Skip to content

gh-150411: fix gc_generation.count race #150413

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
LindaSummer wants to merge 3 commits into
base: main
Choose a base branch
from
+34 −3
Open

gh-150411: fix gc_generation.count race #150413

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
3 commits
Select commit Hold shift + click to select a range
1407cda
fix `gc_generation.count` race
LindaSummer May 25, 2026
9908633
add blurb
LindaSummer May 25, 2026
b71383e
Merge branch 'main' into gc_count_tsan
LindaSummer May 25, 2026
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
29 changes: 29 additions & 0 deletions Lib/test/test_free_threading/test_gc.py
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -94,6 +94,35 @@ def evil():
thread.start()
thread.join()

def test_get_count(self):
class CyclicReference:
def __init__(self):
self.ref = self

NUM_ALLOCATORS = 7
NUM_READERS = 1
NUM_THREADS = NUM_ALLOCATORS + NUM_READERS
NUM_ITERS = 200_000

barrier = threading.Barrier(NUM_THREADS)

def allocator():
barrier.wait()
for _ in range(NUM_ITERS):
CyclicReference()


def reader():
barrier.wait()
for _ in range(NUM_ITERS):
gc.get_count()

threads = [Thread(target=allocator) for _ in range(NUM_ALLOCATORS)]
threads.extend(Thread(target=reader) for _ in range(NUM_READERS))

with threading_helper.start_threads(threads):
pass


if __name__ == "__main__":
unittest.main()
2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Core_and_Builtins/2026-05-26-00-06-30.gh-issue-150411.u-d-_5.rst
View file
Open in desktop
Original file line number Diff line number Diff line change
@@ -0,0 +1,2 @@
Fix a data race in the free-threaded build when :func:`gc.get_count` reads
the young generation allocation count while another thread updates it.
6 changes: 3 additions & 3 deletions Modules/gcmodule.c
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -230,9 +230,9 @@ gc_get_count_impl(PyObject *module)
gcstate->generations[2].count);
#else
return Py_BuildValue("(iii)",
gcstate->young.count,
gcstate->old[0].count,
gcstate->old[1].count);
_Py_atomic_load_int_relaxed(&gcstate->young.count),
_Py_atomic_load_int_relaxed(&gcstate->old[0].count),
_Py_atomic_load_int_relaxed(&gcstate->old[1].count));
#endif
}

Expand Down
Loading