From 375b723d5873f948696c7e85a97f4778d9e00ff0 Mon Sep 17 00:00:00 2001 From: Sam Gross Date: Mon, 24 Jun 2024 14:15:15 -0400 Subject: [PATCH] gh-120858: PyDict_Next should not lock the dict (#120859) PyDict_Next no longer locks the dictionary in the free-threaded build. Locking around individual PyDict_Next calls is not sufficient because the function returns borrowed references and because it allows concurrent modifications during the iteraiton loop. The internal locking also interferes with correct external synchronization because it may suspend outer critical sections created by the caller. --- Doc/c-api/dict.rst | 11 ++++++++++ Doc/howto/free-threading-extensions.rst | 20 ++++++++++++++++++- ...-06-21-16-41-21.gh-issue-120858.Z5_-Mn.rst | 3 +++ Objects/dictobject.c | 8 +------- 4 files changed, 34 insertions(+), 8 deletions(-) create mode 100644 Misc/NEWS.d/next/C API/2024-06-21-16-41-21.gh-issue-120858.Z5_-Mn.rst diff --git a/Doc/c-api/dict.rst b/Doc/c-api/dict.rst index 49a78583a6f..b4fdf47bc91 100644 --- a/Doc/c-api/dict.rst +++ b/Doc/c-api/dict.rst @@ -290,6 +290,17 @@ Dictionary Objects Py_DECREF(o); } + The function is not thread-safe in the :term:`free-threaded ` + build without external synchronization. You can use + :c:macro:`Py_BEGIN_CRITICAL_SECTION` to lock the dictionary while iterating + over it:: + + Py_BEGIN_CRITICAL_SECTION(self->dict); + while (PyDict_Next(self->dict, &pos, &key, &value)) { + ... + } + Py_END_CRITICAL_SECTION(); + .. c:function:: int PyDict_Merge(PyObject *a, PyObject *b, int override) diff --git a/Doc/howto/free-threading-extensions.rst b/Doc/howto/free-threading-extensions.rst index 080170de1e5..1ba91b09516 100644 --- a/Doc/howto/free-threading-extensions.rst +++ b/Doc/howto/free-threading-extensions.rst @@ -114,6 +114,24 @@ Containers like :c:struct:`PyListObject`, in the free-threaded build. For example, the :c:func:`PyList_Append` will lock the list before appending an item. +.. _PyDict_Next: + +``PyDict_Next`` +''''''''''''''' + +A notable exception is :c:func:`PyDict_Next`, which does not lock the +dictionary. You should use :c:macro:`Py_BEGIN_CRITICAL_SECTION` to protect +the dictionary while iterating over it if the dictionary may be concurrently +modified:: + + Py_BEGIN_CRITICAL_SECTION(dict); + PyObject *key, *value; + Py_ssize_t pos = 0; + while (PyDict_Next(dict, &pos, &key, &value)) { + ... + } + Py_END_CRITICAL_SECTION(); + Borrowed References =================== @@ -141,7 +159,7 @@ that return :term:`strong references `. +-----------------------------------+-----------------------------------+ | :c:func:`PyDict_SetDefault` | :c:func:`PyDict_SetDefaultRef` | +-----------------------------------+-----------------------------------+ -| :c:func:`PyDict_Next` | no direct replacement | +| :c:func:`PyDict_Next` | none (see :ref:`PyDict_Next`) | +-----------------------------------+-----------------------------------+ | :c:func:`PyWeakref_GetObject` | :c:func:`PyWeakref_GetRef` | +-----------------------------------+-----------------------------------+ diff --git a/Misc/NEWS.d/next/C API/2024-06-21-16-41-21.gh-issue-120858.Z5_-Mn.rst b/Misc/NEWS.d/next/C API/2024-06-21-16-41-21.gh-issue-120858.Z5_-Mn.rst new file mode 100644 index 00000000000..b5df2a567b9 --- /dev/null +++ b/Misc/NEWS.d/next/C API/2024-06-21-16-41-21.gh-issue-120858.Z5_-Mn.rst @@ -0,0 +1,3 @@ +:c:func:`PyDict_Next` no longer locks the dictionary in the free-threaded +build. The locking needs to be done by the caller around the entire iteration +loop. diff --git a/Objects/dictobject.c b/Objects/dictobject.c index 5529e527d8b..5d325465608 100644 --- a/Objects/dictobject.c +++ b/Objects/dictobject.c @@ -2808,8 +2808,6 @@ _PyDict_Next(PyObject *op, Py_ssize_t *ppos, PyObject **pkey, if (!PyDict_Check(op)) return 0; - ASSERT_DICT_LOCKED(op); - mp = (PyDictObject *)op; i = *ppos; if (_PyDict_HasSplitTable(mp)) { @@ -2882,11 +2880,7 @@ _PyDict_Next(PyObject *op, Py_ssize_t *ppos, PyObject **pkey, int PyDict_Next(PyObject *op, Py_ssize_t *ppos, PyObject **pkey, PyObject **pvalue) { - int res; - Py_BEGIN_CRITICAL_SECTION(op); - res = _PyDict_Next(op, ppos, pkey, pvalue, NULL); - Py_END_CRITICAL_SECTION(); - return res; + return _PyDict_Next(op, ppos, pkey, pvalue, NULL); }