Re: [PATCH 1/2] mm: introduce put_user_page*(), placeholder versions

[Jerome Glisse]

On Tue, Jan 15, 2019 at 06:01:09PM -0800, Dan Williams wrote:
> On Tue, Jan 15, 2019 at 5:56 PM Jerome Glisse <jglisse@xxxxxxxxxx> wrote:
> > On Tue, Jan 15, 2019 at 04:44:41PM -0800, John Hubbard wrote:
> [..]
> > To make it clear.
> >
> > Lock code:
> >     GUP()
> >         ...
> >         lock_page(page);
> >         if (PageWriteback(page)) {
> >             unlock_page(page);
> >             wait_stable_page(page);
> >             goto retry;
> >         }
> >         atomic_add(page->refcount, PAGE_PIN_BIAS);
> >         unlock_page(page);
> >
> >     test_set_page_writeback()
> >         bool pinned = false;
> >         ...
> >         pinned = page_is_pin(page); // could be after TestSetPageWriteback
> >         TestSetPageWriteback(page);
> >         ...
> >         return pinned;
> >
> > Memory barrier:
> >     GUP()
> >         ...
> >         atomic_add(page->refcount, PAGE_PIN_BIAS);
> >         smp_mb();
> >         if (PageWriteback(page)) {
> >             atomic_add(page->refcount, -PAGE_PIN_BIAS);
> >             wait_stable_page(page);
> >             goto retry;
> >         }
> >
> >     test_set_page_writeback()
> >         bool pinned = false;
> >         ...
> >         TestSetPageWriteback(page);
> >         smp_wmb();
> >         pinned = page_is_pin(page);
> >         ...
> >         return pinned;
> >
> >
> > One is not more complex than the other. One can contend, the other
> > will _never_ contend.
> 
> The complexity is in the validation of lockless algorithms. It's
> easier to reason about locks than barriers for the long term
> maintainability of this code. I'm with Jan and John on wanting to
> explore lock_page() before a barrier-based scheme.

How is the above hard to validate ? Either GUP see racing
test_set_page_writeback because it test write back after
incrementing the refcount, or test_set_page_writeback sees
GUP because it checks for pin after setting the write back
bits.

So if GUP see !PageWriteback() then test_set_page_writeback
see page_pin(page) as true. If test_set_page_writeback sees
page_pin(page) as false then GUP did see PageWriteback() as
true.

You _never_ have !PageWriteback() in GUP and !page_pin() in
test_set_page_writeback() if they are both racing. This is
an impossible scenario because of memory barrier.

Cheers,
Jérôme