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gh-117657: Make frame clearing thread-safe #120542

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gh-117657: Make frame clearing thread-safe #120542

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825efee
add test
Fidget-Spinner Jun 15, 2024
c4966e2
Make frame clearing thread safe
Fidget-Spinner Jun 15, 2024
fc7510b
Fix EOF
Fidget-Spinner Jun 15, 2024
21da242
Cleanup test
Fidget-Spinner Jun 19, 2024
acc4644
Revert generator fixes
Fidget-Spinner Jun 21, 2024
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31 changes: 31 additions & 0 deletions Lib/test/test_free_threading/test_frame.py
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Original file line number Diff line number Diff line change
@@ -0,0 +1,31 @@
import sys
import unittest
import threading

from test.support import threading_helper

NTHREADS = 6

@threading_helper.requires_working_threading()
class TestFrame(unittest.TestCase):
def test_frame_clear_simultaneous(self):
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def gen():
for _ in range(10000):
return sys._getframe()

foo = gen()
def work():
for _ in range(4000):
frame1 = foo
frame1.clear()


threads = []
for i in range(NTHREADS):
thread = threading.Thread(target=work)
thread.start()
threads.append(thread)

for thread in threads:
thread.join()
15 changes: 13 additions & 2 deletions Objects/frameobject.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7,7 +7,7 @@
#include "pycore_moduleobject.h" // _PyModule_GetDict()
#include "pycore_object.h" // _PyObject_GC_UNTRACK()
#include "pycore_opcode_metadata.h" // _PyOpcode_Deopt, _PyOpcode_Caches

#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION

#include "frameobject.h" // PyFrameObject
#include "pycore_frame.h"
Expand Down Expand Up @@ -1662,7 +1662,7 @@ frame_tp_clear(PyFrameObject *f)
}

static PyObject *
frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
frame_clear_unlocked(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
{
if (f->f_frame->owner == FRAME_OWNED_BY_GENERATOR) {
PyGenObject *gen = _PyFrame_GetGenerator(f->f_frame);
Expand Down Expand Up @@ -1692,6 +1692,17 @@ frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
return NULL;
}

static PyObject *
frame_clear(PyFrameObject *f, PyObject *Py_UNUSED(ignored))
{
PyObject *res;
// Another materialized frame might be racing on clearing the frame.
Py_BEGIN_CRITICAL_SECTION(f);
res = frame_clear_unlocked(f, NULL);
Py_END_CRITICAL_SECTION();
return res;
}

PyDoc_STRVAR(clear__doc__,
"F.clear(): clear most references held by the frame");

Expand Down
12 changes: 11 additions & 1 deletion Objects/genobject.c
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Expand Up @@ -14,6 +14,7 @@
#include "pycore_pyatomic_ft_wrappers.h" // FT_ATOMIC_*
#include "pycore_pyerrors.h" // _PyErr_ClearExcState()
#include "pycore_pystate.h" // _PyThreadState_GET()
#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION

#include "pystats.h"

Expand Down Expand Up @@ -119,7 +120,7 @@ _PyGen_Finalize(PyObject *self)
}

static void
gen_dealloc(PyGenObject *gen)
gen_dealloc_no_lock(PyGenObject *gen)
{
PyObject *self = (PyObject *) gen;

Expand Down Expand Up @@ -158,6 +159,15 @@ gen_dealloc(PyGenObject *gen)
PyObject_GC_Del(gen);
}

static void
gen_dealloc(PyGenObject *gen)
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This doesn't seem right to me. tp_dealloc can't be called concurrently on an object.

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In this case the racing part is mainly _PyFrame_ClearExceptCode and setting the generator's flags. _PyEval_FrameClearAndPop can race with this as it also clears a generator, but instead it's done by ceval.c.

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I don't understand -- how can _PyFrame_ClearExceptCode race with gen_dealloc?

It looks like gen_dealloc is only called from the tp_dealloc handler. If an object is being destroyed, then no other thread has access to it (or there's a separate serious bug).

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Hmm I'm not too sure either. Let me revert this fix for now and try to hunt it down again.

{
// Another generator's finalizer might race on clearing the frame.
Py_BEGIN_CRITICAL_SECTION(gen);
gen_dealloc_no_lock(gen);
Py_END_CRITICAL_SECTION();
}

static PySendResult
gen_send_ex2(PyGenObject *gen, PyObject *arg, PyObject **presult,
int exc, int closing)
Expand Down
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