Relacy is notable for being the only one in c++ that knows what a fence is, at least last time I looked.
It told me a structure might not make forward progress on some thread interleaving. I.e. if only one thread ever runs, the others don't do anything. That's true but uninformative. I wasn't able to coax it into using a vaguely fair scheduler to get more useful information out than that.
It's been around a while and is easy to work with. Written by Dmitry Vyukov, an expert in the concurrency world.