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gh-111926: Make weakrefs thread-safe in free-threaded builds (#117168) 

Most mutable data is protected by a striped lock that is keyed on the
referenced object's address. The weakref's hash is protected using the
weakref's per-object lock.
 
Note that this only affects free-threaded builds. Apart from some minor
refactoring, the added code is all either gated by `ifdef`s or is a no-op
(e.g. `Py_BEGIN_CRITICAL_SECTION`).
This commit is contained in:
mpage 2024-04-08 07:58:38 -07:00 committed by GitHub
parent e16062dd34
commit df73179048
No known key found for this signature in database
GPG Key ID: B5690EEEBB952194
17 changed files with 490 additions and 326 deletions
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8
Include/cpython/weakrefobject.h
View File

@ -30,6 +30,14 @@ struct _PyWeakReference {
PyWeakReference *wr_prev;
PyWeakReference *wr_next;
vectorcallfunc vectorcall;
#ifdef Py_GIL_DISABLED
/* Pointer to the lock used when clearing in free-threaded builds.
* Normally this can be derived from wr_object, but in some cases we need
* to lock after wr_object has been set to Py_None.
*/
struct _PyMutex *weakrefs_lock;
#endif
};
Py_DEPRECATED(3.13) static inline PyObject* PyWeakref_GET_OBJECT(PyObject *ref_obj)

7
Include/internal/pycore_interp.h
View File

@ -59,6 +59,12 @@ struct _stoptheworld_state {
PyThreadState *requester; // Thread that requested the pause (may be NULL).
};
#ifdef Py_GIL_DISABLED
// This should be prime but otherwise the choice is arbitrary. A larger value
// increases concurrency at the expense of memory.
# define NUM_WEAKREF_LIST_LOCKS 127
#endif
/* cross-interpreter data registry */
/* Tracks some rare events per-interpreter, used by the optimizer to turn on/off
@ -203,6 +209,7 @@ struct _is {
#if defined(Py_GIL_DISABLED)
struct _mimalloc_interp_state mimalloc;
struct _brc_state brc; // biased reference counting state
PyMutex weakref_locks[NUM_WEAKREF_LIST_LOCKS];
#endif
// Per-interpreter state for the obmalloc allocator. For the main

40
Include/internal/pycore_object.h
View File

@ -426,7 +426,7 @@ _Py_TryIncRefShared(PyObject *op)
/* Tries to incref the object op and ensures that *src still points to it. */
static inline int
_Py_TryIncref(PyObject **src, PyObject *op)
_Py_TryIncrefCompare(PyObject **src, PyObject *op)
{
if (_Py_TryIncrefFast(op)) {
return 1;
@ -452,7 +452,7 @@ _Py_XGetRef(PyObject **ptr)
if (value == NULL) {
return value;
}
if (_Py_TryIncref(ptr, value)) {
if (_Py_TryIncrefCompare(ptr, value)) {
return value;
}
}
@ -467,7 +467,7 @@ _Py_TryXGetRef(PyObject **ptr)
if (value == NULL) {
return value;
}
if (_Py_TryIncref(ptr, value)) {
if (_Py_TryIncrefCompare(ptr, value)) {
return value;
}
return NULL;
@ -506,8 +506,42 @@ _Py_XNewRefWithLock(PyObject *obj)
return _Py_NewRefWithLock(obj);
}
static inline void
_PyObject_SetMaybeWeakref(PyObject *op)
{
if (_Py_IsImmortal(op)) {
return;
}
for (;;) {
Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&op->ob_ref_shared);
if ((shared & _Py_REF_SHARED_FLAG_MASK) != 0) {
// Nothing to do if it's in WEAKREFS, QUEUED, or MERGED states.
return;
}
if (_Py_atomic_compare_exchange_ssize(
&op->ob_ref_shared, &shared, shared | _Py_REF_MAYBE_WEAKREF)) {
return;
}
}
}
#endif
/* Tries to incref op and returns 1 if successful or 0 otherwise. */
static inline int
_Py_TryIncref(PyObject *op)
{
#ifdef Py_GIL_DISABLED
return _Py_TryIncrefFast(op) || _Py_TryIncRefShared(op);
#else
if (Py_REFCNT(op) > 0) {
Py_INCREF(op);
return 1;
}
return 0;
#endif
}
#ifdef Py_REF_DEBUG
extern void _PyInterpreterState_FinalizeRefTotal(PyInterpreterState *);
extern void _Py_FinalizeRefTotal(_PyRuntimeState *);

5
Include/internal/pycore_pyatomic_ft_wrappers.h
View File

@ -20,9 +20,12 @@ extern "C" {
#endif
#ifdef Py_GIL_DISABLED
#define FT_ATOMIC_LOAD_PTR(value) _Py_atomic_load_ptr(&value)
#define FT_ATOMIC_LOAD_SSIZE(value) _Py_atomic_load_ssize(&value)
#define FT_ATOMIC_LOAD_SSIZE_RELAXED(value) \
_Py_atomic_load_ssize_relaxed(&value)
#define FT_ATOMIC_STORE_PTR(value, new_value) \
_Py_atomic_store_ptr(&value, new_value)
#define FT_ATOMIC_STORE_PTR_RELAXED(value, new_value) \
_Py_atomic_store_ptr_relaxed(&value, new_value)
#define FT_ATOMIC_STORE_PTR_RELEASE(value, new_value) \
@ -30,8 +33,10 @@ extern "C" {
#define FT_ATOMIC_STORE_SSIZE_RELAXED(value, new_value) \
_Py_atomic_store_ssize_relaxed(&value, new_value)
#else
#define FT_ATOMIC_LOAD_PTR(value) value
#define FT_ATOMIC_LOAD_SSIZE(value) value
#define FT_ATOMIC_LOAD_SSIZE_RELAXED(value) value
#define FT_ATOMIC_STORE_PTR(value, new_value) value = new_value
#define FT_ATOMIC_STORE_PTR_RELAXED(value, new_value) value = new_value
#define FT_ATOMIC_STORE_PTR_RELEASE(value, new_value) value = new_value
#define FT_ATOMIC_STORE_SSIZE_RELAXED(value, new_value) value = new_value

73
Include/internal/pycore_weakref.h
View File

@ -9,7 +9,35 @@ extern "C" {
#endif
#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION()
#include "pycore_lock.h"
#include "pycore_object.h" // _Py_REF_IS_MERGED()
#include "pycore_pyatomic_ft_wrappers.h"
#ifdef Py_GIL_DISABLED
#define WEAKREF_LIST_LOCK(obj) \
_PyInterpreterState_GET() \
->weakref_locks[((uintptr_t)obj) % NUM_WEAKREF_LIST_LOCKS]
// Lock using the referenced object
#define LOCK_WEAKREFS(obj) \
PyMutex_LockFlags(&WEAKREF_LIST_LOCK(obj), _Py_LOCK_DONT_DETACH)
#define UNLOCK_WEAKREFS(obj) PyMutex_Unlock(&WEAKREF_LIST_LOCK(obj))
// Lock using a weakref
#define LOCK_WEAKREFS_FOR_WR(wr) \
PyMutex_LockFlags(wr->weakrefs_lock, _Py_LOCK_DONT_DETACH)
#define UNLOCK_WEAKREFS_FOR_WR(wr) PyMutex_Unlock(wr->weakrefs_lock)
#else
#define LOCK_WEAKREFS(obj)
#define UNLOCK_WEAKREFS(obj)
#define LOCK_WEAKREFS_FOR_WR(wr)
#define UNLOCK_WEAKREFS_FOR_WR(wr)
#endif
static inline int _is_dead(PyObject *obj)
{
@ -30,53 +58,64 @@ static inline int _is_dead(PyObject *obj)
static inline PyObject* _PyWeakref_GET_REF(PyObject *ref_obj)
{
assert(PyWeakref_Check(ref_obj));
PyObject *ret = NULL;
Py_BEGIN_CRITICAL_SECTION(ref_obj);
PyWeakReference *ref = _Py_CAST(PyWeakReference*, ref_obj);
PyObject *obj = ref->wr_object;
PyObject *obj = FT_ATOMIC_LOAD_PTR(ref->wr_object);
if (obj == Py_None) {
// clear_weakref() was called
goto end;
return NULL;
}
if (_is_dead(obj)) {
goto end;
LOCK_WEAKREFS(obj);
#ifdef Py_GIL_DISABLED
if (ref->wr_object == Py_None) {
// clear_weakref() was called
UNLOCK_WEAKREFS(obj);
return NULL;
}
#if !defined(Py_GIL_DISABLED)
assert(Py_REFCNT(obj) > 0);
#endif
ret = Py_NewRef(obj);
end:
Py_END_CRITICAL_SECTION();
return ret;
if (_Py_TryIncref(obj)) {
UNLOCK_WEAKREFS(obj);
return obj;
}
UNLOCK_WEAKREFS(obj);
return NULL;
}
static inline int _PyWeakref_IS_DEAD(PyObject *ref_obj)
{
assert(PyWeakref_Check(ref_obj));
int ret = 0;
Py_BEGIN_CRITICAL_SECTION(ref_obj);
PyWeakReference *ref = _Py_CAST(PyWeakReference*, ref_obj);
PyObject *obj = ref->wr_object;
PyObject *obj = FT_ATOMIC_LOAD_PTR(ref->wr_object);
if (obj == Py_None) {
// clear_weakref() was called
ret = 1;
}
else {
LOCK_WEAKREFS(obj);
// See _PyWeakref_GET_REF() for the rationale of this test
#ifdef Py_GIL_DISABLED
ret = (ref->wr_object == Py_None) || _is_dead(obj);
#else
ret = _is_dead(obj);
#endif
UNLOCK_WEAKREFS(obj);
}
Py_END_CRITICAL_SECTION();
return ret;
}
extern Py_ssize_t _PyWeakref_GetWeakrefCount(PyWeakReference *head);
extern Py_ssize_t _PyWeakref_GetWeakrefCount(PyObject *obj);
// Clear all the weak references to obj but leave their callbacks uncalled and
// intact.
extern void _PyWeakref_ClearWeakRefsExceptCallbacks(PyObject *obj);
extern void _PyWeakref_ClearRef(PyWeakReference *self);
PyAPI_FUNC(int) _PyWeakref_IsDead(PyObject *weakref);
#ifdef __cplusplus
}
#endif
#endif /* !Py_INTERNAL_WEAKREF_H */

8
Lib/test/test_sys.py
View File

@ -1708,11 +1708,15 @@ class SizeofTest(unittest.TestCase):
# TODO: add check that forces layout of unicodefields
# weakref
import weakref
check(weakref.ref(int), size('2Pn3P'))
if support.Py_GIL_DISABLED:
expected = size('2Pn4P')
else:
expected = size('2Pn3P')
check(weakref.ref(int), expected)
# weakproxy
# XXX
# weakcallableproxy
check(weakref.proxy(int), size('2Pn3P'))
check(weakref.proxy(int), expected)
def check_slots(self, obj, base, extra):
expected = sys.getsizeof(base) + struct.calcsize(extra)

19
Lib/test/test_weakref.py
View File

@ -1907,6 +1907,25 @@ class MappingTestCase(TestBase):
self.assertEqual(len(d), 1)
o = None # lose ref
@support.cpython_only
def test_weak_valued_consistency(self):
# A single-threaded, deterministic repro for issue #28427: old keys
# should not remove new values from WeakValueDictionary. This relies on
# an implementation detail of CPython's WeakValueDictionary (its
# underlying dictionary of KeyedRefs) to reproduce the issue.
d = weakref.WeakValueDictionary()
with support.disable_gc():
d[10] = RefCycle()
# Keep the KeyedRef alive after it's replaced so that GC will invoke
# the callback.
wr = d.data[10]
# Replace the value with something that isn't cyclic garbage
o = RefCycle()
d[10] = o
# Trigger GC, which will invoke the callback for `wr`
gc.collect()
self.assertEqual(len(d), 1)
def check_threaded_weak_dict_copy(self, type_, deepcopy):
# `type_` should be either WeakKeyDictionary or WeakValueDictionary.
# `deepcopy` should be either True or False.

5
Modules/_sqlite/blob.c
View File

@ -4,7 +4,6 @@
#include "blob.h"
#include "util.h"
#include "pycore_weakref.h" // _PyWeakref_GET_REF()
#define clinic_state() (pysqlite_get_state_by_type(Py_TYPE(self)))
#include "clinic/blob.c.h"
@ -102,8 +101,8 @@ pysqlite_close_all_blobs(pysqlite_Connection *self)
{
for (int i = 0; i < PyList_GET_SIZE(self->blobs); i++) {
PyObject *weakref = PyList_GET_ITEM(self->blobs, i);
PyObject *blob = _PyWeakref_GET_REF(weakref);
if (blob == NULL) {
PyObject *blob;
if (!PyWeakref_GetRef(weakref, &blob)) {
continue;
}
close_blob((pysqlite_Blob *)blob);

4
Modules/_sqlite/connection.c
View File

@ -38,7 +38,7 @@
#include "pycore_modsupport.h" // _PyArg_NoKeywords()
#include "pycore_pyerrors.h" // _PyErr_ChainExceptions1()
#include "pycore_pylifecycle.h" // _Py_IsInterpreterFinalizing()
#include "pycore_weakref.h" // _PyWeakref_IS_DEAD()
#include "pycore_weakref.h"
#include <stdbool.h>
@ -1065,7 +1065,7 @@ static void _pysqlite_drop_unused_cursor_references(pysqlite_Connection* self)
for (Py_ssize_t i = 0; i < PyList_Size(self->cursors); i++) {
PyObject* weakref = PyList_GetItem(self->cursors, i);
if (_PyWeakref_IS_DEAD(weakref)) {
if (_PyWeakref_IsDead(weakref)) {
continue;
}
if (PyList_Append(new_list, weakref) != 0) {

13
Modules/_ssl.c
View File

@ -29,7 +29,6 @@
#include "pycore_fileutils.h" // _PyIsSelectable_fd()
#include "pycore_pyerrors.h" // _PyErr_ChainExceptions1()
#include "pycore_time.h" // _PyDeadline_Init()
#include "pycore_weakref.h" // _PyWeakref_GET_REF()
/* Include symbols from _socket module */
#include "socketmodule.h"
@ -392,8 +391,8 @@ typedef enum {
// Return a borrowed reference.
static inline PySocketSockObject* GET_SOCKET(PySSLSocket *obj) {
if (obj->Socket) {
PyObject *sock = _PyWeakref_GET_REF(obj->Socket);
if (sock != NULL) {
PyObject *sock;
if (PyWeakref_GetRef(obj->Socket, &sock)) {
// GET_SOCKET() returns a borrowed reference
Py_DECREF(sock);
}
@ -2205,8 +2204,8 @@ PySSL_get_owner(PySSLSocket *self, void *c)
if (self->owner == NULL) {
Py_RETURN_NONE;
}
PyObject *owner = _PyWeakref_GET_REF(self->owner);
if (owner == NULL) {
PyObject *owner;
if (!PyWeakref_GetRef(self->owner, &owner)) {
Py_RETURN_NONE;
}
return owner;
@ -4433,9 +4432,9 @@ _servername_callback(SSL *s, int *al, void *args)
* will be passed. If both do not exist only then the C-level object is
* passed. */
if (ssl->owner)
ssl_socket = _PyWeakref_GET_REF(ssl->owner);
PyWeakref_GetRef(ssl->owner, &ssl_socket);
else if (ssl->Socket)
ssl_socket = _PyWeakref_GET_REF(ssl->Socket);
PyWeakref_GetRef(ssl->Socket, &ssl_socket);
else
ssl_socket = Py_NewRef(ssl);

6
Modules/_ssl/debughelpers.c
View File

@ -28,12 +28,12 @@ _PySSL_msg_callback(int write_p, int version, int content_type,
PyObject *ssl_socket; /* ssl.SSLSocket or ssl.SSLObject */
if (ssl_obj->owner)
ssl_socket = _PyWeakref_GET_REF(ssl_obj->owner);
PyWeakref_GetRef(ssl_obj->owner, &ssl_socket);
else if (ssl_obj->Socket)
ssl_socket = _PyWeakref_GET_REF(ssl_obj->Socket);
PyWeakref_GetRef(ssl_obj->Socket, &ssl_socket);
else
ssl_socket = (PyObject *)Py_NewRef(ssl_obj);
assert(ssl_socket != NULL); // _PyWeakref_GET_REF() can return NULL
assert(ssl_socket != NULL); // PyWeakref_GetRef() can return NULL
/* assume that OpenSSL verifies all payload and buf len is of sufficient
length */

42
Modules/_weakref.c
View File

@ -14,7 +14,6 @@ module _weakref
#include "clinic/_weakref.c.h"
/*[clinic input]
@critical_section object
_weakref.getweakrefcount -> Py_ssize_t
object: object
@ -25,14 +24,9 @@ Return the number of weak references to 'object'.
static Py_ssize_t
_weakref_getweakrefcount_impl(PyObject *module, PyObject *object)
/*[clinic end generated code: output=301806d59558ff3e input=6535a580f1d0ebdc]*/
/*[clinic end generated code: output=301806d59558ff3e input=7d4d04fcaccf64d5]*/
{
if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(object))) {
return 0;
}
PyWeakReference **list = GET_WEAKREFS_LISTPTR(object);
Py_ssize_t count = _PyWeakref_GetWeakrefCount(*list);
return count;
return _PyWeakref_GetWeakrefCount(object);
}
@ -77,7 +71,6 @@ _weakref__remove_dead_weakref_impl(PyObject *module, PyObject *dct,
/*[clinic input]
@critical_section object
_weakref.getweakrefs
object: object
/
@ -86,26 +79,39 @@ Return a list of all weak reference objects pointing to 'object'.
[clinic start generated code]*/
static PyObject *
_weakref_getweakrefs_impl(PyObject *module, PyObject *object)
/*[clinic end generated code: output=5ec268989fb8f035 input=3dea95b8f5b31bbb]*/
_weakref_getweakrefs(PyObject *module, PyObject *object)
/*[clinic end generated code: output=25c7731d8e011824 input=00c6d0e5d3206693]*/
{
if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(object))) {
return PyList_New(0);
}
PyWeakReference **list = GET_WEAKREFS_LISTPTR(object);
Py_ssize_t count = _PyWeakref_GetWeakrefCount(*list);
PyObject *result = PyList_New(count);
PyObject *result = PyList_New(0);
if (result == NULL) {
return NULL;
}
PyWeakReference *current = *list;
for (Py_ssize_t i = 0; i < count; ++i) {
PyList_SET_ITEM(result, i, Py_NewRef(current));
LOCK_WEAKREFS(object);
PyWeakReference *current = *GET_WEAKREFS_LISTPTR(object);
while (current != NULL) {
PyObject *curobj = (PyObject *) current;
if (_Py_TryIncref(curobj)) {
if (PyList_Append(result, curobj)) {
UNLOCK_WEAKREFS(object);
Py_DECREF(curobj);
Py_DECREF(result);
return NULL;
}
else {
// Undo our _Py_TryIncref. This is safe to do with the lock
// held in free-threaded builds; the list holds a reference to
// curobj so we're guaranteed not to invoke the destructor.
Py_DECREF(curobj);
}
}
current = current->wr_next;
}
UNLOCK_WEAKREFS(object);
return result;
}

20
Modules/clinic/_weakref.c.h generated
View File

@ -2,7 +2,6 @@
preserve
[clinic start generated code]*/
#include "pycore_critical_section.h"// Py_BEGIN_CRITICAL_SECTION()
#include "pycore_modsupport.h" // _PyArg_CheckPositional()
PyDoc_STRVAR(_weakref_getweakrefcount__doc__,
@ -23,9 +22,7 @@ _weakref_getweakrefcount(PyObject *module, PyObject *object)
PyObject *return_value = NULL;
Py_ssize_t _return_value;
Py_BEGIN_CRITICAL_SECTION(object);
_return_value = _weakref_getweakrefcount_impl(module, object);
Py_END_CRITICAL_SECTION();
if ((_return_value == -1) && PyErr_Occurred()) {
goto exit;
}
@ -79,21 +76,6 @@ PyDoc_STRVAR(_weakref_getweakrefs__doc__,
#define _WEAKREF_GETWEAKREFS_METHODDEF \
{"getweakrefs", (PyCFunction)_weakref_getweakrefs, METH_O, _weakref_getweakrefs__doc__},
static PyObject *
_weakref_getweakrefs_impl(PyObject *module, PyObject *object);
static PyObject *
_weakref_getweakrefs(PyObject *module, PyObject *object)
{
PyObject *return_value = NULL;
Py_BEGIN_CRITICAL_SECTION(object);
return_value = _weakref_getweakrefs_impl(module, object);
Py_END_CRITICAL_SECTION();
return return_value;
}
PyDoc_STRVAR(_weakref_proxy__doc__,
"proxy($module, object, callback=None, /)\n"
"--\n"
@ -130,4 +112,4 @@ skip_optional:
exit:
return return_value;
}
/*[clinic end generated code: output=d5d30707212a9870 input=a9049054013a1b77]*/
/*[clinic end generated code: output=60f59adc1dc9eab8 input=a9049054013a1b77]*/

8
Objects/dictobject.c
View File

@ -1286,7 +1286,7 @@ Py_ssize_t compare_unicode_generic_threadsafe(PyDictObject *mp, PyDictKeysObject
assert(!PyUnicode_CheckExact(key));
if (startkey != NULL) {
if (!_Py_TryIncref(&ep->me_key, startkey)) {
if (!_Py_TryIncrefCompare(&ep->me_key, startkey)) {
return DKIX_KEY_CHANGED;
}
@ -1334,7 +1334,7 @@ compare_unicode_unicode_threadsafe(PyDictObject *mp, PyDictKeysObject *dk,
return unicode_get_hash(startkey) == hash && unicode_eq(startkey, key);
}
else {
if (!_Py_TryIncref(&ep->me_key, startkey)) {
if (!_Py_TryIncrefCompare(&ep->me_key, startkey)) {
return DKIX_KEY_CHANGED;
}
if (unicode_get_hash(startkey) == hash && unicode_eq(startkey, key)) {
@ -1364,7 +1364,7 @@ Py_ssize_t compare_generic_threadsafe(PyDictObject *mp, PyDictKeysObject *dk,
}
Py_ssize_t ep_hash = _Py_atomic_load_ssize_relaxed(&ep->me_hash);
if (ep_hash == hash) {
if (startkey == NULL || !_Py_TryIncref(&ep->me_key, startkey)) {
if (startkey == NULL || !_Py_TryIncrefCompare(&ep->me_key, startkey)) {
return DKIX_KEY_CHANGED;
}
int cmp = PyObject_RichCompareBool(startkey, key, Py_EQ);
@ -5308,7 +5308,7 @@ acquire_key_value(PyObject **key_loc, PyObject *value, PyObject **value_loc,
}
if (out_value) {
if (!_Py_TryIncref(value_loc, value)) {
if (!_Py_TryIncrefCompare(value_loc, value)) {
if (out_key) {
Py_DECREF(*out_key);
}

12
Objects/typeobject.c
View File

@ -378,7 +378,7 @@ _PyType_GetMRO(PyTypeObject *self)
if (mro == NULL) {
return NULL;
}
if (_Py_TryIncref(&self->tp_mro, mro)) {
if (_Py_TryIncrefCompare(&self->tp_mro, mro)) {
return mro;
}
@ -2193,15 +2193,7 @@ subtype_dealloc(PyObject *self)
finalizers since they might rely on part of the object
being finalized that has already been destroyed. */
if (type->tp_weaklistoffset && !base->tp_weaklistoffset) {
/* Modeled after GET_WEAKREFS_LISTPTR().
This is never triggered for static types so we can avoid the
(slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR(). */
PyWeakReference **list = \
_PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(self);
while (*list) {
_PyWeakref_ClearRef(*list);
}
_PyWeakref_ClearWeakRefsExceptCallbacks(self);
}
}

517
Objects/weakrefobject.c
View File

@ -1,24 +1,58 @@
#include "Python.h"
#include "pycore_critical_section.h"
#include "pycore_lock.h"
#include "pycore_modsupport.h" // _PyArg_NoKwnames()
#include "pycore_object.h" // _PyObject_GET_WEAKREFS_LISTPTR()
#include "pycore_pyerrors.h" // _PyErr_ChainExceptions1()
#include "pycore_pystate.h"
#include "pycore_weakref.h" // _PyWeakref_GET_REF()
#ifdef Py_GIL_DISABLED
/*
* Thread-safety for free-threaded builds
* ======================================
*
* In free-threaded builds we need to protect mutable state of:
*
* - The weakref (wr_object, hash, wr_callback)
* - The referenced object (its head-of-list pointer)
* - The linked list of weakrefs
*
* For now we've chosen to address this in a straightforward way:
*
* - The weakref's hash is protected using the weakref's per-object lock.
* - The other mutable is protected by a striped lock keyed on the referenced
* object's address.
* - The striped lock must be locked using `_Py_LOCK_DONT_DETACH` in order to
* support atomic deletion from WeakValueDictionaries. As a result, we must
* be careful not to perform any operations that could suspend while the
* lock is held.
*
* Since the world is stopped when the GC runs, it is free to clear weakrefs
* without acquiring any locks.
*/
#endif
#define GET_WEAKREFS_LISTPTR(o) \
((PyWeakReference **) _PyObject_GET_WEAKREFS_LISTPTR(o))
Py_ssize_t
_PyWeakref_GetWeakrefCount(PyWeakReference *head)
_PyWeakref_GetWeakrefCount(PyObject *obj)
{
Py_ssize_t count = 0;
if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(obj))) {
return 0;
}
LOCK_WEAKREFS(obj);
Py_ssize_t count = 0;
PyWeakReference *head = *GET_WEAKREFS_LISTPTR(obj);
while (head != NULL) {
++count;
head = head->wr_next;
}
UNLOCK_WEAKREFS(obj);
return count;
}
@ -33,54 +67,55 @@ init_weakref(PyWeakReference *self, PyObject *ob, PyObject *callback)
self->wr_next = NULL;
self->wr_callback = Py_XNewRef(callback);
self->vectorcall = weakref_vectorcall;
#ifdef Py_GIL_DISABLED
self->weakrefs_lock = &WEAKREF_LIST_LOCK(ob);
_PyObject_SetMaybeWeakref(ob);
_PyObject_SetMaybeWeakref((PyObject *)self);
#endif
}
static PyWeakReference *
new_weakref(PyObject *ob, PyObject *callback)
{
PyWeakReference *result;
result = PyObject_GC_New(PyWeakReference, &_PyWeakref_RefType);
if (result) {
init_weakref(result, ob, callback);
PyObject_GC_Track(result);
}
return result;
}
/* This function clears the passed-in reference and removes it from the
* list of weak references for the referent. This is the only code that
* removes an item from the doubly-linked list of weak references for an
* object; it is also responsible for clearing the callback slot.
*/
// Clear the weakref and steal its callback into `callback`, if provided.
static void
clear_weakref(PyWeakReference *self)
clear_weakref_lock_held(PyWeakReference *self, PyObject **callback)
{
PyObject *callback = self->wr_callback;
if (self->wr_object != Py_None) {
PyWeakReference **list = GET_WEAKREFS_LISTPTR(self->wr_object);
if (*list == self)
/* If 'self' is the end of the list (and thus self->wr_next == NULL)
then the weakref list itself (and thus the value of *list) will
end up being set to NULL. */
*list = self->wr_next;
self->wr_object = Py_None;
if (self->wr_prev != NULL)
if (*list == self) {
/* If 'self' is the end of the list (and thus self->wr_next ==
NULL) then the weakref list itself (and thus the value of *list)
will end up being set to NULL. */
FT_ATOMIC_STORE_PTR(*list, self->wr_next);
}
FT_ATOMIC_STORE_PTR(self->wr_object, Py_None);
if (self->wr_prev != NULL) {
self->wr_prev->wr_next = self->wr_next;
if (self->wr_next != NULL)
}
if (self->wr_next != NULL) {
self->wr_next->wr_prev = self->wr_prev;
}
self->wr_prev = NULL;
self->wr_next = NULL;
}
if (callback != NULL) {
Py_DECREF(callback);
*callback = self->wr_callback;
self->wr_callback = NULL;
}
}
// Clear the weakref and its callback
static void
clear_weakref(PyWeakReference *self)
{
PyObject *callback = NULL;
// self->wr_object may be Py_None if the GC cleared the weakref, so lock
// using the pointer in the weakref.
LOCK_WEAKREFS_FOR_WR(self);
clear_weakref_lock_held(self, &callback);
UNLOCK_WEAKREFS_FOR_WR(self);
Py_XDECREF(callback);
}
/* Cyclic gc uses this to *just* clear the passed-in reference, leaving
* the callback intact and uncalled. It must be possible to call self's
* tp_dealloc() after calling this, so self has to be left in a sane enough
@ -95,15 +130,9 @@ clear_weakref(PyWeakReference *self)
void
_PyWeakref_ClearRef(PyWeakReference *self)
{
PyObject *callback;
assert(self != NULL);
assert(PyWeakref_Check(self));
/* Preserve and restore the callback around clear_weakref. */
callback = self->wr_callback;
self->wr_callback = NULL;
clear_weakref(self);
self->wr_callback = callback;
clear_weakref_lock_held(self, NULL);
}
static void
@ -126,7 +155,11 @@ gc_traverse(PyWeakReference *self, visitproc visit, void *arg)
static int
gc_clear(PyWeakReference *self)
{
clear_weakref(self);
PyObject *callback;
// The world is stopped during GC in free-threaded builds. It's safe to
// call this without holding the lock.
clear_weakref_lock_held(self, &callback);
Py_XDECREF(callback);
return 0;
}
@ -150,7 +183,7 @@ weakref_vectorcall(PyObject *self, PyObject *const *args,
}
static Py_hash_t
weakref_hash(PyWeakReference *self)
weakref_hash_lock_held(PyWeakReference *self)
{
if (self->hash != -1)
return self->hash;
@ -164,6 +197,15 @@ weakref_hash(PyWeakReference *self)
return self->hash;
}
static Py_hash_t
weakref_hash(PyWeakReference *self)
{
Py_hash_t hash;
Py_BEGIN_CRITICAL_SECTION(self);
hash = weakref_hash_lock_held(self);
Py_END_CRITICAL_SECTION();
return hash;
}
static PyObject *
weakref_repr(PyObject *self)
@ -276,6 +318,128 @@ insert_head(PyWeakReference *newref, PyWeakReference **list)
*list = newref;
}
/* See if we can reuse either the basic ref or proxy in list instead of
* creating a new weakref
*/
static PyWeakReference *
try_reuse_basic_ref(PyWeakReference *list, PyTypeObject *type,
PyObject *callback)
{
if (callback != NULL) {
return NULL;
}
PyWeakReference *ref, *proxy;
get_basic_refs(list, &ref, &proxy);
PyWeakReference *cand = NULL;
if (type == &_PyWeakref_RefType) {
cand = ref;
}
if ((type == &_PyWeakref_ProxyType) ||
(type == &_PyWeakref_CallableProxyType)) {
cand = proxy;
}
if (cand != NULL && _Py_TryIncref((PyObject *) cand)) {
return cand;
}
return NULL;
}
static int
is_basic_ref(PyWeakReference *ref)
{
return (ref->wr_callback == NULL) && PyWeakref_CheckRefExact(ref);
}
static int
is_basic_proxy(PyWeakReference *proxy)
{
return (proxy->wr_callback == NULL) && PyWeakref_CheckProxy(proxy);
}
static int
is_basic_ref_or_proxy(PyWeakReference *wr)
{
return is_basic_ref(wr) || is_basic_proxy(wr);
}
/* Insert `newref` in the appropriate position in `list` */
static void
insert_weakref(PyWeakReference *newref, PyWeakReference **list)
{
PyWeakReference *ref, *proxy;
get_basic_refs(*list, &ref, &proxy);
PyWeakReference *prev;
if (is_basic_ref(newref)) {
prev = NULL;
}
else if (is_basic_proxy(newref)) {
prev = ref;
}
else {
prev = (proxy == NULL) ? ref : proxy;
}
if (prev == NULL) {
insert_head(newref, list);
}
else {
insert_after(newref, prev);
}
}
static PyWeakReference *
allocate_weakref(PyTypeObject *type, PyObject *obj, PyObject *callback)
{
PyWeakReference *newref = (PyWeakReference *) type->tp_alloc(type, 0);
if (newref == NULL) {
return NULL;
}
init_weakref(newref, obj, callback);
return newref;
}
static PyWeakReference *
get_or_create_weakref(PyTypeObject *type, PyObject *obj, PyObject *callback)
{
if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(obj))) {
PyErr_Format(PyExc_TypeError,
"cannot create weak reference to '%s' object",
Py_TYPE(obj)->tp_name);
return NULL;
}
if (callback == Py_None)
callback = NULL;
PyWeakReference **list = GET_WEAKREFS_LISTPTR(obj);
if ((type == &_PyWeakref_RefType) ||
(type == &_PyWeakref_ProxyType) ||
(type == &_PyWeakref_CallableProxyType))
{
LOCK_WEAKREFS(obj);
PyWeakReference *basic_ref = try_reuse_basic_ref(*list, type, callback);
if (basic_ref != NULL) {
UNLOCK_WEAKREFS(obj);
return basic_ref;
}
PyWeakReference *newref = allocate_weakref(type, obj, callback);
insert_weakref(newref, list);
UNLOCK_WEAKREFS(obj);
return newref;
}
else {
// We may not be able to safely allocate inside the lock
PyWeakReference *newref = allocate_weakref(type, obj, callback);
LOCK_WEAKREFS(obj);
insert_weakref(newref, list);
UNLOCK_WEAKREFS(obj);
return newref;
}
}
static int
parse_weakref_init_args(const char *funcname, PyObject *args, PyObject *kwargs,
PyObject **obp, PyObject **callbackp)
@ -286,54 +450,11 @@ parse_weakref_init_args(const char *funcname, PyObject *args, PyObject *kwargs,
static PyObject *
weakref___new__(PyTypeObject *type, PyObject *args, PyObject *kwargs)
{
PyWeakReference *self = NULL;
PyObject *ob, *callback = NULL;
if (parse_weakref_init_args("__new__", args, kwargs, &ob, &callback)) {
PyWeakReference *ref, *proxy;
PyWeakReference **list;
if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(ob))) {
PyErr_Format(PyExc_TypeError,
"cannot create weak reference to '%s' object",
Py_TYPE(ob)->tp_name);
return (PyObject *)get_or_create_weakref(type, ob, callback);
}
return NULL;
}
if (callback == Py_None)
callback = NULL;
list = GET_WEAKREFS_LISTPTR(ob);
get_basic_refs(*list, &ref, &proxy);
if (callback == NULL && type == &_PyWeakref_RefType) {
if (ref != NULL) {
/* We can re-use an existing reference. */
return Py_NewRef(ref);
}
}
/* We have to create a new reference. */
/* Note: the tp_alloc() can trigger cyclic GC, so the weakref
list on ob can be mutated. This means that the ref and
proxy pointers we got back earlier may have been collected,
so we need to compute these values again before we use
them. */
self = (PyWeakReference *) (type->tp_alloc(type, 0));
if (self != NULL) {
init_weakref(self, ob, callback);
if (callback == NULL && type == &_PyWeakref_RefType) {
insert_head(self, list);
}
else {
PyWeakReference *prev;
get_basic_refs(*list, &ref, &proxy);
prev = (proxy == NULL) ? ref : proxy;
if (prev == NULL)
insert_head(self, list);
else
insert_after(self, prev);
}
}
}
return (PyObject *)self;
}
static int
@ -562,8 +683,6 @@ static void
proxy_dealloc(PyWeakReference *self)
{
PyObject_GC_UnTrack(self);
if (self->wr_callback != NULL)
PyObject_GC_UnTrack((PyObject *)self);
clear_weakref(self);
PyObject_GC_Del(self);
}
@ -784,104 +903,21 @@ _PyWeakref_CallableProxyType = {
proxy_iternext, /* tp_iternext */
};
PyObject *
PyWeakref_NewRef(PyObject *ob, PyObject *callback)
{
PyWeakReference *result = NULL;
PyWeakReference **list;
PyWeakReference *ref, *proxy;
if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(ob))) {
PyErr_Format(PyExc_TypeError,
"cannot create weak reference to '%s' object",
Py_TYPE(ob)->tp_name);
return NULL;
}
list = GET_WEAKREFS_LISTPTR(ob);
get_basic_refs(*list, &ref, &proxy);
if (callback == Py_None)
callback = NULL;
if (callback == NULL)
/* return existing weak reference if it exists */
result = ref;
if (result != NULL)
Py_INCREF(result);
else {
/* We do not need to recompute ref/proxy; new_weakref() cannot
trigger GC.
*/
result = new_weakref(ob, callback);
if (result != NULL) {
if (callback == NULL) {
assert(ref == NULL);
insert_head(result, list);
}
else {
PyWeakReference *prev;
prev = (proxy == NULL) ? ref : proxy;
if (prev == NULL)
insert_head(result, list);
else
insert_after(result, prev);
}
}
}
return (PyObject *) result;
return (PyObject *)get_or_create_weakref(&_PyWeakref_RefType, ob,
callback);
}
PyObject *
PyWeakref_NewProxy(PyObject *ob, PyObject *callback)
{
PyWeakReference *result = NULL;
PyWeakReference **list;
PyWeakReference *ref, *proxy;
if (!_PyType_SUPPORTS_WEAKREFS(Py_TYPE(ob))) {
PyErr_Format(PyExc_TypeError,
"cannot create weak reference to '%s' object",
Py_TYPE(ob)->tp_name);
return NULL;
}
list = GET_WEAKREFS_LISTPTR(ob);
get_basic_refs(*list, &ref, &proxy);
if (callback == Py_None)
callback = NULL;
if (callback == NULL)
/* attempt to return an existing weak reference if it exists */
result = proxy;
if (result != NULL)
Py_INCREF(result);
else {
/* We do not need to recompute ref/proxy; new_weakref cannot
trigger GC.
*/
result = new_weakref(ob, callback);
if (result != NULL) {
PyWeakReference *prev;
PyTypeObject *type = &_PyWeakref_ProxyType;
if (PyCallable_Check(ob)) {
Py_SET_TYPE(result, &_PyWeakref_CallableProxyType);
type = &_PyWeakref_CallableProxyType;
}
else {
Py_SET_TYPE(result, &_PyWeakref_ProxyType);
}
if (callback == NULL) {
prev = ref;
}
else
prev = (proxy == NULL) ? ref : proxy;
if (prev == NULL)
insert_head(result, list);
else
insert_after(result, prev);
}
}
return (PyObject *) result;
return (PyObject *)get_or_create_weakref(type, ob, callback);
}
@ -950,68 +986,73 @@ PyObject_ClearWeakRefs(PyObject *object)
PyErr_BadInternalCall();
return;
}
list = GET_WEAKREFS_LISTPTR(object);
/* Remove the callback-less basic and proxy references */
if (*list != NULL && (*list)->wr_callback == NULL) {
clear_weakref(*list);
if (*list != NULL && (*list)->wr_callback == NULL)
clear_weakref(*list);
if (FT_ATOMIC_LOAD_PTR(list) == NULL) {
// Fast path for the common case
return;
}
if (*list != NULL) {
PyWeakReference *current = *list;
Py_ssize_t count = _PyWeakref_GetWeakrefCount(current);
/* Remove the callback-less basic and proxy references, which always appear
at the head of the list.
*/
for (int done = 0; !done;) {
LOCK_WEAKREFS(object);
if (*list != NULL && is_basic_ref_or_proxy(*list)) {
PyObject *callback;
clear_weakref_lock_held(*list, &callback);
assert(callback == NULL);
}
done = (*list == NULL) || !is_basic_ref_or_proxy(*list);
UNLOCK_WEAKREFS(object);
}
/* Deal with non-canonical (subtypes or refs with callbacks) references. */
Py_ssize_t num_weakrefs = _PyWeakref_GetWeakrefCount(object);
if (num_weakrefs == 0) {
return;
}
PyObject *exc = PyErr_GetRaisedException();
if (count == 1) {
PyObject *callback = current->wr_callback;
current->wr_callback = NULL;
clear_weakref(current);
if (callback != NULL) {
if (Py_REFCNT((PyObject *)current) > 0) {
handle_callback(current, callback);
}
Py_DECREF(callback);
}
}
else {
PyObject *tuple;
Py_ssize_t i = 0;
tuple = PyTuple_New(count * 2);
PyObject *tuple = PyTuple_New(num_weakrefs * 2);
if (tuple == NULL) {
_PyErr_ChainExceptions1(exc);
return;
}
for (i = 0; i < count; ++i) {
PyWeakReference *next = current->wr_next;
Py_ssize_t num_items = 0;
for (int done = 0; !done;) {
PyObject *callback = NULL;
LOCK_WEAKREFS(object);
PyWeakReference *cur = *list;
if (cur != NULL) {
clear_weakref_lock_held(cur, &callback);
if (_Py_TryIncref((PyObject *) cur)) {
assert(num_items / 2 < num_weakrefs);
PyTuple_SET_ITEM(tuple, num_items, (PyObject *) cur);
PyTuple_SET_ITEM(tuple, num_items + 1, callback);
num_items += 2;
callback = NULL;
}
}
done = (*list == NULL);
UNLOCK_WEAKREFS(object);
if (Py_REFCNT((PyObject *)current) > 0) {
PyTuple_SET_ITEM(tuple, i * 2, Py_NewRef(current));
PyTuple_SET_ITEM(tuple, i * 2 + 1, current->wr_callback);
Py_XDECREF(callback);
}
else {
Py_DECREF(current->wr_callback);
}
current->wr_callback = NULL;
clear_weakref(current);
current = next;
}
for (i = 0; i < count; ++i) {
PyObject *callback = PyTuple_GET_ITEM(tuple, i * 2 + 1);
/* The tuple may have slots left to NULL */
for (Py_ssize_t i = 0; i < num_items; i += 2) {
PyObject *callback = PyTuple_GET_ITEM(tuple, i + 1);
if (callback != NULL) {
PyObject *item = PyTuple_GET_ITEM(tuple, i * 2);
handle_callback((PyWeakReference *)item, callback);
PyObject *weakref = PyTuple_GET_ITEM(tuple, i);
handle_callback((PyWeakReference *)weakref, callback);
}
}
Py_DECREF(tuple);
}
assert(!PyErr_Occurred());
PyErr_SetRaisedException(exc);
}
}
/* This function is called by _PyStaticType_Dealloc() to clear weak references.
@ -1025,10 +1066,30 @@ _PyStaticType_ClearWeakRefs(PyInterpreterState *interp, PyTypeObject *type)
{
static_builtin_state *state = _PyStaticType_GetState(interp, type);
PyObject **list = _PyStaticType_GET_WEAKREFS_LISTPTR(state);
while (*list != NULL) {
/* Note that clear_weakref() pops the first ref off the type's
weaklist before clearing its wr_object and wr_callback.
That is how we're able to loop over the list. */
clear_weakref((PyWeakReference *)*list);
// This is safe to do without holding the lock in free-threaded builds;
// there is only one thread running and no new threads can be created.
while (*list) {
_PyWeakref_ClearRef((PyWeakReference *)*list);
}
}
void
_PyWeakref_ClearWeakRefsExceptCallbacks(PyObject *obj)
{
/* Modeled after GET_WEAKREFS_LISTPTR().
This is never triggered for static types so we can avoid the
(slightly) more costly _PyObject_GET_WEAKREFS_LISTPTR(). */
PyWeakReference **list = _PyObject_GET_WEAKREFS_LISTPTR_FROM_OFFSET(obj);
LOCK_WEAKREFS(obj);
while (*list) {
_PyWeakref_ClearRef(*list);
}
UNLOCK_WEAKREFS(obj);
}
int
_PyWeakref_IsDead(PyObject *weakref)
{
return _PyWeakref_IS_DEAD(weakref);
}

9
Python/pystate.c
View File

@ -506,6 +506,15 @@ _PyRuntimeState_ReInitThreads(_PyRuntimeState *runtime)
for (size_t i = 0; i < Py_ARRAY_LENGTH(locks); i++) {
_PyMutex_at_fork_reinit(locks[i]);
}
#ifdef Py_GIL_DISABLED
for (PyInterpreterState *interp = runtime->interpreters.head;
interp != NULL; interp = interp->next)
{
for (int i = 0; i < NUM_WEAKREF_LIST_LOCKS; i++) {
_PyMutex_at_fork_reinit(&interp->weakref_locks[i]);
}
}
#endif
_PyTypes_AfterFork();