Re: [PATCH v2 11/11] powerpc/mm/book3s64/pgtable: Uses counting method to skip serializing

[Leonardo Bras <leonardo@linux.ibm.com>]

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On Fri, 2019-09-20 at 17:48 -0700, John Hubbard wrote:
> 
[...]
> So it seems that full memory barriers (not just compiler barriers) are required.
> If the irq enable/disable somehow provides that, then your new code just goes
> along for the ride and Just Works. (You don't have any memory barriers in
> start_lockless_pgtbl_walk() / end_lockless_pgtbl_walk(), just the compiler
> barriers provided by the atomic inc/dec.)
> 
> So it's really a pre-existing question about the correctness of the gup_fast()
> irq disabling approach.

I am not experienced in other archs, and I am still pretty new to
Power, but by what I could understand, this behavior is better
explained in serialize_against_pte_lookup. 

What happens here is that, before doing a THP split/collapse, the
function does a update of the pmd and a serialize_against_pte_lookup,
in order do avoid a invalid output on a lockless pagetable walk.

Serialize basically runs a do_nothing in every cpu related to the
process, and wait for it to return. 

This running depends on interrupt being enabled, so disabling it before
gup_pgd_range() and re-enabling after the end, makes the THP
split/collapse wait for gup_pgd_range() completion in every cpu before
continuing. (here happens the lock)

(As told before, every gup_pgd_range() that occurs after it uses a
updated pmd, so no problem.)

I am sure other archs may have a similar mechanism using
local_irq_{disable,enable}.

Did it answer your questions?

Best regards,

Leonardo Bras

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