MRI implements a write barrier. You can check out the write barrier implementation here:

  https://github.com/ruby/ruby/blob/7bd93293621b85a87e7e117317612bb0a84efb7a/gc.c#L7802

The behavior for writes on a black object is defined in this function:

  https://github.com/ruby/ruby/blob/7bd93293621b85a87e7e117317612bb0a84efb7a/gc.c#L7766

Basically when a black -> white reference is written during incremental marking, the white object will be grey'd and added to the mark stack.

Happy Thursday!

Alternatively, you can also move the gc "backwards", changing the black object. In Lua, sometimes the write barrier recolors the white child object to grey and sometimes it recolors the black parent object to grey.

IIRC, the motivation for going backwards is that if you assign a large number of white children, recoloring the parent once is faster than recoloring all the children.