gh-124375: Avoid calling _PyMem_ProcessDelayed on other thread states

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.

Python/gc_free_threading.c

@@ -420,18 +420,24 @@ merge_queued_objects(_PyThreadStateImpl *tstate, struct collection_state *state)
 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(&current_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.