2423,7 @@ _PyArg_UnpackKeywordsWithVararg(PyObject *const *args, Py_ssize_t nargs,
         args = buf;
     }
 
-    if (!parser_init(parser)) {
+    if (parser_init(parser) < 0) {
         return NULL;
     }
 

Python/lock.c

@@ -295,3 +295,61 @@ PyEvent_WaitTimed(PyEvent *evt, _PyTime_t timeout_ns)
         return _Py_atomic_load_uint8(&evt->v) == _Py_LOCKED;
     }
 }
+
+static int
+unlock_once(_PyOnceFlag *o, int res)
+{
+    // On success (res=0), we set the state to _Py_ONCE_INITIALIZED.
+    // On failure (res=-1), we reset the state to _Py_UNLOCKED.
+    uint8_t new_value;
+    switch (res) {
+        case -1: new_value = _Py_UNLOCKED; break;
+        case  0: new_value = _Py_ONCE_INITIALIZED; break;
+        default: {
+            Py_FatalError("invalid result from _PyOnceFlag_CallOnce");
+            Py_UNREACHABLE();
+            break;
+        }
+    }
+
+    uint8_t old_value = _Py_atomic_exchange_uint8(&o->v, new_value);
+    if ((old_value & _Py_HAS_PARKED) != 0) {
+        // wake up anyone waiting on the once flag
+        _PyParkingLot_UnparkAll(&o->v);
+    }
+    return res;
+}
+
+int
+_PyOnceFlag_CallOnceSlow(_PyOnceFlag *flag, _Py_once_fn_t *fn, void *arg)
+{
+    uint8_t v = _Py_atomic_load_uint8(&flag->v);
+    for (;;) {
+        if (v == _Py_UNLOCKED) {
+            if (!_Py_atomic_compare_exchange_uint8(&flag->v, &v, _Py_LOCKED)) {
+                continue;
+            }
+            int res = fn(arg);
+            return unlock_once(flag, res);
+        }
+
+        if (v == _Py_ONCE_INITIALIZED) {
+            return 0;
+        }
+
+        // The once flag is initializing (locked).
+        assert((v & _Py_LOCKED));
+        if (!(v & _Py_HAS_PARKED)) {
+            // We are the first waiter. Set the _Py_HAS_PARKED flag.
+            uint8_t new_value = v | _Py_HAS_PARKED;
+            if (!_Py_atomic_compare_exchange_uint8(&flag->v, &v, new_value)) {
+                continue;
+            }
+            v = new_value;
+        }
+
+        // Wait for initialization to finish.
+        _PyParkingLot_Park(&flag->v, &v, sizeof(v), -1, NULL, 1);
+        v = _Py_atomic_load_uint8(&flag->v);
+    }
+}

Python/pystate.c

@@ -379,12 +379,11 @@ _Py_COMP_DIAG_IGNORE_DEPR_DECLS
 static const _PyRuntimeState initial = _PyRuntimeState_INIT(_PyRuntime);
 _Py_COMP_DIAG_POP
 
-#define NUMLOCKS 9
+#define NUMLOCKS 8
 #define LOCKS_INIT(runtime) \
     { \
         &(runtime)->interpreters.mutex, \
         &(runtime)->xi.registry.mutex, \
-        &(runtime)->getargs.mutex, \
         &(runtime)->unicode_state.ids.lock, \
         &(runtime)->imports.extensions.mutex, \
         &(runtime)->ceval.pending_mainthread.lock, \