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https://github.com/python/cpython/commit/b6c62c79e7d9592ca1ea6b93f6ce3dd3829939d0
commit: b6c62c79e7d9592ca1ea6b93f6ce3dd3829939d0
branch: main
author: mpage <mpage@meta.com>
committer: DinoV <dinoviehland@gmail.com>
date: 2024-04-17T09:42:56-07:00
summary:

gh-117657: Fix data races in the method cache in free-threaded builds (#117954)

Fix data races in the method cache in free-threaded builds

These are technically data races, but I think they're benign (to
the extent that that is actually possible). We update cache entries
non-atomically but read them atomically from another thread, and there's
nothing that establishes a happens-before relationship between the
reads and writes that I can see.

files:
M Objects/typeobject.c
M Tools/tsan/suppressions_free_threading.txt

diff --git a/Objects/typeobject.c b/Objects/typeobject.c
index 1cb53516a9ae76..6304ee178fd038 100644
--- a/Objects/typeobject.c
+++ b/Objects/typeobject.c
@@ -4974,13 +4974,15 @@ is_dunder_name(PyObject *name)
static void
update_cache(struct type_cache_entry *entry, PyObject *name, unsigned int version_tag, PyObject *value)
{
- entry->version = version_tag;
- entry->value = value; /* borrowed */
+ _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);
// We're releasing this under the lock for simplicity sake because it's always a
// exact unicode object or Py_None so it's safe to do so.
- Py_SETREF(entry->name, Py_NewRef(name));
+ PyObject *old_name = entry->name;
+ _Py_atomic_store_ptr_relaxed(&entry->name, Py_NewRef(name));
+ Py_DECREF(old_name);
}

#if Py_GIL_DISABLED
diff --git a/Tools/tsan/suppressions_free_threading.txt b/Tools/tsan/suppressions_free_threading.txt
index 889b62e59b14a6..6e2bdc1ea85cd6 100644
--- a/Tools/tsan/suppressions_free_threading.txt
+++ b/Tools/tsan/suppressions_free_threading.txt
@@ -47,5 +47,4 @@ race:set_inheritable
race:start_the_world
race:tstate_set_detached
race:unicode_hash
-race:update_cache
race:update_cache_gil_disabled

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