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gh-135832: implement Py_DECREF specializations for Py_GIL_DISABLED build #135833

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gh-135832: implement Py_DECREF specializations for Py_GIL_DISABLED build #135833

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82 changes: 79 additions & 3 deletions Include/internal/pycore_object.h
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Original file line number Diff line number Diff line change
Expand Up @@ -272,17 +272,93 @@ _Py_DECREF_NO_DEALLOC(PyObject *op)
}

#else
// TODO: implement Py_DECREF specializations for Py_GIL_DISABLED build

static inline void
_Py_DECREF_SPECIALIZED(PyObject *op, const destructor destruct)
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This function is barely used now, but there's a similar function PyStackRef_CLOSE_SPECIALIZED.

There are two possible reasons to implement this as something that's not just Py_DECREF:

  1. Performance, but it's not really clear that it's going to be any faster
  2. To avoid _Py_Dealloc so that you can enable this assertion

    cpython/Objects/object.c

    Lines 3166 to 3170 in 6227662

    #if !defined(Py_GIL_DISABLED) && !defined(Py_STACKREF_DEBUG)
    /* This assertion doesn't hold for the free-threading build, as
    * PyStackRef_CLOSE_SPECIALIZED is not implemented */
    assert(tstate->current_frame == NULL || tstate->current_frame->stackpointer != NULL);
    #endif
    , but that means you can't call _Py_MergeZeroLocalRefcount because that internally may call _Py_Dealloc

I think (2) would be difficult to achieve because of _Py_brc_queue_object.

{
Py_DECREF(op);
uint32_t local = _Py_atomic_load_uint32_relaxed(&op->ob_ref_local);
if (local == _Py_IMMORTAL_REFCNT_LOCAL) {
_Py_DECREF_IMMORTAL_STAT_INC();
return;
}
_Py_DECREF_STAT_INC();
#ifdef Py_REF_DEBUG
_Py_DECREF_DecRefTotal();
#endif
if (_Py_IsOwnedByCurrentThread(op)) {
local--;
_Py_atomic_store_uint32_relaxed(&op->ob_ref_local, local);
if (local == 0) {
_Py_MergeZeroLocalRefcount(op);
}
}
else {
Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared);
for (;;) {
Py_ssize_t new_shared = shared - (1 << _Py_REF_SHARED_SHIFT);
if (_Py_atomic_compare_exchange_ssize(
Comment on lines +298 to +299
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This is missing important parts of _Py_DecRefShared: if the shared reference count becomes negative, the object must be queued so that the owning thread merges the two reference count fields.

&op->ob_ref_shared,
&shared,
new_shared)) {
if (new_shared == 0) {
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You can only safely deallocate if the reference count is zero and it's merged (i.e., new_shared is _Py_REF_MERGED)

#ifdef Py_TRACE_REFS
_Py_ForgetReference(op);
#endif
_PyReftracerTrack(op, PyRefTracer_DESTROY);
destruct(op);
}
break;
}
}
}
}

static inline void
_Py_DECREF_NO_DEALLOC(PyObject *op)
Comment on lines 316 to 317
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I'm not sure this function is used anymore.

{
Py_DECREF(op);
uint32_t local = _Py_atomic_load_uint32_relaxed(&op->ob_ref_local);
if (local == _Py_IMMORTAL_REFCNT_LOCAL) {
_Py_DECREF_IMMORTAL_STAT_INC();
return;
}
_Py_DECREF_STAT_INC();
#ifdef Py_REF_DEBUG
_Py_DECREF_DecRefTotal();
#endif
if (_Py_IsOwnedByCurrentThread(op)) {
local--;
_Py_atomic_store_uint32_relaxed(&op->ob_ref_local, local);
if (local == 0) {
_Py_MergeZeroLocalRefcount(op);
}
}
else {
Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared);
for (;;) {
Py_ssize_t new_shared = shared - (1 << _Py_REF_SHARED_SHIFT);
if (_Py_atomic_compare_exchange_ssize(
&op->ob_ref_shared,
&shared,
new_shared)) {
break;
}
}
}
#ifdef Py_DEBUG
// Check that the refcount is still positive after decrement
if (_Py_IsOwnedByCurrentThread(op)) {
uint32_t final_local = _Py_atomic_load_uint32_relaxed(&op->ob_ref_local);
if (final_local == 0) {
_Py_FatalRefcountError("Expected a positive remaining refcount");
}
}
else {
Py_ssize_t final_shared = _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared);
if ((final_shared >> _Py_REF_SHARED_SHIFT) <= 0) {
_Py_FatalRefcountError("Expected a positive remaining refcount");
}
}
#endif
}

static inline int
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