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Commit9ad0c7b

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gh-132641: fix race inlru_cache under free-threading (#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.
1 parent35f47d0 commit9ad0c7b

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5 files changed

+94
-5
lines changed

5 files changed

+94
-5
lines changed

‎Include/internal/pycore_dict.h

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@@ -150,6 +150,8 @@ extern int _PyDict_Pop_KnownHash(
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Py_hash_thash,
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PyObject**result);
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externvoid_PyDict_Clear_LockHeld(PyObject*op);
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#ifdefPy_GIL_DISABLED
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PyAPI_FUNC(void)_PyDict_EnsureSharedOnRead(PyDictObject*mp);
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#endif
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@@ -0,0 +1,75 @@
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importrandom
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importunittest
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fromfunctoolsimportlru_cache
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fromthreadingimportBarrier,Thread
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fromtest.supportimportthreading_helper
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@threading_helper.requires_working_threading()
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classTestLRUCache(unittest.TestCase):
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def_test_concurrent_operations(self,maxsize):
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num_threads=10
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b=Barrier(num_threads)
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@lru_cache(maxsize=maxsize)
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deffunc(arg=0):
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returnobject()
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defthread_func():
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b.wait()
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foriinrange(1000):
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r=random.randint(0,1000)
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ifi<800:
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func(i)
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elifi<900:
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func.cache_info()
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else:
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func.cache_clear()
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threads= []
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foriinrange(num_threads):
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t=Thread(target=thread_func)
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threads.append(t)
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withthreading_helper.start_threads(threads):
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pass
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deftest_concurrent_operations_unbounded(self):
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self._test_concurrent_operations(maxsize=None)
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deftest_concurrent_operations_bounded(self):
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self._test_concurrent_operations(maxsize=128)
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def_test_reentrant_cache_clear(self,maxsize):
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num_threads=10
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b=Barrier(num_threads)
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@lru_cache(maxsize=maxsize)
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deffunc(arg=0):
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func.cache_clear()
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returnobject()
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defthread_func():
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b.wait()
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foriinrange(1000):
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func(random.randint(0,10000))
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threads= []
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foriinrange(num_threads):
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t=Thread(target=thread_func)
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threads.append(t)
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withthreading_helper.start_threads(threads):
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pass
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deftest_reentrant_cache_clear_unbounded(self):
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self._test_reentrant_cache_clear(maxsize=None)
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deftest_reentrant_cache_clear_bounded(self):
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self._test_reentrant_cache_clear(maxsize=128)
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if__name__=="__main__":
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unittest.main()
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@@ -0,0 +1 @@
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Fixed a race in:func:`functools.lru_cache` under free-threading.

‎Modules/_functoolsmodule.c

Lines changed: 11 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -1383,8 +1383,8 @@ bounded_lru_cache_update_lock_held(lru_cache_object *self,
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this same key, then this setitem call will update the cache dict
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with this new link, leaving the old link as an orphan (i.e. not
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having a cache dict entry that refers to it). */
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if (_PyDict_SetItem_KnownHash(self->cache,key, (PyObject*)link,
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hash)<0) {
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if (_PyDict_SetItem_KnownHash_LockHeld((PyDictObject*)self->cache,key,
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(PyObject*)link,hash)<0) {
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Py_DECREF(link);
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returnNULL;
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}
@@ -1453,8 +1453,8 @@ bounded_lru_cache_update_lock_held(lru_cache_object *self,
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for successful insertion in the cache dict before adding the
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link to the linked list. Otherwise, the potentially reentrant
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__eq__ call could cause the then orphan link to be visited. */
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if (_PyDict_SetItem_KnownHash(self->cache,key, (PyObject*)link,
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hash)<0) {
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if (_PyDict_SetItem_KnownHash_LockHeld((PyDictObject*)self->cache,key,
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(PyObject*)link,hash)<0) {
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/* Somehow the cache dict update failed. We no longer can
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restore the old link. Let the error propagate upward and
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leave the cache short one link. */
@@ -1689,7 +1689,13 @@ _functools__lru_cache_wrapper_cache_clear_impl(PyObject *self)
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lru_list_elem*list=lru_cache_unlink_list(_self);
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FT_ATOMIC_STORE_SSIZE_RELAXED(_self->hits,0);
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FT_ATOMIC_STORE_SSIZE_RELAXED(_self->misses,0);
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PyDict_Clear(_self->cache);
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if (_self->wrapper==bounded_lru_cache_wrapper) {
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/* The critical section on the lru cache itself protects the dictionary
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for bounded_lru_cache instances. */
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_PyDict_Clear_LockHeld(_self->cache);
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}else {
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PyDict_Clear(_self->cache);
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}
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lru_cache_clear_list(list);
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Py_RETURN_NONE;
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}

‎Objects/dictobject.c

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

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