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gh-144295: Fix data race in dict method lookup and global load#144312

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22 changes: 22 additions & 0 deletionsLib/test/test_free_threading/test_dict.py
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Original file line numberDiff line numberDiff line change
Expand Up@@ -245,5 +245,27 @@ def reader():
withthreading_helper.start_threads([t1,t2]):
pass

deftest_racing_dict_update_and_method_lookup(self):
# gh-144295: test race between dict modifications and method lookups.
# Uses BytesIO because the race requires a type without Py_TPFLAGS_INLINE_VALUES
# for the _PyDict_GetMethodStackRef code path.
importio
obj=io.BytesIO()

defwriter():
for_inrange(10000):
obj.x=1
delobj.x

defreader():
for_inrange(10000):
obj.getvalue()

t1=Thread(target=writer)
t2=Thread(target=reader)

withthreading_helper.start_threads([t1,t2]):
pass

if__name__=="__main__":
unittest.main()
36 changes: 23 additions & 13 deletionsObjects/dictobject.c
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Original file line numberDiff line numberDiff line change
Expand Up@@ -1615,7 +1615,9 @@ lookup_threadsafe_unicode(PyDictKeysObject *dk, PyObject *key, Py_hash_t hash, _
Py_ssize_t
_Py_dict_lookup_threadsafe_stackref(PyDictObject *mp, PyObject *key, Py_hash_t hash, _PyStackRef *value_addr)
{
PyDictKeysObject *dk = _Py_atomic_load_ptr(&mp->ma_keys);
ensure_shared_on_read(mp);

PyDictKeysObject *dk = _Py_atomic_load_ptr_acquire(&mp->ma_keys);
if (dk->dk_kind == DICT_KEYS_UNICODE && PyUnicode_CheckExact(key)) {
Py_ssize_t ix = lookup_threadsafe_unicode(dk, key, hash, value_addr);
if (ix != DKIX_KEY_CHANGED) {
Expand DownExpand Up@@ -1669,19 +1671,27 @@ _PyDict_GetMethodStackRef(PyDictObject *mp, PyObject *key, _PyStackRef *method)
Py_hash_t hash = hash_unicode_key(key);

#ifdef Py_GIL_DISABLED
PyDictKeysObject *dk = _Py_atomic_load_ptr_acquire(&mp->ma_keys);
if (dk->dk_kind == DICT_KEYS_UNICODE) {
_PyStackRef ref;
Py_ssize_t ix = lookup_threadsafe_unicode(dk, key, hash, &ref);
if (ix >= 0) {
assert(!PyStackRef_IsNull(ref));
PyStackRef_XSETREF(*method, ref);
return 1;
}
else if (ix == DKIX_EMPTY) {
return 0;
// NOTE: We can only do the fast-path lookup if we are on the owning
// thread or if the dict is already marked as shared so that the load
// of ma_keys is safe without a lock. We cannot call ensure_shared_on_read()
// in this code path without incref'ing the dict because the dict is a
// borrowed reference protected by QSBR, and acquiring the lock could lead
// to a quiescent state (allowing the dict to be freed).
if (_Py_IsOwnedByCurrentThread((PyObject *)mp) || IS_DICT_SHARED(mp)) {
PyDictKeysObject *dk = _Py_atomic_load_ptr_acquire(&mp->ma_keys);
if (dk->dk_kind == DICT_KEYS_UNICODE) {
_PyStackRef ref;
Py_ssize_t ix = lookup_threadsafe_unicode(dk, key, hash, &ref);
if (ix >= 0) {
assert(!PyStackRef_IsNull(ref));
PyStackRef_XSETREF(*method, ref);
return 1;
}
else if (ix == DKIX_EMPTY) {
return 0;
}
assert(ix == DKIX_KEY_CHANGED);
}
assert(ix == DKIX_KEY_CHANGED);
}
#endif

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