gh-137433: Fix deadlock with stop-the-world and daemon threads (gh-137735)

There was a deadlock originally seen by Memray when a daemon thread
enabled or disabled profiling while the interpreter was shutting down.
I think this could also happen with garbage collection, but I haven't
seen that in practice.

The daemon thread could be hung while trying acquire the global rwmutex
that prevents overlapping global and per-interpreter stop-the-world events.
Since it already held the main interpreter's stop-the-world lock, it
also deadlocked the main thread, which is trying to perform interpreter
finalization.

Swap the order of lock acquisition to prevent this deadlock.
Additionally, refactor `_PyParkingLot_Park` so that the global buckets
hashtable is left in a clean state if the thread is hung in
`PyEval_AcquireThread`.

Python/lock.c

@@ -227,7 +227,7 @@ _PyRawMutex_LockSlow(_PyRawMutex *m)
 
         // Wait for us to be woken up. Note that we still have to lock the
         // mutex ourselves: it is NOT handed off to us.
-        _PySemaphore_Wait(&waiter.sema, -1, /*detach=*/0);
+        _PySemaphore_Wait(&waiter.sema, -1);
     }
 
     _PySemaphore_Destroy(&waiter.sema);

Python/parking_lot.c

@@ -91,8 +91,8 @@ _PySemaphore_Destroy(_PySemaphore *sema)
 #endif
 }
 
-static int
-_PySemaphore_PlatformWait(_PySemaphore *sema, PyTime_t timeout)
+int
+_PySemaphore_Wait(_PySemaphore *sema, PyTime_t timeout)
 {
     int res;
 #if defined(MS_WINDOWS)
@@ -225,27 +225,6 @@ _PySemaphore_PlatformWait(_PySemaphore *sema, PyTime_t timeout)
     return res;
 }
 
-int
-_PySemaphore_Wait(_PySemaphore *sema, PyTime_t timeout, int detach)
-{
-    PyThreadState *tstate = NULL;
-    if (detach) {
-        tstate = _PyThreadState_GET();
-        if (tstate && _PyThreadState_IsAttached(tstate)) {
-            // Only detach if we are attached
-            PyEval_ReleaseThread(tstate);
-        }
-        else {
-            tstate = NULL;
-        }
-    }
-    int res = _PySemaphore_PlatformWait(sema, timeout);
-    if (tstate) {
-        PyEval_AcquireThread(tstate);
-    }
-    return res;
-}
-
 void
 _PySemaphore_Wakeup(_PySemaphore *sema)
 {
@@ -342,7 +321,19 @@ _PyParkingLot_Park(const void *addr, const void *expected, size_t size,
     enqueue(bucket, addr, &wait);
     _PyRawMutex_Unlock(&bucket->mutex);
 
-    int res = _PySemaphore_Wait(&wait.sema, timeout_ns, detach);
+    PyThreadState *tstate = NULL;
+    if (detach) {
+        tstate = _PyThreadState_GET();
+        if (tstate && _PyThreadState_IsAttached(tstate)) {
+            // Only detach if we are attached
+            PyEval_ReleaseThread(tstate);
+        }
+        else {
+            tstate = NULL;
+        }
+    }
+
+    int res = _PySemaphore_Wait(&wait.sema, timeout_ns);
     if (res == Py_PARK_OK) {
         goto done;
     }
@@ -354,7 +345,7 @@ _PyParkingLot_Park(const void *addr, const void *expected, size_t size,
         // Another thread has started to unpark us. Wait until we process the
         // wakeup signal.
         do {
-            res = _PySemaphore_Wait(&wait.sema, -1, detach);
+            res = _PySemaphore_Wait(&wait.sema, -1);
         } while (res != Py_PARK_OK);
         goto done;
     }
@@ -366,6 +357,9 @@ _PyParkingLot_Park(const void *addr, const void *expected, size_t size,
 
 done:
     _PySemaphore_Destroy(&wait.sema);
+    if (tstate) {
+        PyEval_AcquireThread(tstate);
+    }
     return res;
 
 }

Python/pystate.c

@@ -2253,13 +2253,15 @@ stop_the_world(struct _stoptheworld_state *stw)
 {
     _PyRuntimeState *runtime = &_PyRuntime;
 
-    PyMutex_Lock(&stw->mutex);
+    // gh-137433: Acquire the rwmutex first to avoid deadlocks with daemon
+    // threads that may hang when blocked on lock acquisition.
     if (stw->is_global) {
         _PyRWMutex_Lock(&runtime->stoptheworld_mutex);
     }
     else {
         _PyRWMutex_RLock(&runtime->stoptheworld_mutex);
     }
+    PyMutex_Lock(&stw->mutex);
 
     HEAD_LOCK(runtime);
     stw->requested = 1;
@@ -2325,13 +2327,13 @@ start_the_world(struct _stoptheworld_state *stw)
     }
     stw->requester = NULL;
     HEAD_UNLOCK(runtime);
+    PyMutex_Unlock(&stw->mutex);
     if (stw->is_global) {
         _PyRWMutex_Unlock(&runtime->stoptheworld_mutex);
     }
     else {
         _PyRWMutex_RUnlock(&runtime->stoptheworld_mutex);
     }
-    PyMutex_Unlock(&stw->mutex);
 }
 #endif  // Py_GIL_DISABLED