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gh-132942: Fix races in type lookup cache (gh-133032)

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Two races related to the type lookup cache, when used in the
free-threaded build.  This caused test_opcache to sometimes fail (as
well as other hard to re-produce failures).
This commit is contained in:
Neil Schemenauer 2025-04-28 11:52:08 -07:00 • committed by GitHub
parent fe462f5a91
commit 31d1342de9
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GPG key ID: B5690EEEBB952194
2 changed files with 13 additions and 3 deletions
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14
Objects/typeobject.c
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@ -5678,7 +5678,6 @@ is_dunder_name(PyObject *name)
static PyObject *
update_cache(struct type_cache_entry *entry, PyObject *name, unsigned int version_tag, PyObject *value)
{
_Py_atomic_store_uint32_relaxed(&entry->version, version_tag);
_Py_atomic_store_ptr_relaxed(&entry->value, value); /* borrowed */
assert(_PyASCIIObject_CAST(name)->hash != -1);
OBJECT_STAT_INC_COND(type_cache_collisions, entry->name != Py_None && entry->name != name);
@ -5686,6 +5685,12 @@ update_cache(struct type_cache_entry *entry, PyObject *name, unsigned int versio
// exact unicode object or Py_None so it's safe to do so.
PyObject *old_name = entry->name;
_Py_atomic_store_ptr_relaxed(&entry->name, Py_NewRef(name));
// We must write the version last to avoid _Py_TryXGetStackRef()
// operating on an invalid (already deallocated) value inside
// _PyType_LookupRefAndVersion(). If we write the version first then a
// reader could pass the "entry_version == type_version" check but could
// be using the old entry value.
_Py_atomic_store_uint32_release(&entry->version, version_tag);
return old_name;
}
@ -5762,7 +5767,7 @@ _PyType_LookupStackRefAndVersion(PyTypeObject *type, PyObject *name, _PyStackRef
// synchronize-with other writing threads by doing an acquire load on the sequence
while (1) {
uint32_t sequence = _PySeqLock_BeginRead(&entry->sequence);
uint32_t entry_version = _Py_atomic_load_uint32_relaxed(&entry->version);
uint32_t entry_version = _Py_atomic_load_uint32_acquire(&entry->version);
uint32_t type_version = _Py_atomic_load_uint32_acquire(&type->tp_version_tag);
if (entry_version == type_version &&
_Py_atomic_load_ptr_relaxed(&entry->name) == name) {
@ -5809,11 +5814,14 @@ _PyType_LookupStackRefAndVersion(PyTypeObject *type, PyObject *name, _PyStackRef
int has_version = 0;
unsigned int assigned_version = 0;
BEGIN_TYPE_LOCK();
res = find_name_in_mro(type, name, &error);
// We must assign the version before doing the lookup. If
// find_name_in_mro() blocks and releases the critical section
// then the type version can change.
if (MCACHE_CACHEABLE_NAME(name)) {
has_version = assign_version_tag(interp, type);
assigned_version = type->tp_version_tag;
}
res = find_name_in_mro(type, name, &error);
END_TYPE_LOCK();
/* Only put NULL results into cache if there was no error. */