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SubjectRe: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in GUP-fast
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On 8/30/22 12:57, Jason Gunthorpe wrote:
> I don't like the use of smb_mb very much, I deliberately choose the
> more modern language of release/acquire because it makes it a lot
> clearer what barriers are doing..
>
> So, if we dig into it, using what I said above, the atomic refcount is:
>
> gup_pte_range()
> try_grab_folio()
> try_get_folio()
> folio_ref_try_add_rcu()
> folio_ref_add_unless()
> page_ref_add_unless()
> atomic_add_unless()
>
> So that wants to be an acquire
>
> The pairing release is in the page table code that does the put_page,
> it wants to be an atomic_dec_return() as a release.

Thanks for making that a lot clearer, at least for me anyway!

>
> Now, we go and look at Documentation/atomic_t.txt to try to understand
> what are the ordering semantics of the atomics we are using and become
> dazed-confused like me:
>
> ORDERING (go read memory-barriers.txt first)
> --------
>
> - RMW operations that have a return value are fully ordered;
>
> - RMW operations that are conditional are unordered on FAILURE,
> otherwise the above rules apply.
>
> Fully ordered primitives are ordered against everything prior and everything
> subsequent. Therefore a fully ordered primitive is like having an smp_mb()
> before and an smp_mb() after the primitive.
>
> So, I take that to mean that both atomic_add_unless() and
> atomic_dec_return() are "fully ordered" and "fully ordered" is a super
> set of acquire/release.
>
> Thus, we already have the necessary barriers integrated into the
> atomic being used.

As long as we continue to sort-of-accidentally use atomic_add_unless(),
which returns a value, instead of atomic_add(), which does not. :)

Likewise on the put_page() side: we are depending on the somewhat
accidental (from the perspective of memory barriers) use of
atomics that return values.

Maybe it would be good to add a little note at each site, to that
effect?

>
> The smb_mb_after_atomic stuff is to be used with atomics that don't
> return values, there are some examples in the doc
>
> Jason

thanks,

--
John Hubbard
NVIDIA

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