GH-125174: Make immortality "sticky" (GH-131355)

* Use a higher threshold for immortality for INCREF than DECREF

diff --git a/Include/refcount.h b/Include/refcount.h
index 3efb7e5fee4a06a3a5e5d738e5d34420dc3bf365..417d91bfa0da92774768545dd273dff4484f6474 100644 (file)
--- a/Include/refcount.h
+++ b/Include/refcount.h
@@ -41,6 +41,10 @@ having all the lower 32 bits set, which will avoid the reference count to go
 beyond the refcount limit. Immortality checks for reference count decreases will
 be done by checking the bit sign flag in the lower 32 bits.
 
+To ensure that once an object becomes immortal, it remains immortal, the threshold
+for omitting increfs is much higher than for omitting decrefs. Consequently, once
+the refcount for an object exceeds _Py_IMMORTAL_MINIMUM_REFCNT it will gradually
+increase over time until it reaches _Py_IMMORTAL_INITIAL_REFCNT.
 */
 #define _Py_IMMORTAL_INITIAL_REFCNT (3ULL << 30)
 #define _Py_IMMORTAL_MINIMUM_REFCNT (1ULL << 31)
@@ -288,7 +292,7 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
     }
 #elif SIZEOF_VOID_P > 4
     PY_UINT32_T cur_refcnt = op->ob_refcnt;
-    if (((int32_t)cur_refcnt) < 0) {
+    if (cur_refcnt >= _Py_IMMORTAL_INITIAL_REFCNT) {
         // the object is immortal
         _Py_INCREF_IMMORTAL_STAT_INC();
         return;
@@ -303,7 +307,10 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
 #endif
     _Py_INCREF_STAT_INC();
 #ifdef Py_REF_DEBUG
-    _Py_INCREF_IncRefTotal();
+    // Don't count the incref if the object is immortal.
+    if (!_Py_IsImmortal(op)) {
+        _Py_INCREF_IncRefTotal();
+    }
 #endif
 #endif
 }

diff --git a/Lib/test/test_bigmem.py b/Lib/test/test_bigmem.py
index c9ab1c1de9e1865ac17ca55b510b36dc55f78565..6d0879ad65d53c61ddae5accea0491e046c135b7 100644 (file)
--- a/Lib/test/test_bigmem.py
+++ b/Lib/test/test_bigmem.py
@@ -9,7 +9,8 @@ high memory limit to regrtest, with the -M option.
 """
 
 from test import support
-from test.support import bigmemtest, _1G, _2G, _4G
+from test.support import bigmemtest, _1G, _2G, _4G, import_helper
+_testcapi = import_helper.import_module('_testcapi')
 
 import unittest
 import operator
@@ -1257,6 +1258,27 @@ class DictTest(unittest.TestCase):
         d[size] = 1
 
 
+class ImmortalityTest(unittest.TestCase):
+
+    @bigmemtest(size=_2G, memuse=pointer_size * 9/8)
+    def test_stickiness(self, size):
+        """Check that immortality is "sticky", so that
+           once an object is immortal it remains so."""
+        if size < _2G:
+            # Not enough memory to cause immortality on overflow
+            return
+        o1 = o2 = o3 = o4 = o5 = o6 = o7 = o8 = object()
+        l = [o1] * (size-20)
+        self.assertFalse(_testcapi.is_immortal(o1))
+        for _ in range(30):
+            l.append(l[0])
+        self.assertTrue(_testcapi.is_immortal(o1))
+        del o2, o3, o4, o5, o6, o7, o8
+        self.assertTrue(_testcapi.is_immortal(o1))
+        del l
+        self.assertTrue(_testcapi.is_immortal(o1))
+
+
 if __name__ == '__main__':
     if len(sys.argv) > 1:
         support.set_memlimit(sys.argv[1])