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

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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
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17 changed files with 490 additions and 326 deletions
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537
Objects/weakrefobject.c
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@ -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 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 *)get_or_create_weakref(type, ob, callback);
}
return (PyObject *)self;
return NULL;
}
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;
PyTypeObject *type = &_PyWeakref_ProxyType;
if (PyCallable_Check(ob)) {
type = &_PyWeakref_CallableProxyType;
}
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;
if (PyCallable_Check(ob)) {
Py_SET_TYPE(result, &_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);
PyObject *exc = PyErr_GetRaisedException();
if (count == 1) {
PyObject *callback = current->wr_callback;
/* 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);
}
current->wr_callback = NULL;
clear_weakref(current);
if (callback != NULL) {
if (Py_REFCNT((PyObject *)current) > 0) {
handle_callback(current, callback);
}
Py_DECREF(callback);
/* 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();
PyObject *tuple = PyTuple_New(num_weakrefs * 2);
if (tuple == NULL) {
_PyErr_ChainExceptions1(exc);
return;
}
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;
}
}
else {
PyObject *tuple;
Py_ssize_t i = 0;
done = (*list == NULL);
UNLOCK_WEAKREFS(object);
tuple = PyTuple_New(count * 2);
if (tuple == NULL) {
_PyErr_ChainExceptions1(exc);
return;
}
for (i = 0; i < count; ++i) {
PyWeakReference *next = current->wr_next;
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);
}
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 */
if (callback != NULL) {
PyObject *item = PyTuple_GET_ITEM(tuple, i * 2);
handle_callback((PyWeakReference *)item, callback);
}
}
Py_DECREF(tuple);
}
assert(!PyErr_Occurred());
PyErr_SetRaisedException(exc);
Py_XDECREF(callback);
}
for (Py_ssize_t i = 0; i < num_items; i += 2) {
PyObject *callback = PyTuple_GET_ITEM(tuple, i + 1);
if (callback != NULL) {
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);
}