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    SubjectRe: [PATCH] mm: fix hang on anon_vma->root->lock
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    On Fri, Aug 27, 2010 at 1:56 PM, Christoph Lameter <cl@linux.com> wrote:
    > On Fri, 27 Aug 2010, Hugh Dickins wrote:
    >
    >> Nothing ensures that the root pointer was not changed after the
    >> ACCESS_ONCE, that's exactly why we use ACCESS_ONCE there: once we've
    >> got the lock and realize that what we've locked may not be what we
    >> wanted (or may change from what we were wanting at any moment, the
    >> page no longer being mapped there - but in that case we no longer want
    >> it), we have to be sure to unlock the one we locked, rather than the
    >> one which anon_vma->root might subsequently point to.
    >
    > I do not see any check after we have taken the lock to verify that we
    > locked the correct object. Was there a second version of the patch?

    No second version of the patch, no. As I said already, it's that
    second page_mapped check which gives the guarantee that the anon_vma
    has not yet been freed, hence we've locked the correct object.

    >
    >> > Since there is no lock taken before the mapped check none of the
    >> > earlier reads from the anon vma structure nor the page mapped check
    >> > necessarily reflect a single state of the anon_vma.
    >>
    >> There's no lock (other than RCU's read "lock")  taken before the
    >> original mapped check, and that's important, otherwise our attempt to
    >> lock might actually spinon or corrupt something that was long ago an
    >> anon_vma.  But we do take the anon_vma->root->lock before the second
    >> mapped check which I added.  If the page is still mapped at the point
    >
    > You then are using an object from the anon_vma (the pointer) without a
    > lock!

    Yes. (not counting RCU's read "lock" as a lock).

    > This is unstable therefore unless there are other constraints. The
    > anon_vma->lock must be taken before derefencing that pointer.

    No, SLAB_DESTROY_BY_RCU gives us just the stablity we need to take the lock.

    > The page may
    > have been unmapped and mapped again between the two checks. Unlikely but
    > possible.

    Yes, unlikely but possible. (Well, actually, is it possible? It can
    be unmapped on exit without any lock, but unmapping for pageout would
    require the page lock, would insert a swp_entry_t, and mapping again
    would go to do_swap_page which would again require the page lock. But
    never mind that, let's assume there is a way it can be unmapped and
    mapped again.) The thing is, page->mapping will point to the same
    anon_vma throughout, that only gets reset when the page is freed, and
    there should be nowhere else that modifies page->mapping once it's
    been set to anon_vma - if you know of somewhere, please point to it,
    we do need to examine that case.

    >
    >> of that second check, then we know that we got the right anon_vma,
    >
    > I do not see a second check (*after* taking the lock) in the patch






    > and the
    > way the lock is taken can be a problem in itself.
    >
    >
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