I think you understood it. To your last point: > it doesn’t really prove that the only cause of x != y issue [...] is caused by the memory barrier problem (eg

I think you understood it. To your last point: > it doesn’t really prove that the only cause of x != y issue [...] is caused by the memory barrier problem (eg maybe a rare buffer overflow sets done to true too early). I mean, you can never prove that, and that goes for pretty much any bug. Hypothetically, a hardware issue could be in the mix. Or a cosmic ray may hit the device (that's a real thing). You can't prove a negative after all. But while this does not prove that there's no other cause with the same effect, this at least makes it extremely likely that the missing barrier was an issue: Breaking with the debugger suddenly makes it work without any other intervention, and also they've just looked at the machine code in the first place, with the knowledge of how CPUs work. The likelihood that there's also a rare buffer overflow consistently reproducing the same issue is negligibly small. If there is, that's a problem you can look at once it actually happens again. And then stare at it in bewildered disbelief.