Update comments

Author: colesbury

Modules/_testinternalcapi/test_critical_sections.c

@@ -405,9 +405,9 @@ thread_stw(void *arg)
     }
 
     // All threads: acquire critical section and hold it long enough to
-    // trigger TIME_TO_BE_FAIR_NS, which causes direct handoff on unlock.
+    // trigger TIME_TO_BE_FAIR_NS (1 ms), which causes direct handoff on unlock.
     Py_BEGIN_CRITICAL_SECTION(test_data->obj);
-    pysleep(10);
+    pysleep(10);  // 10 ms = 10 x TIME_TO_BE_FAIR_NS
     Py_END_CRITICAL_SECTION();
 
     PyGILState_Release(gil);

Python/critical_section.c

@@ -45,6 +45,8 @@ _PyCriticalSection_BeginSlow(PyThreadState *tstate, PyCriticalSection *c, PyMute
     }
     // If the world is stopped, we don't need to acquire the lock because
     // there are no other threads that could be accessing the object.
+    // Without this check, acquiring a critical section while the world is
+    // stopped could lead to a deadlock.
     if (tstate->interp->stoptheworld.world_stopped) {
         c->_cs_mutex = NULL;
         c->_cs_prev = 0;