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SubjectRe: data dependency naming inconsistency
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On Mon, 11 Oct 2021 07:07:08 -0400, Michael S. Tsirkin wrote:
> Hello Paul, all!

Hello Michael,

I thought Paul would respond soon, but looks like he has not
done so.
So, I'm trying to give some hint to your findings.

> I've been reading with interest Paul's posts about Rust interactions with LKMM
> https://paulmck.livejournal.com/63316.html
> and in particular it states:
> A data dependency involves a load whose return value directly or
> indirectly determine the value stored by a later store, which results in
> the load being ordered before the store.
>
> This matches the perf book:
> A data dependency occurs when the value returned by
> a load instruction is used to compute the data stored by
> a later store instruction.

You might likely be aware, but these concern "data dependency",
not a _barrier_.

>
> however, memory-barriers.txt states:
>
> A data dependency barrier is a partial ordering on interdependent loads
> only; it is not required to have any effect on stores, independent loads
> or overlapping loads.
>
> It also says:
> A data-dependency barrier is not required to order dependent writes
> because the CPUs that the Linux kernel supports don't do writes
> until they are certain (1) that the write will actually happen, (2)
> of the location of the write, and (3) of the value to be written.

These concern the historic "data-dependency barrier", or
[smp_]read_barrier_depends(), which existed until Linux kernel v4.14.

>
> so the result it the same: writes are ordered without a barrier,
> reads are ordered by a barrier.
>
> However, it would seem that a bit more consistency in naming won't
> hurt.

So, I don't think the historic term of "data-dependency barrier"
can be changed.

I guess the right approach would be to further de-emphasize
"data-dependency barrier"/"data dependency barrier" in
memory-barriers.txt.

Rewrite by commit 8ca924aeb4f2 ("Documentation/barriers: Remove
references to [smp_]read_barrier_depends()") did some of such
changes, but it failed to update the introductory section of
"VARIETIES OF MEMORY BARRIER".
The part Michael quoted above belongs to it.
I don't think it has any merit keeping it around.

Also, there remain a couple of ascii-art diagrams concerning
<data dependency barrier> in the first part of "EXAMPLES OF MEMORY
BARRIER SEQUENCES" section, which, I think, can be removed as well.

Hope this helps clarify the circumstances.

Paul, what is your take on the naming of "data dependency"/
"data dependency barrier"?

Thanks, Akira

>
> Thanks,
>
> --
> MST

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