Re: [PATCH] mm: fix hang on anon_vma->root->lock

[Hugh Dickins]

On Fri, Aug 27, 2010 at 2:55 AM, Andrea Arcangeli <aarcange@xxxxxxxxxx> wrote:
> On Thu, Aug 26, 2010 at 06:43:31PM -0700, Hugh Dickins wrote:
>> some light., I think you're mistaking the role that RCU plays here.
>
> That's exactly correct, I thought it prevented reuse of the slab
> entry, not only of the whole slab... SLAB_DESTROY_BY_RCU is a lot more
> tricky to use than I though...
>
> However at the light of this, I think page_lock_anon_vma could have
> returned a freed and reused anon_vma well before the anon-vma changes.
>
> The anon_vma could have been freed after the first page_mapped check
> succeed but before taking the spinlock. I think, it worked fine
> because the rmap walks are robust enough just not to fall apart on a
> reused anon_vma while the lock is hold. It become a visible problem
> now because we were unlocking the wrong lock leading to a
> deadlock. But I guess it wasn't too intentional to return a reused
> anon_vma out of page_lock_anon_vma.

What you say there is all exactly right, except for "I guess it wasn't
too intentional": it was intentional, and known that it all worked out
okay in the rare case when a reused anon_vma got fed into the loops -
the anon_vma, after all, is nothing more than a list of places where
you may find the page mapped, it has never asserted that a page will
be found everywhere that the anon_vma lists.

I would have liked to say "well known" above, but perhaps well known
only to me: you're certainly not the first to be surprised by this.
IIRC both Christoph and Peter have at different times proposed patches
to tighten up page_lock_anon_vma() to avoid returning a stale/reused
anon_vma, probably both were dropped because neither was actually
necessary, until now: I guess it's a good thing for understandability
that anon_vma->root->lock now requires that we weed out that case.

Hugh
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