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gh-129533: Update PyGC_Enable/Disable/IsEnabled to use atomic operation #129563

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Feb 6, 2025
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gh-129533: Update PyGC_Enable/Disable/IsEnabled to use atomic operation #129563

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gh-129533: Update PyGC_Enable/Disable/IsEnabled to use CAS operation
corona10 Feb 2, 2025
0624809
fix
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nit
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Update NEWS.d
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/C_API/2025-02-02-12-58-21.gh-issue-129533.dFfqkT.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Update :c:func:`PyGC_Enable()`, :c:func:`PyGC_Disable()`,
:c:func:`PyGC_IsEnabled()` to use CAS operation for thread-safety at
free-threading build. Patch by Donghee Na.
18 changes: 9 additions & 9 deletions Python/gc_free_threading.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1476,7 +1476,8 @@ gc_should_collect(GCState *gcstate)
{
int count = _Py_atomic_load_int_relaxed(&gcstate->young.count);
int threshold = gcstate->young.threshold;
if (count <= threshold || threshold == 0 || !gcstate->enabled) {
int gc_enabled = _Py_atomic_load_int_relaxed(&gcstate->enabled);
if (count <= threshold || threshold == 0 || !gc_enabled) {
return false;
}
// Avoid quadratic behavior by scaling threshold to the number of live
Expand Down Expand Up @@ -1952,25 +1953,25 @@ int
PyGC_Enable(void)
{
GCState *gcstate = get_gc_state();
int old_state = gcstate->enabled;
gcstate->enabled = 1;
int old_state = _Py_atomic_load_int_relaxed(&gcstate->enabled);
while (!_Py_atomic_compare_exchange_int(&gcstate->enabled, &old_state, 1));
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return old_state;
}

int
PyGC_Disable(void)
{
GCState *gcstate = get_gc_state();
int old_state = gcstate->enabled;
gcstate->enabled = 0;
int old_state = _Py_atomic_load_int_relaxed(&gcstate->enabled);
while (!_Py_atomic_compare_exchange_int(&gcstate->enabled, &old_state, 0));
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We don't need to update old_state explicitly since __atomic_compare_exchange_n automatically handle.
https://gcc.gnu.org/onlinedocs/gcc-8.4.0/gcc/_005f_005fatomic-Builtins.html

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return old_state;
}

int
PyGC_IsEnabled(void)
{
GCState *gcstate = get_gc_state();
return gcstate->enabled;
return _Py_atomic_load_int_relaxed(&gcstate->enabled);
}

/* Public API to invoke gc.collect() from C */
Expand All @@ -1980,7 +1981,7 @@ PyGC_Collect(void)
PyThreadState *tstate = _PyThreadState_GET();
GCState *gcstate = &tstate->interp->gc;

if (!gcstate->enabled) {
if (!_Py_atomic_load_int_relaxed(&gcstate->enabled)) {
return 0;
}

Expand Down Expand Up @@ -2139,8 +2140,7 @@ _PyObject_GC_Link(PyObject *op)
void
_Py_RunGC(PyThreadState *tstate)
{
GCState *gcstate = get_gc_state();
if (!gcstate->enabled) {
if (!PyGC_IsEnabled()) {
return;
}
gc_collect_main(tstate, 0, _Py_GC_REASON_HEAP);
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