https://github.com/python/cpython/commit/9881fc5483d6298dc969919dddecf2b72ddf8d43
commit: 9881fc5483d6298dc969919dddecf2b72ddf8d43
branch: main
author: Mark Shannon <m...@hotpy.org>
committer: markshannon <m...@hotpy.org>
date: 2025-03-17T17:22:35Z
summary:
GH-125174: Make immortality "sticky" (GH-131355)
* Use a higher threshold for immortality for INCREF than DECREF
files:
M Include/refcount.h
M Lib/test/test_bigmem.py
diff --git a/Include/refcount.h b/Include/refcount.h
index 3efb7e5fee4a06..417d91bfa0da92 100644
--- 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 c9ab1c1de9e186..6d0879ad65d53c 100644
--- a/Lib/test/test_bigmem.py
+++ b/Lib/test/test_bigmem.py
@@ -9,7 +9,8 @@
"""
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 @@ def test_dict(self, size):
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])
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