https://github.com/python/cpython/commit/3ab94d684286b49144bf2e43cc1041f3e4c0cda8
commit: 3ab94d684286b49144bf2e43cc1041f3e4c0cda8
branch: main
author: Sam Gross <[email protected]>
committer: colesbury <[email protected]>
date: 2026-04-12T10:40:41-04:00
summary:
gh-148393: Use atomic ops on _ma_watcher_tag in free threading build (gh-148397)
Fixes data races between dict mutation and watch/unwatch on the same dict.
files:
A
Misc/NEWS.d/next/Core_and_Builtins/2026-04-11-17-28-52.gh-issue-148393.lX6gwN.rst
M Include/internal/pycore_dict.h
M Include/internal/pycore_pyatomic_ft_wrappers.h
M Lib/test/test_free_threading/test_dict.py
M Objects/dictobject.c
M Python/optimizer_analysis.c
diff --git a/Include/internal/pycore_dict.h b/Include/internal/pycore_dict.h
index d58539fa846563..5bbea187394db6 100644
--- a/Include/internal/pycore_dict.h
+++ b/Include/internal/pycore_dict.h
@@ -292,7 +292,7 @@ _PyDict_NotifyEvent(PyDict_WatchEvent event,
PyObject *value)
{
assert(Py_REFCNT((PyObject*)mp) > 0);
- int watcher_bits = mp->_ma_watcher_tag & DICT_WATCHER_MASK;
+ int watcher_bits = FT_ATOMIC_LOAD_UINT64_RELAXED(mp->_ma_watcher_tag) &
DICT_WATCHER_MASK;
if (watcher_bits) {
RARE_EVENT_STAT_INC(watched_dict_modification);
_PyDict_SendEvent(watcher_bits, event, mp, key, value);
@@ -368,7 +368,7 @@ PyDictObject
*_PyObject_MaterializeManagedDict_LockHeld(PyObject *);
static inline Py_ssize_t
_PyDict_UniqueId(PyDictObject *mp)
{
- return (Py_ssize_t)(mp->_ma_watcher_tag >> DICT_UNIQUE_ID_SHIFT);
+ return (Py_ssize_t)(FT_ATOMIC_LOAD_UINT64_RELAXED(mp->_ma_watcher_tag) >>
DICT_UNIQUE_ID_SHIFT);
}
static inline void
diff --git a/Include/internal/pycore_pyatomic_ft_wrappers.h
b/Include/internal/pycore_pyatomic_ft_wrappers.h
index c0f859a23e10b8..3155481bb5c36b 100644
--- a/Include/internal/pycore_pyatomic_ft_wrappers.h
+++ b/Include/internal/pycore_pyatomic_ft_wrappers.h
@@ -49,6 +49,8 @@ extern "C" {
_Py_atomic_load_uint16_relaxed(&value)
#define FT_ATOMIC_LOAD_UINT32_RELAXED(value) \
_Py_atomic_load_uint32_relaxed(&value)
+#define FT_ATOMIC_LOAD_UINT64_RELAXED(value) \
+ _Py_atomic_load_uint64_relaxed(&value)
#define FT_ATOMIC_LOAD_ULONG_RELAXED(value) \
_Py_atomic_load_ulong_relaxed(&value)
#define FT_ATOMIC_STORE_PTR_RELAXED(value, new_value) \
@@ -71,6 +73,12 @@ extern "C" {
_Py_atomic_store_uint16_relaxed(&value, new_value)
#define FT_ATOMIC_STORE_UINT32_RELAXED(value, new_value) \
_Py_atomic_store_uint32_relaxed(&value, new_value)
+#define FT_ATOMIC_AND_UINT64(value, new_value) \
+ (void)_Py_atomic_and_uint64(&value, new_value)
+#define FT_ATOMIC_OR_UINT64(value, new_value) \
+ (void)_Py_atomic_or_uint64(&value, new_value)
+#define FT_ATOMIC_ADD_UINT64(value, new_value) \
+ (void)_Py_atomic_add_uint64(&value, new_value)
#define FT_ATOMIC_STORE_CHAR_RELAXED(value, new_value) \
_Py_atomic_store_char_relaxed(&value, new_value)
#define FT_ATOMIC_LOAD_CHAR_RELAXED(value) \
@@ -146,6 +154,7 @@ extern "C" {
#define FT_ATOMIC_LOAD_UINT8_RELAXED(value) value
#define FT_ATOMIC_LOAD_UINT16_RELAXED(value) value
#define FT_ATOMIC_LOAD_UINT32_RELAXED(value) value
+#define FT_ATOMIC_LOAD_UINT64_RELAXED(value) value
#define FT_ATOMIC_LOAD_ULONG_RELAXED(value) value
#define FT_ATOMIC_STORE_PTR_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_STORE_PTR_RELEASE(value, new_value) value = new_value
@@ -157,6 +166,9 @@ extern "C" {
#define FT_ATOMIC_STORE_UINT8_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_STORE_UINT16_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_STORE_UINT32_RELAXED(value, new_value) value = new_value
+#define FT_ATOMIC_AND_UINT64(value, new_value) (void)(value &= new_value)
+#define FT_ATOMIC_OR_UINT64(value, new_value) (void)(value |= new_value)
+#define FT_ATOMIC_ADD_UINT64(value, new_value) (void)(value += new_value)
#define FT_ATOMIC_LOAD_CHAR_RELAXED(value) value
#define FT_ATOMIC_STORE_CHAR_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_LOAD_UCHAR_RELAXED(value) value
diff --git a/Lib/test/test_free_threading/test_dict.py
b/Lib/test/test_free_threading/test_dict.py
index 1ffd924e9f477a..55272a00c3ad50 100644
--- a/Lib/test/test_free_threading/test_dict.py
+++ b/Lib/test/test_free_threading/test_dict.py
@@ -245,6 +245,29 @@ def reader():
with threading_helper.start_threads([t1, t2]):
pass
+ @unittest.skipIf(_testcapi is None, "requires _testcapi")
+ def test_racing_watch_unwatch_dict(self):
+ # gh-148393: race between PyDict_Watch / PyDict_Unwatch
+ # and concurrent dict mutation reading _ma_watcher_tag.
+ wid = _testcapi.add_dict_watcher(0)
+ try:
+ d = {}
+ ITERS = 1000
+
+ def writer():
+ for i in range(ITERS):
+ d[i] = i
+ del d[i]
+
+ def watcher():
+ for _ in range(ITERS):
+ _testcapi.watch_dict(wid, d)
+ _testcapi.unwatch_dict(wid, d)
+
+ threading_helper.run_concurrently([writer, watcher])
+ finally:
+ _testcapi.clear_dict_watcher(wid)
+
def test_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
diff --git
a/Misc/NEWS.d/next/Core_and_Builtins/2026-04-11-17-28-52.gh-issue-148393.lX6gwN.rst
b/Misc/NEWS.d/next/Core_and_Builtins/2026-04-11-17-28-52.gh-issue-148393.lX6gwN.rst
new file mode 100644
index 00000000000000..33c4b75bfb944c
--- /dev/null
+++
b/Misc/NEWS.d/next/Core_and_Builtins/2026-04-11-17-28-52.gh-issue-148393.lX6gwN.rst
@@ -0,0 +1,2 @@
+Fix data races between :c:func:`PyDict_Watch` / :c:func:`PyDict_Unwatch`
+and concurrent dict mutation in the :term:`free-threaded build`.
diff --git a/Objects/dictobject.c b/Objects/dictobject.c
index 67bc4319e0bae2..b5300eb410c69c 100644
--- a/Objects/dictobject.c
+++ b/Objects/dictobject.c
@@ -8028,7 +8028,7 @@ PyDict_Watch(int watcher_id, PyObject* dict)
if (validate_watcher_id(interp, watcher_id)) {
return -1;
}
- ((PyDictObject*)dict)->_ma_watcher_tag |= (1LL << watcher_id);
+ FT_ATOMIC_OR_UINT64(((PyDictObject*)dict)->_ma_watcher_tag, (1LL <<
watcher_id));
return 0;
}
@@ -8043,7 +8043,7 @@ PyDict_Unwatch(int watcher_id, PyObject* dict)
if (validate_watcher_id(interp, watcher_id)) {
return -1;
}
- ((PyDictObject*)dict)->_ma_watcher_tag &= ~(1LL << watcher_id);
+ FT_ATOMIC_AND_UINT64(((PyDictObject*)dict)->_ma_watcher_tag, ~(1LL <<
watcher_id));
return 0;
}
diff --git a/Python/optimizer_analysis.c b/Python/optimizer_analysis.c
index ca9bcc8a40c35e..6742488a0d06c2 100644
--- a/Python/optimizer_analysis.c
+++ b/Python/optimizer_analysis.c
@@ -119,14 +119,15 @@ static int
get_mutations(PyObject* dict) {
assert(PyDict_CheckExact(dict));
PyDictObject *d = (PyDictObject *)dict;
- return (d->_ma_watcher_tag >> DICT_MAX_WATCHERS) & ((1 <<
DICT_WATCHED_MUTATION_BITS)-1);
+ uint64_t tag = FT_ATOMIC_LOAD_UINT64_RELAXED(d->_ma_watcher_tag);
+ return (tag >> DICT_MAX_WATCHERS) & ((1 << DICT_WATCHED_MUTATION_BITS) -
1);
}
static void
increment_mutations(PyObject* dict) {
assert(PyDict_CheckExact(dict));
PyDictObject *d = (PyDictObject *)dict;
- d->_ma_watcher_tag += (1 << DICT_MAX_WATCHERS);
+ FT_ATOMIC_ADD_UINT64(d->_ma_watcher_tag, (1 << DICT_MAX_WATCHERS));
}
/* The first two dict watcher IDs are reserved for CPython,
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