The bacon-rajan-cc link has only -implemented- a stop-the-world version, but notes right at the top of the README that it -can- be concurrent, and the stop-the-world-only-ness is only 'Currently.'
Samsara is implementing the same algorithm and seems to be further along. Though there's also 'shredder' https://github.com/Others/shredder with a different overall approach.
https://trout.me.uk/gc/ has two Recycler papers if you want more details.