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gh-123358: Use _PyStackRef in LOAD_DEREF (gh-130064)

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Concurrent accesses from multiple threads to the same `cell` object did not
scale well in the free-threaded build. Use `_PyStackRef` and optimistically
avoid locking to improve scaling.

With the locks around cell reads gone, some of the free threading tests were
prone to starvation: the readers were able to run in a tight loop and the
writer threads weren't scheduled frequently enough to make timely progress.
Adjust the tests to avoid this.

Co-authored-by: Donghee Na <donghee.na@python.org>
This commit is contained in:
Sam Gross 2025-03-26 12:08:20 -04:00 • committed by GitHub
parent 1b8bb1ed0c
commit 3d4ac1a2c2
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12 changed files with 90 additions and 44 deletions
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Python/bytecodes.c
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@ -1822,12 +1822,11 @@ dummy_func(
inst(LOAD_DEREF, ( -- value)) {
PyCellObject *cell = (PyCellObject *)PyStackRef_AsPyObjectBorrow(GETLOCAL(oparg));
PyObject *value_o = PyCell_GetRef(cell);
if (value_o == NULL) {
value = _PyCell_GetStackRef(cell);
if (PyStackRef_IsNull(value)) {
_PyEval_FormatExcUnbound(tstate, _PyFrame_GetCode(frame), oparg);
ERROR_IF(true, error);
}
value = PyStackRef_FromPyObjectSteal(value_o);
}
inst(STORE_DEREF, (v --)) {