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gh-129824: Fix some data races with types in subinterpreters #129829

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gh-129824: Fix some data races with types in subinterpreters #129829

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a7df3c2
gh-129824: Fix data races in type_ready with subinterpreters
colesbury Feb 7, 2025
e6109fb
Fix additional data races
colesbury Feb 7, 2025
32e2acd
Remove test_interpreters from TSAN list
colesbury Feb 7, 2025
829abfa
Fix unused variable warning
colesbury Feb 7, 2025
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2 changes: 2 additions & 0 deletions Lib/test/test_interpreters/test_api.py
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Original file line number Diff line number Diff line change
Expand Up @@ -703,6 +703,7 @@ def test_not_shareable(self):
with self.assertRaises(ExecutionFailed):
interp.exec('print(spam)')

@support.skip_if_sanitizer('gh-129824: file descriptor race', thread=True)
def test_running(self):
interp = interpreters.create()
interp.prepare_main({'spam': True})
Expand Down Expand Up @@ -1530,6 +1531,7 @@ def test_whence(self):
whence = eval(text)
self.assertEqual(whence, _interpreters.WHENCE_LEGACY_CAPI)

@support.skip_if_sanitizer('gh-129824: file descriptor race', thread=True)
def test_is_running(self):
def check(interpid, expected):
with self.assertRaisesRegex(InterpreterError, 'unrecognized'):
Expand Down
3 changes: 2 additions & 1 deletion Objects/exceptions.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4231,7 +4231,8 @@ _PyExc_InitTypes(PyInterpreterState *interp)
return -1;
}
if (exc->tp_new == BaseException_new
&& exc->tp_init == (initproc)BaseException_init)
&& exc->tp_init == (initproc)BaseException_init
&& _Py_IsMainInterpreter(interp))
{
exc->tp_vectorcall = BaseException_vectorcall;
}
Expand Down
34 changes: 27 additions & 7 deletions Objects/typeobject.c
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Expand Up @@ -234,10 +234,10 @@ managed_static_type_state_init(PyInterpreterState *interp, PyTypeObject *self,
? index
: index + _Py_MAX_MANAGED_STATIC_BUILTIN_TYPES;

assert((initial == 1) ==
(_PyRuntime.types.managed_static.types[full_index].interp_count == 0));
(void)_Py_atomic_add_int64(
int64_t prev_interp_count = _Py_atomic_add_int64(
&_PyRuntime.types.managed_static.types[full_index].interp_count, 1);
assert((initial == 1) == (prev_interp_count == 0));
(void)prev_interp_count;

if (initial) {
assert(_PyRuntime.types.managed_static.types[full_index].type == NULL);
Expand Down Expand Up @@ -8506,7 +8506,12 @@ type_ready_set_new(PyTypeObject *type, int initial)
&& base == &PyBaseObject_Type
&& !(type->tp_flags & Py_TPFLAGS_HEAPTYPE))
{
type->tp_flags |= Py_TPFLAGS_DISALLOW_INSTANTIATION;
if (initial) {
type->tp_flags |= Py_TPFLAGS_DISALLOW_INSTANTIATION;
}
else {
assert(_PyType_HasFeature(type, Py_TPFLAGS_DISALLOW_INSTANTIATION));
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I'm worried that these assertions might be racy. If two interpreters are trying to initialize the type at the same time, TSan might start complaining, because the store is non-atomic.

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There's no race because tp_flags is only set in the initial interpreter (when initial=1), and all static types are initialized at interpreter startup.

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I might not understand the type lifecycle correctly. I'm imagining a case where:

  • Two subinterpreters attempt to import a module that has not been initialized by any interpreter, and that module contains static types.
  • Interpreter A will get initial atomically, and then interpreter B falls back to the assertion.
  • If interpreter A sets tp_flags at the same time as interpreter B calls PyType_HasFeature, wouldn't that race?

Is there some extra layer of synchronization?

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I think that modules are always imported in the main interpreter first, but @ericsnowcurrently would know better:

cpython/Python/import.c

Lines 1973 to 1979 in 8d9d3e4

* To avoid all of that, we make sure the module's init function
* is always run first with the main interpreter active. If it was
* already the main interpreter then we can continue loading the
* module like normal. Otherwise, right after the init function,
* we take care of some import state bookkeeping, switch back
* to the subinterpreter, check for single-phase init,
* and then continue loading like normal. */

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Hmm. I think that just means it will have a thread state under the main interpreter, which should synchronize it on a GILicious build, but there would be no synchronization under FT. It's probably worth investigating that once we get the rest of the type races fixed. I'll yield to you on whether to keep the assertions in this PR.

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Yeah, this is more complicated than I originally thought. I'm going to close this for now.

}
}

if (!(type->tp_flags & Py_TPFLAGS_DISALLOW_INSTANTIATION)) {
Expand All @@ -8521,12 +8526,22 @@ type_ready_set_new(PyTypeObject *type, int initial)
}
else {
// tp_new is NULL: inherit tp_new from base
type->tp_new = base->tp_new;
if (initial) {
type->tp_new = base->tp_new;
}
else {
assert(type->tp_new == base->tp_new);
}
}
}
else {
// Py_TPFLAGS_DISALLOW_INSTANTIATION sets tp_new to NULL
type->tp_new = NULL;
if (initial) {
type->tp_new = NULL;
}
else {
assert(type->tp_new == NULL);
}
}
return 0;
}
Expand Down Expand Up @@ -8659,7 +8674,12 @@ type_ready(PyTypeObject *type, int initial)
}

/* All done -- set the ready flag */
type->tp_flags |= Py_TPFLAGS_READY;
if (initial) {
type->tp_flags |= Py_TPFLAGS_READY;
}
else {
assert(_PyType_HasFeature(type, Py_TPFLAGS_READY));
}
stop_readying(type);

assert(_PyType_CheckConsistency(type));
Expand Down
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