diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-10-15-15-29-41.gh-issue-124375.wNrWVa.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-10-15-15-29-41.gh-issue-124375.wNrWVa.rst
new file mode 100644
index 00000000000000..4bd17a6e683bc7
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2024-10-15-15-29-41.gh-issue-124375.wNrWVa.rst
@@ -0,0 +1 @@
+Fix a crash in the free threading build when the GC runs concurrently with a new thread starting.
diff --git a/Python/gc_free_threading.c b/Python/gc_free_threading.c
index 140f1eaa88bfc2..1d8641b91da662 100644
--- a/Python/gc_free_threading.c
+++ b/Python/gc_free_threading.c
@@ -340,18 +340,24 @@ merge_all_queued_objects(PyInterpreterState *interp, struct collection_state *st
static void
process_delayed_frees(PyInterpreterState *interp)
{
- // In STW status, we can observe the latest write sequence by
- // advancing the write sequence immediately.
+ // While we are in a "stop the world" pause, we can observe the latest
+ // write sequence by advancing the write sequence immediately.
_Py_qsbr_advance(&interp->qsbr);
_PyThreadStateImpl *current_tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
_Py_qsbr_quiescent_state(current_tstate->qsbr);
+
+ // Merge the queues from other threads into our own queue so that we can
+ // process all of the pending delayed free requests at once.
HEAD_LOCK(&_PyRuntime);
- PyThreadState *tstate = interp->threads.head;
- while (tstate != NULL) {
- _PyMem_ProcessDelayed(tstate);
- tstate = (PyThreadState *)tstate->next;
+ for (PyThreadState *p = interp->threads.head; p != NULL; p = p->next) {
+ _PyThreadStateImpl *other = (_PyThreadStateImpl *)p;
+ if (other != current_tstate) {
+ llist_concat(¤t_tstate->mem_free_queue, &other->mem_free_queue);
+ }
}
HEAD_UNLOCK(&_PyRuntime);
+
+ _PyMem_ProcessDelayed((PyThreadState *)current_tstate);
}
// Subtract an incoming reference from the computed "gc_refs" refcount.
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