[3.9] GH-102126: fix deadlock at shutdown when clearing thread states (GH-102222) (#102236)

(cherry picked from commit 5f11478ce7fda826d399530af4c5ca96c592f144)

1 files changed, 10 insertions, 2 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index c3520c3..1cf9be1 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -288,11 +288,19 @@ PyInterpreterState_Clear(PyInterpreterState *interp)
         _PyErr_Clear(tstate);
     }
 
+    // Clear the current/main thread state last.
     HEAD_LOCK(runtime);
-    for (PyThreadState *p = interp->tstate_head; p != NULL; p = p->next) {
+    PyThreadState *p = interp->tstate_head;
+    HEAD_UNLOCK(runtime);
+    while (p != NULL) {
+        // See https://github.com/python/cpython/issues/102126
+        // Must be called without HEAD_LOCK held as it can deadlock
+        // if any finalizer tries to acquire that lock.
         PyThreadState_Clear(p);
+        HEAD_LOCK(runtime);
+        p = p->next;
+        HEAD_UNLOCK(runtime);
     }
-    HEAD_UNLOCK(runtime);
 
     Py_CLEAR(interp->audit_hooks);