barriers vs. reads

[Werner Almesberger <wa@almesberger.net>]

I'm working on an elevator with priorities, and I'm wondering what
semantics are expected from barriers when it comes to reads.

My problem with read barriers is that they can upset priorities
quite a bit, by forcing the entire queue to be processed before
any new (possibly timing-critical) reads are allowed.

So, is there anything that actually depends on barriers also
constraining read - or, more likely, read vs. write - order ?
If not, will there be ?

Also, it seems, but is never quite explicitly spelt out, that an
elevator is never really supposed to look for barriers in
rq->flags, but can solely rely on the insertion position as an
indication for barriers. Is this true ?

Thanks,
- Werner

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 / Werner Almesberger, Buenos Aires, Argentina         wa@almesberger.net /
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