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    SubjectRe: [PATCH 2/4] Add replace_page(), change the mapping of pte from one page into another
    On Thu, Nov 13, 2008 at 03:00:59AM +0100, Andrea Arcangeli wrote:
    > CPU0 migrate.c CPU1 filemap.c
    > ------- ----------
    > find_get_page
    > radix_tree_lookup_slot returns the oldpage
    > page_count still = expected_count
    > freeze_ref (oldpage->count = 0)
    > radix_tree_replace (too late, other side already got the oldpage)
    > unfreeze_ref (oldpage->count = 2)
    > page_cache_get_speculative(old_page)
    > set count to 3 and succeeds

    After reading more of this lockless radix tree code, I realized this
    below check is the one that was intended to restart find_get_page and
    prevent it to return the oldpage:

    if (unlikely(page != *pagep)) {

    But there's no barrier there, atomic_add_unless would need to provide
    an atomic smp_mb() _after_ atomic_add_unless executed. In the old days
    the atomic_* routines had no implied memory barriers, you had to use
    smp_mb__after_atomic_add_unless if you wanted to avoid the race. I
    don't see much in the ppc implementation of atomic_add_unless that
    would provide an implicit smb_mb after the page_cache_get_speculative
    returns, so I can't see why the pagep can't be by find_get_page read
    before the other cpu executes radix_tree_replace in the above
    timeline.

    I guess you intended to put an smp_mb() in between the
    page_cache_get_speculative and the *pagep to make the code safe on ppc
    too, but there isn't, and I think it must be fixed, either that or I
    don't understand ppc assembly right. The other side has a smp_wmb
    implicit inside radix_tree_replace_slot so it should be ok already to
    ensure we see the refcount going to 0 before we see the pagep changed
    (the fact the other side has a memory barrier, further confirms this
    side needs it too).

    BTW, the radix_tree_deref_slot might miss a rcu_barrier_depends()
    after radix_tree_deref_slot returns but I'm not entirely sure and only
    alpha would be affected in the worst case.


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