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[WIP] bpo-40533: Make PyObject.ob_refcnt atomic in subinterpreters #19958

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[WIP] bpo-40533: Make PyObject.ob_refcnt atomic in subinterpreters #19958

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7 changes: 7 additions & 0 deletions Include/object.h
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Original file line number Diff line number Diff line change
Expand Up @@ -104,7 +104,14 @@ typedef struct _typeobject PyTypeObject;
*/
typedef struct _object {
_PyObject_HEAD_EXTRA
#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
/* bpo-40533: Use an atomic variable for PyObject.ob_refcnt, to ensure that
Py_INCREF() and Py_DECREF() are safe when called in parallel, until
subinterpreters stop sharing Python objects. */
_Atomic Py_ssize_t ob_refcnt;
Comment on lines +107 to +111
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@aeros aeros May 6, 2020 •
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Perhaps this is somewhat emphasized by the "experimental" part, but could we possibly make it a bit more crystal clear that this is only a temporary stopgap measure? I personally find that "until subinterpreters stop sharing Python objects" could be interpreted as a near indefinite amount of time; I'm not confident that most readers will go over the full context in the bpo issue and related discussions.

Normally, this wouldn't be a huge concern, but I think object.h is likely one of the most public-facing header files in CPython, so it seems important to communicate this as clearly as possible.

#else
Py_ssize_t ob_refcnt;
#endif
PyTypeObject *ob_type;
} PyObject;

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6 changes: 6 additions & 0 deletions Objects/dict-common.h
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Expand Up @@ -20,7 +20,13 @@ typedef Py_ssize_t (*dict_lookup_func)

/* See dictobject.c for actual layout of DictKeysObject */
struct _dictkeysobject {
#ifdef EXPERIMENTAL_ISOLATED_SUBINTERPRETERS
/* bpo-40533: Use an atomic variables until subinterpreters stop sharing
Python dictionaries. */
_Atomic Py_ssize_t dk_refcnt;
#else
Py_ssize_t dk_refcnt;
#endif

/* Size of the hash table (dk_indices). It must be a power of 2. */
Py_ssize_t dk_size;
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