gh-159558: Avoid un-needed nested locks in set operations

Misc/NEWS.d/next/Core_and_Builtins/2026-10-16-17-55-45.gh-issue-159558.ftCsRe.rst

@@ -0,0 +1,3 @@
+Speed up ``set >= set``, ``set > set``, :meth:`set.issuperset`, :meth:`set.discard`
+and :meth:`set.remove` on the free-threaded build by not re-entering a critical
+section already held on the set.

Objects/setobject.c

@@ -411,27 +411,39 @@ set_insert_clean(setentry *table, size_t mask, PyObject *key, Py_hash_t hash)
 /* ======================================================================== */
 
 static int
-set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash, setentry **epp)
+set_lookkey_lock_held(PySetObject *so, PyObject *key, Py_hash_t hash,
+                      setentry **epp)
 {
     int status;
     if (PyFrozenSet_CheckExact(so)) {
         status = set_do_lookup(so, so->table, so->mask, key, hash, epp,
                                set_compare_frozenset);
     }
     else {
-        Py_BEGIN_CRITICAL_SECTION(so);
         do {
             status = set_do_lookup(so, so->table, so->mask, key, hash, epp,
                                    set_compare_entry_lock_held);
         } while (status == SET_LOOKKEY_CHANGED);
-        Py_END_CRITICAL_SECTION();
     }
     assert(status == SET_LOOKKEY_FOUND ||
            status == SET_LOOKKEY_NO_MATCH ||
            status == SET_LOOKKEY_ERROR);
     return status;
 }
 
+static int
+set_lookkey(PySetObject *so, PyObject *key, Py_hash_t hash, setentry **epp)
+{
+    if (PyFrozenSet_CheckExact(so)) {
+        return set_lookkey_lock_held(so, key, hash, epp);
+    }
+    int status;
+    Py_BEGIN_CRITICAL_SECTION(so);
+    status = set_lookkey_lock_held(so, key, hash, epp);
+    Py_END_CRITICAL_SECTION();
+    return status;
+}
+
 #ifdef Py_GIL_DISABLED
 static int
 set_lookkey_threadsafe(PySetObject *so, PyObject *key, Py_hash_t hash)
@@ -581,7 +593,7 @@ set_discard_entry(PySetObject *so, PyObject *key, Py_hash_t hash)
 {
     setentry *entry;
     PyObject *old_key;
-    int status = set_lookkey(so, key, hash, &entry);
+    int status = set_lookkey_lock_held(so, key, hash, &entry);
     if (status < 0) {
         return -1;
     }
@@ -2418,6 +2430,9 @@ set_issubset_impl(PySetObject *so, PyObject *other)
     Py_ssize_t pos = 0;
     int rv;
 
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(so);
+    _Py_CRITICAL_SECTION_ASSERT_OBJECT_LOCKED(other);
+
     if (!PyAnySet_Check(other)) {
         PyObject *tmp = set_intersection(so, other);
         if (tmp == NULL) {
@@ -2460,7 +2475,7 @@ set_issuperset_impl(PySetObject *so, PyObject *other)
 /*[clinic end generated code: output=ecf00ce552c09461 input=5f2e1f262e6e4ccc]*/
 {
     if (PyAnySet_Check(other)) {
-        return set_issubset(other, (PyObject *)so);
+        return set_issubset_impl((PySetObject *)other, (PyObject *)so);
     }
 
     PyObject *key, *it = PyObject_GetIter(other);