https://github.com/python/cpython/commit/cd5217283112d41c0244e2d96302cbe33f0b4cb1
commit: cd5217283112d41c0244e2d96302cbe33f0b4cb1
branch: main
author: Sam Gross <[email protected]>
committer: colesbury <[email protected]>
date: 2026-03-12T13:30:36-04:00
summary:
gh-145685: Improve scaling of type attribute lookups (gh-145774)
Avoid locking in the PyType_Lookup cache-miss path if the type's
tp_version_tag is already valid.
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2026-03-10-12-52-06.gh-issue-145685.80B7gK.rst
M Include/internal/pycore_pyatomic_ft_wrappers.h
M Objects/typeobject.c
diff --git a/Include/internal/pycore_pyatomic_ft_wrappers.h
b/Include/internal/pycore_pyatomic_ft_wrappers.h
index 70a32db663b293..c0f859a23e10b8 100644
--- a/Include/internal/pycore_pyatomic_ft_wrappers.h
+++ b/Include/internal/pycore_pyatomic_ft_wrappers.h
@@ -95,6 +95,8 @@ extern "C" {
_Py_atomic_store_int_relaxed(&value, new_value)
#define FT_ATOMIC_LOAD_INT_RELAXED(value) \
_Py_atomic_load_int_relaxed(&value)
+#define FT_ATOMIC_LOAD_UINT(value) \
+ _Py_atomic_load_uint(&value)
#define FT_ATOMIC_STORE_UINT_RELAXED(value, new_value) \
_Py_atomic_store_uint_relaxed(&value, new_value)
#define FT_ATOMIC_LOAD_UINT_RELAXED(value) \
@@ -167,6 +169,7 @@ extern "C" {
#define FT_ATOMIC_STORE_INT(value, new_value) value = new_value
#define FT_ATOMIC_LOAD_INT_RELAXED(value) value
#define FT_ATOMIC_STORE_INT_RELAXED(value, new_value) value = new_value
+#define FT_ATOMIC_LOAD_UINT(value) value
#define FT_ATOMIC_LOAD_UINT_RELAXED(value) value
#define FT_ATOMIC_STORE_UINT_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_LOAD_LONG_RELAXED(value) value
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2026-03-10-12-52-06.gh-issue-145685.80B7gK.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-10-12-52-06.gh-issue-145685.80B7gK.rst
new file mode 100644
index 00000000000000..da34b67c952c7c
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2026-03-10-12-52-06.gh-issue-145685.80B7gK.rst
@@ -0,0 +1,2 @@
+Improve scaling of type attribute lookups in the :term:`free-threaded build` by
+avoiding contention on the internal type lock.
diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index bb473dce68f65b..2a818f5f0205fd 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -52,8 +52,8 @@ class object "PyObject *" "&PyBaseObject_Type"
MCACHE_HASH(FT_ATOMIC_LOAD_UINT_RELAXED((type)->tp_version_tag), \
((Py_ssize_t)(name)) >> 3)
#define MCACHE_CACHEABLE_NAME(name) \
- PyUnicode_CheckExact(name) && \
- (PyUnicode_GET_LENGTH(name) <= MCACHE_MAX_ATTR_SIZE)
+ (PyUnicode_CheckExact(name) && \
+ (PyUnicode_GET_LENGTH(name) <= MCACHE_MAX_ATTR_SIZE))
#define NEXT_VERSION_TAG(interp) \
(interp)->types.next_version_tag
@@ -6134,6 +6134,14 @@ _PyType_LookupRefAndVersion(PyTypeObject *type, PyObject
*name, unsigned int *ve
return PyStackRef_AsPyObjectSteal(out);
}
+static int
+should_assign_version_tag(PyTypeObject *type, PyObject *name, unsigned int
version_tag)
+{
+ return (version_tag == 0
+ && FT_ATOMIC_LOAD_UINT16_RELAXED(type->tp_versions_used) <
MAX_VERSIONS_PER_CLASS
+ && MCACHE_CACHEABLE_NAME(name));
+}
+
unsigned int
_PyType_LookupStackRefAndVersion(PyTypeObject *type, PyObject *name,
_PyStackRef *out)
{
@@ -6182,24 +6190,20 @@ _PyType_LookupStackRefAndVersion(PyTypeObject *type,
PyObject *name, _PyStackRef
/* We may end up clearing live exceptions below, so make sure it's ours. */
assert(!PyErr_Occurred());
- // We need to atomically do the lookup and capture the version before
- // anyone else can modify our mro or mutate the type.
-
int res;
PyInterpreterState *interp = _PyInterpreterState_GET();
- int has_version = 0;
- unsigned int assigned_version = 0;
- BEGIN_TYPE_LOCK();
- // We must assign the version before doing the lookup. If
- // find_name_in_mro() blocks and releases the critical section
- // then the type version can change.
- if (MCACHE_CACHEABLE_NAME(name)) {
- has_version = assign_version_tag(interp, type);
- assigned_version = type->tp_version_tag;
- }
- res = find_name_in_mro(type, name, out);
- END_TYPE_LOCK();
+ unsigned int version_tag = FT_ATOMIC_LOAD_UINT(type->tp_version_tag);
+ if (should_assign_version_tag(type, name, version_tag)) {
+ BEGIN_TYPE_LOCK();
+ assign_version_tag(interp, type);
+ version_tag = type->tp_version_tag;
+ res = find_name_in_mro(type, name, out);
+ END_TYPE_LOCK();
+ }
+ else {
+ res = find_name_in_mro(type, name, out);
+ }
/* Only put NULL results into cache if there was no error. */
if (res < 0) {
@@ -6207,16 +6211,18 @@ _PyType_LookupStackRefAndVersion(PyTypeObject *type,
PyObject *name, _PyStackRef
return 0;
}
- if (has_version) {
- PyObject *res_obj = PyStackRef_AsPyObjectBorrow(*out);
+ if (version_tag == 0 || !MCACHE_CACHEABLE_NAME(name)) {
+ return 0;
+ }
+
+ PyObject *res_obj = PyStackRef_AsPyObjectBorrow(*out);
#if Py_GIL_DISABLED
- update_cache_gil_disabled(entry, name, assigned_version, res_obj);
+ update_cache_gil_disabled(entry, name, version_tag, res_obj);
#else
- PyObject *old_value = update_cache(entry, name, assigned_version,
res_obj);
- Py_DECREF(old_value);
+ PyObject *old_value = update_cache(entry, name, version_tag, res_obj);
+ Py_DECREF(old_value);
#endif
- }
- return has_version ? assigned_version : 0;
+ return version_tag;
}
/* Internal API to look for a name through the MRO.
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