gh-110481: Implement inter-thread queue for biased reference counting (#114824)

Biased reference counting maintains two refcount fields in each object:
`ob_ref_local` and `ob_ref_shared`. The true refcount is the sum of these two
fields. In some cases, when refcounting operations are split across threads,
the ob_ref_shared field can be negative (although the total refcount must be
at least zero). In this case, the thread that decremented the refcount
requests that the owning thread give up ownership and merge the refcount
fields.

1 files changed, 11 insertions, 0 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index e77e5bf..6cd0347 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -611,6 +611,9 @@ init_interpreter(PyInterpreterState *interp,
     _PyGC_InitState(&interp->gc);
     PyConfig_InitPythonConfig(&interp->config);
     _PyType_InitCache(interp);
+#ifdef Py_GIL_DISABLED
+    _Py_brc_init_state(interp);
+#endif
     for (int i = 0; i < _PY_MONITORING_UNGROUPED_EVENTS; i++) {
         interp->monitors.tools[i] = 0;
     }
@@ -1336,6 +1339,11 @@ init_threadstate(_PyThreadStateImpl *_tstate,
     tstate->datastack_limit = NULL;
     tstate->what_event = -1;
 
+#ifdef Py_GIL_DISABLED
+    // Initialize biased reference counting inter-thread queue
+    _Py_brc_init_thread(tstate);
+#endif
+
     if (interp->stoptheworld.requested || _PyRuntime.stoptheworld.requested) {
         // Start in the suspended state if there is an ongoing stop-the-world.
         tstate->state = _Py_THREAD_SUSPENDED;
@@ -1561,6 +1569,9 @@ PyThreadState_Clear(PyThreadState *tstate)
     _PyFreeListState *freelist_state = &((_PyThreadStateImpl*)tstate)->freelist_state;
     _Py_ClearFreeLists(freelist_state, 1);
     _PySlice_ClearCache(freelist_state);
+
+    // Remove ourself from the biased reference counting table of threads.
+    _Py_brc_remove_thread(tstate);
 #endif
 
     _PyThreadState_ClearMimallocHeaps(tstate);