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

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* Use a higher threshold for immortality for INCREF than DECREF
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Mark Shannon 2025-03-17 17:22:35 +00:00 • committed by GitHub
parent aabab76c8a
commit 9881fc5483
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2 changed files with 32 additions and 3 deletions
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9
Include/refcount.h
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@ -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
// Don't count the incref if the object is immortal.
if (!_Py_IsImmortal(op)) {
_Py_INCREF_IncRefTotal();
}
#endif
#endif
}

24
Lib/test/test_bigmem.py
View file

@ -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])