gh-128182: Add per-object memory access synchronization to `ctypes` (GH-128490)

5 files changed, 234 insertions, 47 deletions

diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
index f25bf94..077a279 100644
--- a/Doc/library/ctypes.rst
+++ b/Doc/library/ctypes.rst
@@ -870,6 +870,36 @@ invalid non-\ ``NULL`` pointers would crash Python)::
    ValueError: NULL pointer access
    >>>
 
+.. _ctypes-thread-safety:
+
+Thread safety without the GIL
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+In Python 3.13, the :term:`GIL` may be disabled on :term:`experimental free threaded <free threading>` builds.
+In ctypes, reads and writes to a single object concurrently is safe, but not across multiple objects:
+
+   .. code-block:: pycon
+
+      >>> number = c_int(42)
+      >>> pointer_a = pointer(number)
+      >>> pointer_b = pointer(number)
+
+In the above, it's only safe for one object to read and write to the address at once if the GIL is disabled.
+So, ``pointer_a`` can be shared and written to across multiple threads, but only if ``pointer_b``
+is not also attempting to do the same. If this is an issue, consider using a :class:`threading.Lock`
+to synchronize access to memory:
+
+   .. code-block:: pycon
+
+      >>> import threading
+      >>> lock = threading.Lock()
+      >>> # Thread 1
+      >>> with lock:
+      ...    pointer_a.contents = 24
+      >>> # Thread 2
+      >>> with lock:
+      ...    pointer_b.contents = 42
+
 
 .. _ctypes-type-conversions:
 
diff --git a/Lib/test/test_ctypes/test_arrays.py b/Lib/test/test_ctypes/test_arrays.py
index c80fdff..7f1f6cf 100644
--- a/Lib/test/test_ctypes/test_arrays.py
+++ b/Lib/test/test_ctypes/test_arrays.py
@@ -5,7 +5,7 @@ from ctypes import (Structure, Array, ARRAY, sizeof, addressof,
                     create_string_buffer, create_unicode_buffer,
                     c_char, c_wchar, c_byte, c_ubyte, c_short, c_ushort, c_int, c_uint,
                     c_long, c_ulonglong, c_float, c_double, c_longdouble)
-from test.support import bigmemtest, _2G
+from test.support import bigmemtest, _2G, threading_helper, Py_GIL_DISABLED
 from ._support import (_CData, PyCArrayType, Py_TPFLAGS_DISALLOW_INSTANTIATION,
                        Py_TPFLAGS_IMMUTABLETYPE)
 
@@ -267,6 +267,26 @@ class ArrayTestCase(unittest.TestCase):
     def test_large_array(self, size):
         c_char * size
 
+    @threading_helper.requires_working_threading()
+    @unittest.skipUnless(Py_GIL_DISABLED, "only meaningful if the GIL is disabled")
+    def test_thread_safety(self):
+        from threading import Thread
+
+        buffer = (ctypes.c_char_p * 10)()
+
+        def run():
+            for i in range(100):
+                buffer.value = b"hello"
+                buffer[0] = b"j"
+
+        with threading_helper.catch_threading_exception() as cm:
+            threads = (Thread(target=run) for _ in range(25))
+            with threading_helper.start_threads(threads):
+                pass
+
+            if cm.exc_value:
+                raise cm.exc_value
+
 
 if __name__ == '__main__':
     unittest.main()
diff --git a/Misc/NEWS.d/next/Library/2025-01-04-11-32-46.gh-issue-128182.SJ2Zsa.rst b/Misc/NEWS.d/next/Library/2025-01-04-11-32-46.gh-issue-128182.SJ2Zsa.rst
new file mode 100644
index 0000000..038fecb
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2025-01-04-11-32-46.gh-issue-128182.SJ2Zsa.rst
@@ -0,0 +1,2 @@
+Fix crash when using :mod:`ctypes` pointers concurrently on the :term:`free
+threaded <free threading>` build.
diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
index ede95bd..c4d130a 100644
--- a/Modules/_ctypes/_ctypes.c
+++ b/Modules/_ctypes/_ctypes.c
@@ -598,7 +598,7 @@ StructUnionType_paramfunc(ctypes_state *st, CDataObject *self)
         if (ptr == NULL) {
             return NULL;
         }
-        memcpy(ptr, self->b_ptr, self->b_size);
+        locked_memcpy_from(ptr, self, self->b_size);
 
         /* Create a Python object which calls PyMem_Free(ptr) in
            its deallocator. The object will be destroyed
@@ -907,8 +907,7 @@ CDataType_from_buffer_copy_impl(PyObject *type, PyTypeObject *cls,
 
     result = generic_pycdata_new(st, (PyTypeObject *)type, NULL, NULL);
     if (result != NULL) {
-        memcpy(((CDataObject *)result)->b_ptr,
-               (char *)buffer->buf + offset, info->size);
+        locked_memcpy_to((CDataObject *) result, (char *)buffer->buf + offset, info->size);
     }
     return result;
 }
@@ -1195,7 +1194,7 @@ PyCPointerType_paramfunc(ctypes_state *st, CDataObject *self)
     parg->tag = 'P';
     parg->pffi_type = &ffi_type_pointer;
     parg->obj = Py_NewRef(self);
-    parg->value.p = *(void **)self->b_ptr;
+    parg->value.p = locked_deref(self);
     return parg;
 }
 
@@ -1432,7 +1431,7 @@ CharArray_set_raw(CDataObject *self, PyObject *value, void *Py_UNUSED(ignored))
         goto fail;
     }
 
-    memcpy(self->b_ptr, ptr, size);
+    locked_memcpy_to(self, ptr, size);
 
     PyBuffer_Release(&view);
     return 0;
@@ -1444,18 +1443,26 @@ CharArray_set_raw(CDataObject *self, PyObject *value, void *Py_UNUSED(ignored))
 static PyObject *
 CharArray_get_raw(CDataObject *self, void *Py_UNUSED(ignored))
 {
-    return PyBytes_FromStringAndSize(self->b_ptr, self->b_size);
+    PyObject *res;
+    LOCK_PTR(self);
+    res = PyBytes_FromStringAndSize(self->b_ptr, self->b_size);
+    UNLOCK_PTR(self);
+    return res;
 }
 
 static PyObject *
 CharArray_get_value(CDataObject *self, void *Py_UNUSED(ignored))
 {
     Py_ssize_t i;
+    PyObject *res;
+    LOCK_PTR(self);
     char *ptr = self->b_ptr;
     for (i = 0; i < self->b_size; ++i)
         if (*ptr++ == '\0')
             break;
-    return PyBytes_FromStringAndSize(self->b_ptr, i);
+    res = PyBytes_FromStringAndSize(self->b_ptr, i);
+    UNLOCK_PTR(self);
+    return res;
 }
 
 static int
@@ -1486,9 +1493,11 @@ CharArray_set_value(CDataObject *self, PyObject *value, void *Py_UNUSED(ignored)
     }
 
     ptr = PyBytes_AS_STRING(value);
+    LOCK_PTR(self);
     memcpy(self->b_ptr, ptr, size);
     if (size < self->b_size)
         self->b_ptr[size] = '\0';
+    UNLOCK_PTR(self);
     Py_DECREF(value);
 
     return 0;
@@ -1506,11 +1515,15 @@ static PyObject *
 WCharArray_get_value(CDataObject *self, void *Py_UNUSED(ignored))
 {
     Py_ssize_t i;
+    PyObject *res;
     wchar_t *ptr = (wchar_t *)self->b_ptr;
+    LOCK_PTR(self);
     for (i = 0; i < self->b_size/(Py_ssize_t)sizeof(wchar_t); ++i)
         if (*ptr++ == (wchar_t)0)
             break;
-    return PyUnicode_FromWideChar((wchar_t *)self->b_ptr, i);
+    res = PyUnicode_FromWideChar((wchar_t *)self->b_ptr, i);
+    UNLOCK_PTR(self);
+    return res;
 }
 
 static int
@@ -1540,10 +1553,11 @@ WCharArray_set_value(CDataObject *self, PyObject *value, void *Py_UNUSED(ignored
         PyErr_SetString(PyExc_ValueError, "string too long");
         return -1;
     }
-    if (PyUnicode_AsWideChar(value, (wchar_t *)self->b_ptr, size) < 0) {
-        return -1;
-    }
-    return 0;
+    Py_ssize_t rc;
+    LOCK_PTR(self);
+    rc = PyUnicode_AsWideChar(value, (wchar_t *)self->b_ptr, size);
+    UNLOCK_PTR(self);
+    return rc < 0 ? -1 : 0;
 }
 
 static PyGetSetDef WCharArray_getsets[] = {
@@ -2053,6 +2067,7 @@ c_void_p_from_param_impl(PyObject *type, PyTypeObject *cls, PyObject *value)
         parg->pffi_type = &ffi_type_pointer;
         parg->tag = 'P';
         Py_INCREF(value);
+        // Function pointers don't change their contents, no need to lock
         parg->value.p = *(void **)func->b_ptr;
         parg->obj = value;
         return (PyObject *)parg;
@@ -2079,7 +2094,7 @@ c_void_p_from_param_impl(PyObject *type, PyTypeObject *cls, PyObject *value)
             parg->tag = 'Z';
             parg->obj = Py_NewRef(value);
             /* Remember: b_ptr points to where the pointer is stored! */
-            parg->value.p = *(void **)(((CDataObject *)value)->b_ptr);
+            parg->value.p = locked_deref((CDataObject *)value);
             return (PyObject *)parg;
         }
     }
@@ -2196,7 +2211,7 @@ PyCSimpleType_paramfunc(ctypes_state *st, CDataObject *self)
     parg->tag = fmt[0];
     parg->pffi_type = fd->pffi_type;
     parg->obj = Py_NewRef(self);
-    memcpy(&parg->value, self->b_ptr, self->b_size);
+    locked_memcpy_from(&parg->value, self, self->b_size);
     return parg;
 }
 
@@ -2697,7 +2712,7 @@ PyCFuncPtrType_paramfunc(ctypes_state *st, CDataObject *self)
     parg->tag = 'P';
     parg->pffi_type = &ffi_type_pointer;
     parg->obj = Py_NewRef(self);
-    parg->value.p = *(void **)self->b_ptr;
+    parg->value.p = locked_deref(self);
     return parg;
 }
 
@@ -3017,8 +3032,12 @@ PyCData_reduce_impl(PyObject *myself, PyTypeObject *cls)
     if (dict == NULL) {
         return NULL;
     }
+    PyObject *bytes;
+    LOCK_PTR(self);
+    bytes = PyBytes_FromStringAndSize(self->b_ptr, self->b_size);
+    UNLOCK_PTR(self);
     return Py_BuildValue("O(O(NN))", st->_unpickle, Py_TYPE(myself), dict,
-                         PyBytes_FromStringAndSize(self->b_ptr, self->b_size));
+                         bytes);
 }
 
 static PyObject *
@@ -3036,7 +3055,10 @@ PyCData_setstate(PyObject *myself, PyObject *args)
     }
     if (len > self->b_size)
         len = self->b_size;
+    // XXX Can we use locked_memcpy_to()?
+    LOCK_PTR(self);
     memmove(self->b_ptr, data, len);
+    UNLOCK_PTR(self);
     mydict = PyObject_GetAttrString(myself, "__dict__");
     if (mydict == NULL) {
         return NULL;
@@ -3094,6 +3116,12 @@ static PyType_Spec pycdata_spec = {
 static int
 PyCData_MallocBuffer(CDataObject *obj, StgInfo *info)
 {
+    /* We don't have to lock in this function, because it's only
+     * used in constructors and therefore does not have concurrent
+     * access.
+     */
+   assert (Py_REFCNT(obj) == 1);
+
     if ((size_t)info->size <= sizeof(obj->b_value)) {
         /* No need to call malloc, can use the default buffer */
         obj->b_ptr = (char *)&obj->b_value;
@@ -3219,15 +3247,28 @@ PyObject *
 PyCData_get(ctypes_state *st, PyObject *type, GETFUNC getfunc, PyObject *src,
           Py_ssize_t index, Py_ssize_t size, char *adr)
 {
-    if (getfunc)
-        return getfunc(adr, size);
+#ifdef Py_GIL_DISABLED
+    // This isn't used if the GIL is enabled, so it causes a compiler warning.
+    CDataObject *cdata = (CDataObject *)src;
+#endif
+    if (getfunc) {
+        PyObject *res;
+        LOCK_PTR(cdata);
+        res = getfunc(adr, size);
+        UNLOCK_PTR(cdata);
+        return res;
+    }
     assert(type);
     StgInfo *info;
     if (PyStgInfo_FromType(st, type, &info) < 0) {
         return NULL;
     }
     if (info && info->getfunc && !_ctypes_simple_instance(st, type)) {
-        return info->getfunc(adr, size);
+        PyObject *res;
+        LOCK_PTR(cdata);
+        res = info->getfunc(adr, size);
+        UNLOCK_PTR(cdata);
+        return res;
     }
     return PyCData_FromBaseObj(st, type, src, index, adr);
 }
@@ -3244,15 +3285,24 @@ _PyCData_set(ctypes_state *st,
     int err;
 
     if (setfunc) {
-        return setfunc(ptr, value, size);
+        PyObject *res;
+        LOCK_PTR(dst);
+        res = setfunc(ptr, value, size);
+        UNLOCK_PTR(dst);
+        return res;
     }
     if (!CDataObject_Check(st, value)) {
         StgInfo *info;
         if (PyStgInfo_FromType(st, type, &info) < 0) {
             return NULL;
         }
-        if (info && info->setfunc)
-            return info->setfunc(ptr, value, size);
+        if (info && info->setfunc) {
+            PyObject *res;
+            LOCK_PTR(dst);
+            res = info->setfunc(ptr, value, size);
+            UNLOCK_PTR(dst);
+            return res;
+        }
         /*
            If value is a tuple, we try to call the type with the tuple
            and use the result!
@@ -3272,7 +3322,9 @@ _PyCData_set(ctypes_state *st,
             Py_DECREF(ob);
             return result;
         } else if (value == Py_None && PyCPointerTypeObject_Check(st, type)) {
+            LOCK_PTR(dst);
             *(void **)ptr = NULL;
+            UNLOCK_PTR(dst);
             Py_RETURN_NONE;
         } else {
             PyErr_Format(PyExc_TypeError,
@@ -3288,9 +3340,7 @@ _PyCData_set(ctypes_state *st,
     if (err == -1)
         return NULL;
     if (err) {
-        memcpy(ptr,
-               src->b_ptr,
-               size);
+        locked_memcpy_from(ptr, src, size);
 
         if (PyCPointerTypeObject_Check(st, type)) {
             /* XXX */
@@ -3324,7 +3374,9 @@ _PyCData_set(ctypes_state *st,
                          ((PyTypeObject *)type)->tp_name);
             return NULL;
         }
+        LOCK_PTR(src);
         *(void **)ptr = src->b_ptr;
+        UNLOCK_PTR(src);
 
         keep = GetKeepedObjects(src);
         if (keep == NULL)
@@ -3891,6 +3943,8 @@ PyCFuncPtr_FromDll(PyTypeObject *type, PyObject *args, PyObject *kwds)
 
     self->paramflags = Py_XNewRef(paramflags);
 
+    // No other threads can have this object, no need to
+    // lock it.
     *(void **)self->b_ptr = address;
     Py_INCREF(dll);
     Py_DECREF(ftuple);
@@ -4823,18 +4877,24 @@ Array_subscript(PyObject *myself, PyObject *item)
             if (slicelen <= 0)
                 return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
             if (step == 1) {
-                return PyBytes_FromStringAndSize(ptr + start,
-                                                 slicelen);
+                PyObject *res;
+                LOCK_PTR(self);
+                res = PyBytes_FromStringAndSize(ptr + start,
+                                                slicelen);
+                UNLOCK_PTR(self);
+                return res;
             }
             dest = (char *)PyMem_Malloc(slicelen);
 
             if (dest == NULL)
                 return PyErr_NoMemory();
 
+            LOCK_PTR(self);
             for (cur = start, i = 0; i < slicelen;
                  cur += step, i++) {
                 dest[i] = ptr[cur];
             }
+            UNLOCK_PTR(self);
 
             np = PyBytes_FromStringAndSize(dest, slicelen);
             PyMem_Free(dest);
@@ -4847,8 +4907,12 @@ Array_subscript(PyObject *myself, PyObject *item)
             if (slicelen <= 0)
                 return Py_GetConstant(Py_CONSTANT_EMPTY_STR);
             if (step == 1) {
-                return PyUnicode_FromWideChar(ptr + start,
-                                              slicelen);
+                PyObject *res;
+                LOCK_PTR(self);
+                res = PyUnicode_FromWideChar(ptr + start,
+                                                       slicelen);
+                UNLOCK_PTR(self);
+                return res;
             }
 
             dest = PyMem_New(wchar_t, slicelen);
@@ -4857,10 +4921,12 @@ Array_subscript(PyObject *myself, PyObject *item)
                 return NULL;
             }
 
+            LOCK_PTR(self);
             for (cur = start, i = 0; i < slicelen;
                  cur += step, i++) {
                 dest[i] = ptr[cur];
             }
+            UNLOCK_PTR(self);
 
             np = PyUnicode_FromWideChar(dest, slicelen);
             PyMem_Free(dest);
@@ -5118,7 +5184,9 @@ Simple_set_value(CDataObject *self, PyObject *value, void *Py_UNUSED(ignored))
     assert(info); /* Cannot be NULL for CDataObject instances */
     assert(info->setfunc);
 
+    LOCK_PTR(self);
     result = info->setfunc(self->b_ptr, value, info->size);
+    UNLOCK_PTR(self);
     if (!result)
         return -1;
 
@@ -5147,7 +5215,11 @@ Simple_get_value(CDataObject *self, void *Py_UNUSED(ignored))
     }
     assert(info); /* Cannot be NULL for CDataObject instances */
     assert(info->getfunc);
-    return info->getfunc(self->b_ptr, self->b_size);
+    PyObject *res;
+    LOCK_PTR(self);
+    res = info->getfunc(self->b_ptr, self->b_size);
+    UNLOCK_PTR(self);
+    return res;
 }
 
 static PyGetSetDef Simple_getsets[] = {
@@ -5183,7 +5255,11 @@ static PyMethodDef Simple_methods[] = {
 
 static int Simple_bool(CDataObject *self)
 {
-    return memcmp(self->b_ptr, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", self->b_size);
+    int cmp;
+    LOCK_PTR(self);
+    cmp = memcmp(self->b_ptr, "\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", self->b_size);
+    UNLOCK_PTR(self);
+    return cmp;
 }
 
 /* "%s(%s)" % (self.__class__.__name__, self.value) */
@@ -5239,8 +5315,9 @@ Pointer_item(PyObject *myself, Py_ssize_t index)
     Py_ssize_t size;
     Py_ssize_t offset;
     PyObject *proto;
+    void *deref = locked_deref(self);
 
-    if (*(void **)self->b_ptr == NULL) {
+    if (deref == NULL) {
         PyErr_SetString(PyExc_ValueError,
                         "NULL pointer access");
         return NULL;
@@ -5267,7 +5344,7 @@ Pointer_item(PyObject *myself, Py_ssize_t index)
     offset = index * iteminfo->size;
 
     return PyCData_get(st, proto, stginfo->getfunc, (PyObject *)self,
-                     index, size, (*(char **)self->b_ptr) + offset);
+                       index, size, (char *)((char *)deref + offset));
 }
 
 static int
@@ -5284,7 +5361,8 @@ Pointer_ass_item(PyObject *myself, Py_ssize_t index, PyObject *value)
         return -1;
     }
 
-    if (*(void **)self->b_ptr == NULL) {
+    void *deref = locked_deref(self);
+    if (deref == NULL) {
         PyErr_SetString(PyExc_ValueError,
                         "NULL pointer access");
         return -1;
@@ -5311,13 +5389,14 @@ Pointer_ass_item(PyObject *myself, Py_ssize_t index, PyObject *value)
     offset = index * iteminfo->size;
 
     return PyCData_set(st, (PyObject *)self, proto, stginfo->setfunc, value,
-                     index, size, (*(char **)self->b_ptr) + offset);
+                       index, size, ((char *)deref + offset));
 }
 
 static PyObject *
 Pointer_get_contents(CDataObject *self, void *closure)
 {
-    if (*(void **)self->b_ptr == NULL) {
+    void *deref = locked_deref(self);
+    if (deref == NULL) {
         PyErr_SetString(PyExc_ValueError,
                         "NULL pointer access");
         return NULL;
@@ -5332,7 +5411,7 @@ Pointer_get_contents(CDataObject *self, void *closure)
 
     return PyCData_FromBaseObj(st, stginfo->proto,
                              (PyObject *)self, 0,
-                             *(void **)self->b_ptr);
+                             deref);
 }
 
 static int
@@ -5367,7 +5446,7 @@ Pointer_set_contents(CDataObject *self, PyObject *value, void *closure)
     }
 
     dst = (CDataObject *)value;
-    *(void **)self->b_ptr = dst->b_ptr;
+    locked_deref_assign(self, dst->b_ptr);
 
     /*
        A Pointer instance must keep the value it points to alive.  So, a
@@ -5502,41 +5581,53 @@ Pointer_subscript(PyObject *myself, PyObject *item)
         }
         assert(iteminfo);
         if (iteminfo->getfunc == _ctypes_get_fielddesc("c")->getfunc) {
-            char *ptr = *(char **)self->b_ptr;
+            char *ptr = locked_deref(self);
             char *dest;
 
             if (len <= 0)
                 return Py_GetConstant(Py_CONSTANT_EMPTY_BYTES);
             if (step == 1) {
-                return PyBytes_FromStringAndSize(ptr + start,
-                                                 len);
+                PyObject *res;
+                LOCK_PTR(self);
+                res = PyBytes_FromStringAndSize(ptr + start,
+                                                len);
+                UNLOCK_PTR(self);
+                return res;
             }
             dest = (char *)PyMem_Malloc(len);
             if (dest == NULL)
                 return PyErr_NoMemory();
+            LOCK_PTR(self);
             for (cur = start, i = 0; i < len; cur += step, i++) {
                 dest[i] = ptr[cur];
             }
+            UNLOCK_PTR(self);
             np = PyBytes_FromStringAndSize(dest, len);
             PyMem_Free(dest);
             return np;
         }
         if (iteminfo->getfunc == _ctypes_get_fielddesc("u")->getfunc) {
-            wchar_t *ptr = *(wchar_t **)self->b_ptr;
+            wchar_t *ptr = locked_deref(self);
             wchar_t *dest;
 
             if (len <= 0)
                 return Py_GetConstant(Py_CONSTANT_EMPTY_STR);
             if (step == 1) {
-                return PyUnicode_FromWideChar(ptr + start,
-                                              len);
+                PyObject *res;
+                LOCK_PTR(self);
+                res = PyUnicode_FromWideChar(ptr + start,
+                                             len);
+                UNLOCK_PTR(self);
+                return res;
             }
             dest = PyMem_New(wchar_t, len);
             if (dest == NULL)
                 return PyErr_NoMemory();
+            LOCK_PTR(self);
             for (cur = start, i = 0; i < len; cur += step, i++) {
                 dest[i] = ptr[cur];
             }
+            UNLOCK_PTR(self);
             np = PyUnicode_FromWideChar(dest, len);
             PyMem_Free(dest);
             return np;
@@ -5562,7 +5653,7 @@ Pointer_subscript(PyObject *myself, PyObject *item)
 static int
 Pointer_bool(CDataObject *self)
 {
-    return (*(void **)self->b_ptr != NULL);
+    return locked_deref(self) != NULL;
 }
 
 static PyType_Slot pycpointer_slots[] = {
@@ -5770,7 +5861,7 @@ cast(void *ptr, PyObject *src, PyObject *ctype)
         }
     }
     /* Should we assert that result is a pointer type? */
-    memcpy(result->b_ptr, &ptr, sizeof(void *));
+    locked_memcpy_to(result, &ptr, sizeof(void *));
     return (PyObject *)result;
 
   failed:
diff --git a/Modules/_ctypes/ctypes.h b/Modules/_ctypes/ctypes.h
index 45e00a5..cc09639 100644
--- a/Modules/_ctypes/ctypes.h
+++ b/Modules/_ctypes/ctypes.h
@@ -543,3 +543,47 @@ PyStgInfo_Init(ctypes_state *state, PyTypeObject *type)
     info->initialized = 1;
     return info;
 }
+
+/* See discussion in gh-128490. The plan here is to eventually use a per-object
+ * lock rather than a critical section, but that work is for later. */
+#ifdef Py_GIL_DISABLED
+#  define LOCK_PTR(self) Py_BEGIN_CRITICAL_SECTION(self)
+#  define UNLOCK_PTR(self) Py_END_CRITICAL_SECTION()
+#else
+#  define LOCK_PTR(self)
+#  define UNLOCK_PTR(self)
+#endif
+
+static inline void
+locked_memcpy_to(CDataObject *self, void *buf, Py_ssize_t size)
+{
+    LOCK_PTR(self);
+    (void)memcpy(self->b_ptr, buf, size);
+    UNLOCK_PTR(self);
+}
+
+static inline void
+locked_memcpy_from(void *buf, CDataObject *self, Py_ssize_t size)
+{
+    LOCK_PTR(self);
+    (void)memcpy(buf, self->b_ptr, size);
+    UNLOCK_PTR(self);
+}
+
+static inline void *
+locked_deref(CDataObject *self)
+{
+    void *ptr;
+    LOCK_PTR(self);
+    ptr = *(void **)self->b_ptr;
+    UNLOCK_PTR(self);
+    return ptr;
+}
+
+static inline void
+locked_deref_assign(CDataObject *self, void *new_ptr)
+{
+    LOCK_PTR(self);
+    *(void **)self->b_ptr = new_ptr;
+    UNLOCK_PTR(self);
+}