Doesn't the strict definition of QBSR require a thread to keep no references to shared data when it is in quiescent state, whereas epoch-based reclamation allows threads to simply retain no references to older versions of shared data when it advances its epoch?

I'm referring to the FIFO implementation. On the definition of QSBR though, frankly, I think the term gets overloaded in some of the more recent literature in this area. Both EBR and QSBR require that no hazardous references get carried over across "protected sections" (EBR is explicit read-side protected sections and in QSBR, across a quiescent point).

I think it does fit that description, but for some reason none of the implementations I found were completely without atomic ops. They had at least barriers which aren't free on ARM.

wouldn't you need acquire/release barriers around your load and stores of your token (or alternatively acq/rel load/stores)? Otherwise your other operations won't be ordered with regard the epoch marked by the token.