Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

gh-111926: Set up basic sementics of weakref API for freethreading #113621

Merged
merged 6 commits into from
Jan 3, 2024
Merged
Changes from all commits
Commits
File filter

Filter by extension

Filter by extension

Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
56 changes: 37 additions & 19 deletions Include/internal/pycore_weakref.h
Original file line number Diff line number Diff line change
Expand Up @@ -9,48 +9,66 @@ extern "C" {
#endif

#include "pycore_critical_section.h" // Py_BEGIN_CRITICAL_SECTION()
#include "pycore_object.h" // _Py_REF_IS_MERGED()

static inline PyObject* _PyWeakref_GET_REF(PyObject *ref_obj) {
static inline int _is_dead(PyObject *obj)
{
// Explanation for the Py_REFCNT() check: when a weakref's target is part
// of a long chain of deallocations which triggers the trashcan mechanism,
// clearing the weakrefs can be delayed long after the target's refcount
// has dropped to zero. In the meantime, code accessing the weakref will
// be able to "see" the target object even though it is supposed to be
// unreachable. See issue gh-60806.
#if defined(Py_GIL_DISABLED)
Py_ssize_t shared = _Py_atomic_load_ssize_relaxed(&obj->ob_ref_shared);
return shared == _Py_REF_SHARED(0, _Py_REF_MERGED);
#else
return (Py_REFCNT(obj) == 0);
#endif
}

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;

if (obj == Py_None) {
// clear_weakref() was called
return NULL;
goto end;
}

// Explanation for the Py_REFCNT() check: when a weakref's target is part
// of a long chain of deallocations which triggers the trashcan mechanism,
// clearing the weakrefs can be delayed long after the target's refcount
// has dropped to zero. In the meantime, code accessing the weakref will
// be able to "see" the target object even though it is supposed to be
// unreachable. See issue gh-60806.
Py_ssize_t refcnt = Py_REFCNT(obj);
if (refcnt == 0) {
return NULL;
if (_is_dead(obj)) {
goto end;
}

assert(refcnt > 0);
return Py_NewRef(obj);
#if !defined(Py_GIL_DISABLED)
assert(Py_REFCNT(obj) > 0);
#endif
ret = Py_NewRef(obj);
end:
Py_END_CRITICAL_SECTION();
return ret;
}

static inline int _PyWeakref_IS_DEAD(PyObject *ref_obj) {
static inline int _PyWeakref_IS_DEAD(PyObject *ref_obj)
{
assert(PyWeakref_Check(ref_obj));
int is_dead;
int ret = 0;
Py_BEGIN_CRITICAL_SECTION(ref_obj);
PyWeakReference *ref = _Py_CAST(PyWeakReference*, ref_obj);
PyObject *obj = ref->wr_object;
if (obj == Py_None) {
// clear_weakref() was called
is_dead = 1;
ret = 1;
}
else {
// See _PyWeakref_GET_REF() for the rationale of this test
Copy link
Contributor

@colesbury colesbury Jan 2, 2024

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

EDIT: removed suggestion

Copy link
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Never mind -- that's probably not helpful. Can probably just delete the comment now that the code calls _is_dead(). The reader probably knows to look at the function implementation to understand the call.

is_dead = (Py_REFCNT(obj) == 0);
ret = _is_dead(obj);
}
Py_END_CRITICAL_SECTION();
return is_dead;
return ret;
}

extern Py_ssize_t _PyWeakref_GetWeakrefCount(PyWeakReference *head);
Expand Down
Loading