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[3.14] gh-135557: use atomic stores in heapq operations in free-threading (GH-135601) #135787

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Merged
merged 1 commit into from
Jun 21, 2025
Merged

[3.14] gh-135557: use atomic stores in heapq operations in free-threading (GH-135601) #135787

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29 changes: 28 additions & 1 deletion Lib/test/test_free_threading/test_heapq.py
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Original file line number Diff line number Diff line change
Expand Up @@ -3,7 +3,7 @@
import heapq

from enum import Enum
from threading import Thread, Barrier
from threading import Thread, Barrier, Lock
from random import shuffle, randint

from test.support import threading_helper
Expand Down Expand Up @@ -178,6 +178,33 @@ def heapreplace_max_func(max_heap, replace_items):
self.assertEqual(len(max_heap), OBJECT_COUNT)
self.test_heapq.check_max_invariant(max_heap)

def test_lock_free_list_read(self):
n, n_threads = 1_000, 10
l = []
barrier = Barrier(n_threads * 2)

count = 0
lock = Lock()

def worker():
with lock:
nonlocal count
x = count
count += 1

barrier.wait()
for i in range(n):
if x % 2:
heapq.heappush(l, 1)
heapq.heappop(l)
else:
try:
l[0]
except IndexError:
pass

self.run_concurrently(worker, (), n_threads * 2)

@staticmethod
def is_sorted_ascending(lst):
"""
Expand Down
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Library/2025-06-17-23-13-56.gh-issue-135557.Bfcy4v.rst
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@@ -0,0 +1,2 @@
Fix races on :mod:`heapq` updates and :class:`list` reads on the :term:`free threaded <free threading>`
build.
37 changes: 21 additions & 16 deletions Modules/_heapqmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@ annotated by François Pinard, and converted to C by Raymond Hettinger.

#include "Python.h"
#include "pycore_list.h" // _PyList_ITEMS(), _PyList_AppendTakeRef()
#include "pycore_pyatomic_ft_wrappers.h"

#include "clinic/_heapqmodule.c.h"

Expand Down Expand Up @@ -59,8 +60,8 @@ siftdown(PyListObject *heap, Py_ssize_t startpos, Py_ssize_t pos)
arr = _PyList_ITEMS(heap);
parent = arr[parentpos];
newitem = arr[pos];
arr[parentpos] = newitem;
arr[pos] = parent;
FT_ATOMIC_STORE_PTR_RELAXED(arr[parentpos], newitem);
FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], parent);
pos = parentpos;
}
return 0;
Expand Down Expand Up @@ -108,8 +109,8 @@ siftup(PyListObject *heap, Py_ssize_t pos)
/* Move the smaller child up. */
tmp1 = arr[childpos];
tmp2 = arr[pos];
arr[childpos] = tmp2;
arr[pos] = tmp1;
FT_ATOMIC_STORE_PTR_RELAXED(arr[childpos], tmp2);
FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], tmp1);
pos = childpos;
}
/* Bubble it up to its final resting place (by sifting its parents down). */
Expand Down Expand Up @@ -172,8 +173,9 @@ heappop_internal(PyObject *heap, int siftup_func(PyListObject *, Py_ssize_t))
if (!n)
return lastelt;
returnitem = PyList_GET_ITEM(heap, 0);
PyList_SET_ITEM(heap, 0, lastelt);
if (siftup_func((PyListObject *)heap, 0)) {
PyListObject *list = _PyList_CAST(heap);
FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], lastelt);
if (siftup_func(list, 0)) {
Py_DECREF(returnitem);
return NULL;
}
Expand Down Expand Up @@ -208,8 +210,9 @@ heapreplace_internal(PyObject *heap, PyObject *item, int siftup_func(PyListObjec
}

returnitem = PyList_GET_ITEM(heap, 0);
PyList_SET_ITEM(heap, 0, Py_NewRef(item));
if (siftup_func((PyListObject *)heap, 0)) {
PyListObject *list = _PyList_CAST(heap);
FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], Py_NewRef(item));
if (siftup_func(list, 0)) {
Py_DECREF(returnitem);
return NULL;
}
Expand Down Expand Up @@ -284,8 +287,9 @@ _heapq_heappushpop_impl(PyObject *module, PyObject *heap, PyObject *item)
}

returnitem = PyList_GET_ITEM(heap, 0);
PyList_SET_ITEM(heap, 0, Py_NewRef(item));
if (siftup((PyListObject *)heap, 0)) {
PyListObject *list = _PyList_CAST(heap);
FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], Py_NewRef(item));
if (siftup(list, 0)) {
Py_DECREF(returnitem);
return NULL;
}
Expand Down Expand Up @@ -437,8 +441,8 @@ siftdown_max(PyListObject *heap, Py_ssize_t startpos, Py_ssize_t pos)
arr = _PyList_ITEMS(heap);
parent = arr[parentpos];
newitem = arr[pos];
arr[parentpos] = newitem;
arr[pos] = parent;
FT_ATOMIC_STORE_PTR_RELAXED(arr[parentpos], newitem);
FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], parent);
pos = parentpos;
}
return 0;
Expand Down Expand Up @@ -486,8 +490,8 @@ siftup_max(PyListObject *heap, Py_ssize_t pos)
/* Move the smaller child up. */
tmp1 = arr[childpos];
tmp2 = arr[pos];
arr[childpos] = tmp2;
arr[pos] = tmp1;
FT_ATOMIC_STORE_PTR_RELAXED(arr[childpos], tmp2);
FT_ATOMIC_STORE_PTR_RELAXED(arr[pos], tmp1);
pos = childpos;
}
/* Bubble it up to its final resting place (by sifting its parents down). */
Expand Down Expand Up @@ -621,8 +625,9 @@ _heapq_heappushpop_max_impl(PyObject *module, PyObject *heap, PyObject *item)
}

returnitem = PyList_GET_ITEM(heap, 0);
PyList_SET_ITEM(heap, 0, Py_NewRef(item));
if (siftup_max((PyListObject *)heap, 0) < 0) {
PyListObject *list = _PyList_CAST(heap);
FT_ATOMIC_STORE_PTR_RELAXED(list->ob_item[0], Py_NewRef(item));
if (siftup_max(list, 0) < 0) {
Py_DECREF(returnitem);
return NULL;
}
Expand Down
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