pythongh-132641: fix race in lru_cache under free-threading (python#133787)

Fix race in `lru_cache` by acquiring critical section on the cache object itself and call the lock held variant of dict functions to modify the underlying dict.

Author: hawkinsp

Include/internal/pycore_dict.h

@@ -150,6 +150,8 @@ extern int _PyDict_Pop_KnownHash(
     Py_hash_t hash,
     PyObject **result);
 
+extern void _PyDict_Clear_LockHeld(PyObject *op);
+
 #ifdef Py_GIL_DISABLED
 PyAPI_FUNC(void) _PyDict_EnsureSharedOnRead(PyDictObject *mp);
 #endif

Lib/test/test_free_threading/test_functools.py

@@ -0,0 +1,75 @@
+import random
+import unittest
+
+from functools import lru_cache
+from threading import Barrier, Thread
+
+from test.support import threading_helper
+
+@threading_helper.requires_working_threading()
+class TestLRUCache(unittest.TestCase):
+
+    def _test_concurrent_operations(self, maxsize):
+        num_threads = 10
+        b = Barrier(num_threads)
+        @lru_cache(maxsize=maxsize)
+        def func(arg=0):
+            return object()
+
+
+        def thread_func():
+            b.wait()
+            for i in range(1000):
+                r = random.randint(0, 1000)
+                if i < 800:
+                    func(i)
+                elif i < 900:
+                    func.cache_info()
+                else:
+                    func.cache_clear()
+
+        threads = []
+        for i in range(num_threads):
+            t = Thread(target=thread_func)
+            threads.append(t)
+
+        with threading_helper.start_threads(threads):
+            pass
+
+    def test_concurrent_operations_unbounded(self):
+        self._test_concurrent_operations(maxsize=None)
+
+    def test_concurrent_operations_bounded(self):
+        self._test_concurrent_operations(maxsize=128)
+
+    def _test_reentrant_cache_clear(self, maxsize):
+        num_threads = 10
+        b = Barrier(num_threads)
+        @lru_cache(maxsize=maxsize)
+        def func(arg=0):
+            func.cache_clear()
+            return object()
+
+
+        def thread_func():
+            b.wait()
+            for i in range(1000):
+                func(random.randint(0, 10000))
+
+        threads = []
+        for i in range(num_threads):
+            t = Thread(target=thread_func)
+            threads.append(t)
+
+        with threading_helper.start_threads(threads):
+            pass
+
+    def test_reentrant_cache_clear_unbounded(self):
+        self._test_reentrant_cache_clear(maxsize=None)
+
+    def test_reentrant_cache_clear_bounded(self):
+        self._test_reentrant_cache_clear(maxsize=128)
+
+
+if __name__ == "__main__":
+    unittest.main()

Misc/NEWS.d/next/Library/2025-05-09-20-59-24.gh-issue-132641.3qTw44.rst

@@ -0,0 +1 @@
+Fixed a race in :func:`functools.lru_cache` under free-threading.

Modules/_functoolsmodule.c

@@ -1383,8 +1383,8 @@ bounded_lru_cache_update_lock_held(lru_cache_object *self,
            this same key, then this setitem call will update the cache dict
            with this new link, leaving the old link as an orphan (i.e. not
            having a cache dict entry that refers to it). */
-        if (_PyDict_SetItem_KnownHash(self->cache, key, (PyObject *)link,
-                                      hash) < 0) {
+        if (_PyDict_SetItem_KnownHash_LockHeld((PyDictObject *)self->cache, key,
+                                               (PyObject *)link, hash) < 0) {
             Py_DECREF(link);
             return NULL;
         }
@@ -1453,8 +1453,8 @@ bounded_lru_cache_update_lock_held(lru_cache_object *self,
        for successful insertion in the cache dict before adding the
        link to the linked list.  Otherwise, the potentially reentrant
        __eq__ call could cause the then orphan link to be visited. */
-    if (_PyDict_SetItem_KnownHash(self->cache, key, (PyObject *)link,
-                                  hash) < 0) {
+    if (_PyDict_SetItem_KnownHash_LockHeld((PyDictObject *)self->cache, key,
+                                           (PyObject *)link, hash) < 0) {
         /* Somehow the cache dict update failed.  We no longer can
            restore the old link.  Let the error propagate upward and
            leave the cache short one link. */
@@ -1689,7 +1689,13 @@ _functools__lru_cache_wrapper_cache_clear_impl(PyObject *self)
     lru_list_elem *list = lru_cache_unlink_list(_self);
     FT_ATOMIC_STORE_SSIZE_RELAXED(_self->hits, 0);
     FT_ATOMIC_STORE_SSIZE_RELAXED(_self->misses, 0);
-    PyDict_Clear(_self->cache);
+    if (_self->wrapper == bounded_lru_cache_wrapper) {
+        /* The critical section on the lru cache itself protects the dictionary
+           for bounded_lru_cache instances. */
+        _PyDict_Clear_LockHeld(_self->cache);
+    } else {
+        PyDict_Clear(_self->cache);
+    }
     lru_cache_clear_list(list);
     Py_RETURN_NONE;
 }

Objects/dictobject.c

@@ -2915,6 +2915,11 @@ clear_lock_held(PyObject *op)
     ASSERT_CONSISTENT(mp);
 }
 
+void
+_PyDict_Clear_LockHeld(PyObject *op) {
+    clear_lock_held(op);
+}
+
 void
 PyDict_Clear(PyObject *op)
 {