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gh-127020: Make `PyCode_GetCode` thread-safe for free threading (#127… · python/cpython@3926842 · GitHub

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Commit 3926842

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colesbury
authored
gh-127020: Make PyCode_GetCode thread-safe for free threading (#127043)
Some fields in PyCodeObject are lazily initialized. Use atomics and critical sections to make their initializations and accesses thread-safe.
1 parent dc7a2b6 commit 3926842

3 files changed

Lines changed: 85 additions & 27 deletions

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‎Lib/test/test_free_threading/test_code.py‎

Lines changed: 30 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,30 @@
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import unittest
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from threading import Thread
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from unittest import TestCase
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from test.support import threading_helper
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@threading_helper.requires_working_threading()
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class TestCode(TestCase):
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def test_code_attrs(self):
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"""Test concurrent accesses to lazily initialized code attributes"""
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code_objects = []
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for _ in range(1000):
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code_objects.append(compile("a + b", "<string>", "eval"))
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def run_in_thread():
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for code in code_objects:
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self.assertIsInstance(code.co_code, bytes)
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self.assertIsInstance(code.co_freevars, tuple)
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self.assertIsInstance(code.co_varnames, tuple)
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threads = [Thread(target=run_in_thread) for _ in range(2)]
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for thread in threads:
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thread.start()
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for thread in threads:
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thread.join()
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if __name__ == "__main__":
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unittest.main()

‎Misc/NEWS.d/next/Core_and_Builtins/2024-11-19-21-49-58.gh-issue-127020.5vvI17.rst‎

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Fix a crash in the free threading build when :c:func:`PyCode_GetCode`,
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:c:func:`PyCode_GetVarnames`, :c:func:`PyCode_GetCellvars`, or
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:c:func:`PyCode_GetFreevars` were called from multiple threads at the same
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time.

‎Objects/codeobject.c‎

Lines changed: 51 additions & 27 deletions
Original file line numberDiff line numberDiff line change
@@ -302,21 +302,32 @@ validate_and_copy_tuple(PyObject *tup)
302302
}
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304304
static int
305-
init_co_cached(PyCodeObject *self) {
306-
if (self->_co_cached == NULL) {
307-
self->_co_cached = PyMem_New(_PyCoCached, 1);
308-
if (self->_co_cached == NULL) {
305+
init_co_cached(PyCodeObject *self)
306+
{
307+
_PyCoCached *cached = FT_ATOMIC_LOAD_PTR(self->_co_cached);
308+
if (cached != NULL) {
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return 0;
310+
}
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Py_BEGIN_CRITICAL_SECTION(self);
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cached = self->_co_cached;
314+
if (cached == NULL) {
315+
cached = PyMem_New(_PyCoCached, 1);
316+
if (cached == NULL) {
309317
PyErr_NoMemory();
310-
return -1;
311318
}
312-
self->_co_cached->_co_code = NULL;
313-
self->_co_cached->_co_cellvars = NULL;
314-
self->_co_cached->_co_freevars = NULL;
315-
self->_co_cached->_co_varnames = NULL;
319+
else {
320+
cached->_co_code = NULL;
321+
cached->_co_cellvars = NULL;
322+
cached->_co_freevars = NULL;
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cached->_co_varnames = NULL;
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FT_ATOMIC_STORE_PTR(self->_co_cached, cached);
325+
}
316326
}
317-
return 0;
318-
327+
Py_END_CRITICAL_SECTION();
328+
return cached != NULL ? 0 : -1;
319329
}
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320331
/******************
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* _PyCode_New()
322333
******************/
@@ -1571,16 +1582,21 @@ get_cached_locals(PyCodeObject *co, PyObject **cached_field,
15711582
{
15721583
assert(cached_field != NULL);
15731584
assert(co->_co_cached != NULL);
1574-
if (*cached_field != NULL) {
1575-
return Py_NewRef(*cached_field);
1585+
PyObject *varnames = FT_ATOMIC_LOAD_PTR(*cached_field);
1586+
if (varnames != NULL) {
1587+
return Py_NewRef(varnames);
15761588
}
1577-
assert(*cached_field == NULL);
1578-
PyObject *varnames = get_localsplus_names(co, kind, num);
1589+
1590+
Py_BEGIN_CRITICAL_SECTION(co);
1591+
varnames = *cached_field;
15791592
if (varnames == NULL) {
1580-
return NULL;
1593+
varnames = get_localsplus_names(co, kind, num);
1594+
if (varnames != NULL) {
1595+
FT_ATOMIC_STORE_PTR(*cached_field, varnames);
1596+
}
15811597
}
1582-
*cached_field = Py_NewRef(varnames);
1583-
return varnames;
1598+
Py_END_CRITICAL_SECTION();
1599+
return Py_XNewRef(varnames);
15841600
}
15851601

15861602
PyObject *
@@ -1674,18 +1690,26 @@ _PyCode_GetCode(PyCodeObject *co)
16741690
if (init_co_cached(co)) {
16751691
return NULL;
16761692
}
1677-
if (co->_co_cached->_co_code != NULL) {
1678-
return Py_NewRef(co->_co_cached->_co_code);
1693+
1694+
_PyCoCached *cached = co->_co_cached;
1695+
PyObject *code = FT_ATOMIC_LOAD_PTR(cached->_co_code);
1696+
if (code != NULL) {
1697+
return Py_NewRef(code);
16791698
}
1680-
PyObject *code = PyBytes_FromStringAndSize((const char *)_PyCode_CODE(co),
1681-
_PyCode_NBYTES(co));
1699+
1700+
Py_BEGIN_CRITICAL_SECTION(co);
1701+
code = cached->_co_code;
16821702
if (code == NULL) {
1683-
return NULL;
1703+
code = PyBytes_FromStringAndSize((const char *)_PyCode_CODE(co),
1704+
_PyCode_NBYTES(co));
1705+
if (code != NULL) {
1706+
deopt_code(co, (_Py_CODEUNIT *)PyBytes_AS_STRING(code));
1707+
assert(cached->_co_code == NULL);
1708+
FT_ATOMIC_STORE_PTR(cached->_co_code, code);
1709+
}
16841710
}
1685-
deopt_code(co, (_Py_CODEUNIT *)PyBytes_AS_STRING(code));
1686-
assert(co->_co_cached->_co_code == NULL);
1687-
co->_co_cached->_co_code = Py_NewRef(code);
1688-
return code;
1711+
Py_END_CRITICAL_SECTION();
1712+
return Py_XNewRef(code);
16891713
}
16901714

16911715
PyObject *

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