Extension modules which check for self->weakreflist != NULL in tp_dealloc before calling PyObject_ClearWeakRefs(self) leads to to data race if another thread is concurrently mutating the weaklist.
In free-threading the weaklist is modified atomically as such it can cause data race with the non atomic reads in extension modules. This can be avoided in extension modules by calling PyObject_ClearWeakRefs always and removing checking for weaklist == NULL.

I'll try to find a smaller reproducer for this.

Linked PRs

• gh-135607: Remove null checking of weakref list in dealloc of extension modules and objects #135614
• [3.14] gh-135607: remove null checking of weakref list in dealloc of extension modules and objects (#135614) #136119
• [3.13] gh-135607: remove null checking of weakref list in dealloc of extension modules and objects (#135614) #136126