gh-151646: Fix data race between gc.get_stats() and GC in free-threading builds

Include/internal/pycore_interp_structs.h

@@ -235,6 +235,12 @@ struct _gc_runtime_state {
     /* a permanent generation which won't be collected */
     struct gc_generation permanent_generation;
     struct gc_stats *generation_stats;
+#ifdef Py_GIL_DISABLED
+    /* Protects generation_stats between gc_get_stats_impl() (reader) and
+       gc_collect_main() (writer); both resolve the stats slot (buffer index)
+       under this lock so they agree on which slot to touch (gh-151646). */
+    PyMutex stats_mutex;
+#endif
     /* true if we are currently running the collector */
     int collecting;
     // The frame that started the current collection. It might be NULL even when

Lib/test/test_free_threading/test_gc_get_stats_race.py

@@ -0,0 +1,99 @@
+# Reproduction for the gc.get_stats() data race under free-threading.
+#
+# CPython issue gh-151646: in free-threading builds (``--disable-gil``) the
+# concurrent collector and ``gc.get_stats()`` touch the same per-generation
+# statistics struct (``gcstate->generation_stats``) without any synchronisation:
+#
+#   * READER -- ``gc_get_stats_impl()`` in ``Modules/gcmodule.c`` copies the
+#     ``struct gc_generation_stats`` for each generation (``collections``,
+#     ``collected``, ``uncollectable``, ``candidates``, ``duration``) with a
+#     plain struct assignment and no lock.
+#
+#   * WRITER -- at the end of every collection cycle ``gc_collect_main()`` in
+#     ``Python/gc_free_threading.c`` mutates that very struct in place
+#     (``stats->collections++``, ``stats->collected += m``, ...), again with no
+#     lock.
+#
+# When one thread is collecting while several others read the stats, the
+# unsynchronised read/write to the same memory is a data race.  The values are
+# only statistics, so the race is benign at the Python level (no crash), which
+# is exactly what lets this script double as a regression test: once the fix
+# adds proper synchronisation, ThreadSanitizer should stay quiet while the
+# script keeps exiting cleanly.
+#
+# Running this script under a free-threading CPython build compiled with
+# ThreadSanitizer (``./configure --disable-gil --with-thread-sanitizer``) is
+# expected to print a ``WARNING: ThreadSanitizer: data race`` report whose stack
+# traces point at ``gc_get_stats_impl`` (gcmodule.c) and ``gc_collect_main``
+# (gc_free_threading.c).
+#
+# Standalone usage:
+#
+#     ./python Lib/test/test_free_threading/test_gc_get_stats_race.py
+#
+# or as part of the test suite (only meaningful under a TSAN build):
+#
+#     ./python -m test test_free_threading.test_gc_get_stats_race
+
+import gc
+import threading
+import unittest
+
+from test.support import threading_helper
+
+
+# One thread hammers gc.collect() (the writer side); enough reader threads run
+# gc.get_stats() concurrently to make the race easy to observe.  Readers run
+# until the writer is done so the test duration is controlled by the collection
+# count rather than by reader loop counts.
+NUM_READERS = 4
+ITERATIONS = 50
+
+
+def _stress_get_stats_race(num_readers=NUM_READERS, iterations=ITERATIONS):
+    """Race gc.collect() against gc.get_stats()."""
+
+    # Synchronise the start so the writer and readers overlap for as long as
+    # possible, maximising the chance of the read and write landing on the
+    # statistics struct at the same time.
+    done = threading.Event()
+
+    def collector():
+        try:
+            for _ in range(iterations):
+                # Writer: each full collection updates gcstate->generation_stats.
+                gc.collect()
+        finally:
+            done.set()
+
+    def reader():
+        while not done.is_set():
+            # Reader: copies the per-generation stats structs with no lock.
+            gc.get_stats()
+
+    threading_helper.run_concurrently([collector] + [reader] * num_readers)
+
+
+@threading_helper.requires_working_threading()
+class TestGCGetStatsRace(unittest.TestCase):
+    def test_get_stats_collect_race(self):
+        _stress_get_stats_race()
+
+        # The race is benign at the Python level: gc.get_stats() must still
+        # return well-formed data and the interpreter must not crash.
+        stats = gc.get_stats()
+        self.assertEqual(len(stats), 3)
+        for generation in stats:
+            self.assertIn("collections", generation)
+            self.assertIn("collected", generation)
+            self.assertIn("uncollectable", generation)
+
+
+if __name__ == "__main__":
+    # Standalone reproduction: run the race and exit cleanly so the script can
+    # be reused as a regression check once the fix lands.
+    print(f"Racing 1 gc.collect() thread against "
+          f"{NUM_READERS} gc.get_stats() thread(s), {ITERATIONS} collections...")
+    _stress_get_stats_race()
+    print("Done (no Python-level crash). "
+          "Run under a free-threading + TSAN build to observe the data race.")

Misc/NEWS.d/next/Core_and_Builtins/2026-06-19-12-00-00.gh-issue-151646.Gc5tAt.rst

@@ -0,0 +1,3 @@
+Fix a data race in free-threading builds between :func:`gc.get_stats` and a
+concurrent garbage collection cycle.  Access to the per-generation statistics
+is now serialized with a mutex so the reader observes a consistent snapshot.

Modules/gcmodule.c

@@ -374,9 +374,15 @@ gc_get_stats_impl(PyObject *module)
     /* To get consistent values despite allocations while constructing
        the result list, we use a snapshot of the running stats. */
     GCState *gcstate = get_gc_state();
+#ifdef Py_GIL_DISABLED
+    PyMutex_Lock(&gcstate->stats_mutex);
+#endif
     stats[0] = gcstate->generation_stats->young.items[gcstate->generation_stats->young.index];
     stats[1] = gcstate->generation_stats->old[0].items[gcstate->generation_stats->old[0].index];
     stats[2] = gcstate->generation_stats->old[1].items[gcstate->generation_stats->old[1].index];
+#ifdef Py_GIL_DISABLED
+    PyMutex_Unlock(&gcstate->stats_mutex);
+#endif
 
     PyObject *result = PyList_New(0);
     if (result == NULL)

Python/gc_free_threading.c

@@ -2281,7 +2281,8 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
         }
     }
 
-    /* Update stats */
+    /* Update stats. */
+    PyMutex_Lock(&gcstate->stats_mutex);
     struct gc_generation_stats *stats = get_stats(gcstate, generation);
     stats->ts_start = start;
     stats->ts_stop = stop;
@@ -2290,6 +2291,7 @@ gc_collect_main(PyThreadState *tstate, int generation, _PyGC_Reason reason)
     stats->uncollectable += n;
     stats->duration += duration;
     stats->candidates += state.candidates;
+    PyMutex_Unlock(&gcstate->stats_mutex);
 
     GC_STAT_ADD(generation, objects_collected, m);
 #ifdef Py_STATS