gh-123358: Use _PyStackRef in LOAD_DEREF (gh-130064)

Concurrent accesses from multiple threads to the same `cell` object did not
scale well in the free-threaded build. Use `_PyStackRef` and optimistically
avoid locking to improve scaling.

With the locks around cell reads gone, some of the free threading tests were
prone to starvation: the readers were able to run in a tight loop and the
writer threads weren't scheduled frequently enough to make timely progress.
Adjust the tests to avoid this.

Co-authored-by: Donghee Na <donghee.na@python.org>

Lib/test/test_free_threading/test_monitoring.py

@@ -8,7 +8,7 @@ import weakref
 
 from sys import monitoring
 from test.support import threading_helper
-from threading import Thread, _PyRLock
+from threading import Thread, _PyRLock, Barrier
 from unittest import TestCase
 
 
@@ -194,7 +194,9 @@ class SetProfileMultiThreaded(InstrumentationMultiThreadedMixin, TestCase):
 
 @threading_helper.requires_working_threading()
 class MonitoringMisc(MonitoringTestMixin, TestCase):
-    def register_callback(self):
+    def register_callback(self, barrier):
+        barrier.wait()
+
         def callback(*args):
             pass
 
@@ -206,8 +208,9 @@ class MonitoringMisc(MonitoringTestMixin, TestCase):
     def test_register_callback(self):
         self.refs = []
         threads = []
-        for i in range(50):
-            t = Thread(target=self.register_callback)
+        barrier = Barrier(5)
+        for i in range(5):
+            t = Thread(target=self.register_callback, args=(barrier,))
             t.start()
             threads.append(t)