On Wed, 20 Feb 2019, Will Deacon wrote: > Whilst I completely agree that relying on the ordering provided by "dep ; > rfi" is subtle and error prone, having it forbid the outcome above appeals > to a hardware-based mindset of how memory ordering works. In the kernel > community, I would posit that the majority of developers are writing code > with the underlying hardware in mind and so allowing behaviours in the > memory model which are counter to how a real machine operates is likely to > make things more confusing, rather than simplifying them! > > IIRC, herd has a feature where you can "flag" the result of a litmus test > to highlight certain internal constraint violations (e.g. warning that a > data race is present in a concurrent C11 program). How about we preserve > the existing semantics, but flag any use of "dep; rfi" to indicate that > the ordering guarantees being relied upon are subtle and error-prone, and > therefore should only be considered for fast-path code? Unfortunately, herd can't really tell whether a particular ordering is being _used_; it can only tell when the ordering is present. Therefore such a flag would be prone to false positives. Alan
