Messages in this thread | | | Date | Tue, 30 Aug 2022 19:39:23 -0300 | From | Jason Gunthorpe <> | Subject | Re: [PATCH v1 2/3] mm/gup: use gup_can_follow_protnone() also in GUP-fast |
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On Tue, Aug 30, 2022 at 01:12:53PM -0700, John Hubbard wrote:
> As long as we continue to sort-of-accidentally use atomic_add_unless(), > which returns a value, instead of atomic_add(), which does not. :)
I should say I didn't have time to carefully check what put_page was doing, but IIRC it is an atomic_dec_return to decide if it should free the page.
The conditional is pretty much inherent to the model, because 0 is special it always has to be checked. Not so accidental
But you might make the case that these could be the relaxed versions of the atomic...
> 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?
It would be fantastic if things that are required to be acquire/releases are documented as acquire/release :)
It is incredibly subtle stuff and all carefully inlined for maximum performance.
Jason
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