@@ -279,9 +279,14 @@ impl fmt::Debug for ObjExt {
 }
 
 /// Precomputed offset from PyInner pointer back to ObjExt prefix.
-/// Both ObjExt and PyInner are #[repr(C)] with 8-byte alignment,
-/// so the offset equals size_of::<ObjExt>() with no padding.
+/// ObjExt is #[repr(C, align(8))] and PyInner is #[repr(C)], so as long as
+/// ObjExt's alignment >= PyInner's alignment, Layout::extend adds no padding
+/// and the offset equals size_of::<ObjExt>().
 const EXT_OFFSET: usize = core::mem::size_of::<ObjExt>();
+// Guarantee: ObjExt size is a multiple of its alignment, and its alignment
+// is >= any PyInner alignment, so Layout::extend produces no inter-padding.
+const _: () = assert!(core::mem::size_of::<ObjExt>() % core::mem::align_of::<ObjExt>() == 0);
+const _: () = assert!(core::mem::align_of::<ObjExt>() >= core::mem::align_of::<PyInner<()>>());
 
 /// This is an actual python object. It consists of a `typ` which is the
 /// python class, and carries some rust payload optionally. This rust
@@ -1061,6 +1066,7 @@ impl PyObject {
     /// Returns the first weakref in the weakref list, if any.
     pub(crate) fn get_weakrefs(&self) -> Option<PyObjectRef> {
         let wrl = self.weak_ref_list()?;
+        let _lock = weakref_lock::lock(self as *const PyObject as usize);
         let head_ptr = wrl.head.load(Ordering::Relaxed);
         if head_ptr.is_null() {
             None