Is SGCL your project?

In that case, are you doing atomic writes for managed pointers/the read flag on them? I have read a few of your comments on reddit and your flags seem to be per memory page? Still, the synchronization on them may or may not have a more serious performance impact than alternative methods and without a good way to compare it to something like Java which is the state of the art in GC research we can't really comment much on whether it's a net benefit.

Also, have you perhaps tried modeling your design in something like TLA+?