Update comments and _PyThreadState_Suspend.

_PyThreadState_Suspend now switches to "detached" if there is no active
stop-the-world request.

Include/internal/pycore_pystate.h

@@ -20,23 +20,23 @@ extern "C" {
 // transitions between "attached" and "detached" on its own PyThreadState.
 //
 // The "suspended" state is used to implement stop-the-world pauses, such as
-// for cyclic garbage collection. It is only used in `--disable-gil` builds. It
-// is similar to the "detached" state, but only the thread performing a
-// stop-the-world pause may transition a thread from the "suspended" state back
-// to the "detached" state. A thread trying to "attach" from the "suspended"
-// state will block until it is transitioned back to "detached" when the
-// stop-the-world pause is complete.
+// for cyclic garbage collection. It is only used in `--disable-gil` builds.
+// The "suspended" state is similar to the "detached" state in that in both
+// states the thread is not allowed to call most Python APIs. However, unlike
+// the "detached" state, a thread may not transition itself out from the
+// "suspended" state. Only the thread performing a stop-the-world pause may
+// transition a thread from the "suspended" state back to the "detached" state.
 //
 // State transition diagram:
 //
 //            (bound thread)        (stop-the-world thread)
 // [attached]       <->       [detached]       <->       [suspended]
-//   +                                                        ^
-//   +--------------------------------------------------------+
+//   |                                                        ^
+//   +---------------------------->---------------------------+
 //                          (bound thread)
 //
-// See `_PyThreadState_Attach()`, `_PyThreadState_Detach()`, and
-// `_PyThreadState_Suspend()`.
+// The (bound thread) and (stop-the-world thread) labels indicate which thread
+// is allowed to perform the transition.
 #define _Py_THREAD_DETACHED     0
 #define _Py_THREAD_ATTACHED     1
 #define _Py_THREAD_SUSPENDED    2
@@ -153,10 +153,9 @@ extern void _PyThreadState_Detach(PyThreadState *tstate);
 // Detaches the current thread to the "suspended" state if a stop-the-world
 // pause is in progress.
 //
-// If there is no stop-the-world pause in progress, then this function is
-// a no-op. Returns one if the thread was switched to the "suspended" state and
-// zero otherwise.
-extern int _PyThreadState_Suspend(PyThreadState *tstate);
+// If there is no stop-the-world pause in progress, then the thread switches
+// to the "detached" state.
+extern void _PyThreadState_Suspend(PyThreadState *tstate);
 
 // Perform a stop-the-world pause for all threads in the all interpreters.
 //

Python/ceval_gil.c

@@ -952,11 +952,10 @@ _Py_HandlePending(PyThreadState *tstate)
     /* Stop-the-world */
     if (_Py_eval_breaker_bit_is_set(interp, _PY_EVAL_PLEASE_STOP_BIT)) {
         _Py_set_eval_breaker_bit(interp, _PY_EVAL_PLEASE_STOP_BIT, 0);
-        if (_PyThreadState_Suspend(tstate)) {
-            /* The attach blocks until the stop-the-world event is complete. */
-            _PyThreadState_Attach(tstate);
-        }
-        // else: stale stop-the-world event, ignore it!
+        _PyThreadState_Suspend(tstate);
+
+        /* The attach blocks until the stop-the-world event is complete. */
+        _PyThreadState_Attach(tstate);
     }
 
     /* Pending signals */

Python/pystate.c

@@ -1876,7 +1876,7 @@ _PyThreadState_Detach(PyThreadState *tstate)
     detach_thread(tstate, _Py_THREAD_DETACHED);
 }
 
-int
+void
 _PyThreadState_Suspend(PyThreadState *tstate)
 {
     _PyRuntimeState *runtime = &_PyRuntime;
@@ -1894,8 +1894,10 @@ _PyThreadState_Suspend(PyThreadState *tstate)
     HEAD_UNLOCK(runtime);
 
     if (stw == NULL) {
-        // Don't suspend if we are not in a stop-the-world event.
-        return 0;
+        // Switch directly to "detached" if there is no active stop-the-world
+        // request.
+        detach_thread(tstate, _Py_THREAD_DETACHED);
+        return;
     }
 
     // Switch to "suspended" state.
@@ -1905,7 +1907,6 @@ _PyThreadState_Suspend(PyThreadState *tstate)
     HEAD_LOCK(runtime);
     decrement_stoptheworld_countdown(stw);
     HEAD_UNLOCK(runtime);
-    return 1;
 }
 
 // Decrease stop-the-world counter of remaining number of threads that need to