https://github.com/python/cpython/commit/534b6b062e53f70587840eac9b432442f66c16c6
commit: 534b6b062e53f70587840eac9b432442f66c16c6
branch: main
author: Neil Schemenauer <[email protected]>
committer: kumaraditya303 <[email protected]>
date: 2026-08-01T15:31:26+05:30
summary:
Add comments about _Py_LOCK_DONT_DETACH usage. (#153817)
files:
M Include/internal/pycore_lock.h
M Objects/unicodeobject.c
diff --git a/Include/internal/pycore_lock.h b/Include/internal/pycore_lock.h
index e31d8b4e5c68c92..13224a0572934b4 100644
--- a/Include/internal/pycore_lock.h
+++ b/Include/internal/pycore_lock.h
@@ -34,6 +34,11 @@ _PyMutex_at_fork_reinit(PyMutex *m)
typedef enum _PyLockFlags {
// Do not detach/release the GIL when waiting on the lock.
+ //
+ // Note that code executed while holding a mutex with this flag must
+ // not detach, reach a safepoint or initiate a stop-the-world pause.
+ // Otherwise, a non-detaching waiter may remain waiting for this mutex and
+ // prevent the pause from completing.
_Py_LOCK_DONT_DETACH = 0,
// Detach/release the GIL while waiting on the lock.
diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index eec02f662e79059..4b4f7178ec9faf6 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -14726,6 +14726,15 @@ intern_common(PyInterpreterState *interp, PyObject *s
/* stolen */,
}
#endif
+ // Why _Py_LOCK_DONT_DETACH is used here: waiting for the interned mutex
+ // must not detach the thread state. Extension code is expected to
+ // detach before blocking on opaque external synchronization. However,
+ // the lock used for C++ static initialization is hidden, making
+ // that difficult, and it is common for C++ extensions to call
+ // PyUnicode_InternFromString() from static initializers. Detaching here
+ // can therefore deadlock: a stop-the-world pause may prevent the lock
+ // owner from reattaching while the pause waits for another attached
+ // thread blocked on the hidden lock.
FT_MUTEX_LOCK_FLAGS(INTERN_MUTEX, _Py_LOCK_DONT_DETACH);
PyObject *t;
{
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