Re: Non-reproducible AIO failure

Hi,

I'm a bit confused by this focus on bitfields - both Alexander and Konstantin
stated they could reproduce the issue without the bitfields.

But we have observed the generated code being pretty grotty and it's caused
more than enough confusion - so let's just replace them with plain uint8's and
cast in switches.

> I think the issue is that if the compiler decides to coalesce what we
> think of as distinct (but neighboring) bitfields, then when you update
> one of the bitfields you could be updating the other with stale data
> from an earlier read where the cached stale data is cached in a
> _register_.  Thus the fact that the cache line should have the most up
> to date data for that other field is irrelevant because the stale data
> is in a _register_.
> 
> The very fact that this can happen, that the C specs allow it, argues
> that one must never have adjacent distinct (for some value of
> "distinct") bitfields for anything that requires atomics.

I think the barriers in place should prevent that.

Greetings,

Andres Freund