I kind of knew where this was going in terms of the actual bug, but the way they've "proven" their fix is indeed pretty neat. I would have thoroughly explained

I kind of knew where this was going in terms of the actual bug, but the way they've "proven" their fix is indeed pretty neat. I would have thoroughly explained the bug and fix, but in order to actually verify I honestly most probably would have just done what they suggested (and rejected) first: Run it long enough to show that the issue does not occur anymore. As an aside, I have not stared at the problem long enough to say this with full confidence, but if the barriers they inserted are full barriers, they might be too strong. acquire/release might have been sufficient.