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gh-115103: Update gc.collect to process delayed objects #116238

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Merged
corona10 merged 5 commits into from
Mar 2, 2024
Merged

gh-115103: Update gc.collect to process delayed objects #116238

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c28c79f
gh-115103: Update refleak checker to trigger _PyMem_ProcessDelayed
corona10 Mar 2, 2024
81f333a
Update Lib/test/support/__init__.py
corona10 Mar 2, 2024
59b3fc4
Address code review
corona10 Mar 2, 2024
85b419c
Address code review
corona10 Mar 2, 2024
a187b03
nit
corona10 Mar 2, 2024
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18 changes: 18 additions & 0 deletions Python/gc_free_threading.c
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Original file line number Diff line number Diff line change
Expand Up @@ -324,6 +324,23 @@ merge_all_queued_objects(PyInterpreterState *interp, struct collection_state *st
HEAD_UNLOCK(&_PyRuntime);
}

static void
process_delayed_frees(PyInterpreterState *interp)
{
// In STW status, we can observe the latest write sequence by
// advancing the write sequence immediately.
_Py_qsbr_advance(&interp->qsbr);
_PyThreadStateImpl *current_tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
_Py_qsbr_quiescent_state(current_tstate->qsbr);
HEAD_LOCK(&_PyRuntime);
PyThreadState *tstate = interp->threads.head;
while (tstate != NULL) {
_PyMem_ProcessDelayed(tstate);
tstate = (PyThreadState *)tstate->next;
}
HEAD_UNLOCK(&_PyRuntime);
}

// Subtract an incoming reference from the computed "gc_refs" refcount.
static int
visit_decref(PyObject *op, void *arg)
Expand Down Expand Up @@ -1006,6 +1023,7 @@ gc_collect_internal(PyInterpreterState *interp, struct collection_state *state)
_PyEval_StopTheWorld(interp);
// merge refcounts for all queued objects
merge_all_queued_objects(interp, state);
process_delayed_frees(interp);
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@pablogsal pablogsal Mar 2, 2024

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Nit/question: _PyEval_StopTheWorld also locks _PyRuntime. Maybe is worth moving the lock up?

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@colesbury colesbury Mar 2, 2024

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_PyEval_StopTheWorld locks _PyRuntime temporarily to loop over threads, but does not hold the lock

We want process_delayed_frees after the stop-the-world call because it guarantees that we can free all the "delayed free" pointers. If we moved it up before the _PyEval_StopTheWorld there might be some memory blocks we don't free.

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@pablogsal pablogsal Mar 2, 2024

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Makes sense, thanks for clarifying

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@pablogsal pablogsal Mar 2, 2024

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If we moved it up before the _PyEval_StopTheWorld there might be some memory blocks we don't free.

With "moving the lock up" i meant to have a unique locking for both functions, not to move this call before the other. But your comment still applies


// Find unreachable objects
int err = deduce_unreachable_heap(interp, state);
Expand Down