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

[Andrea Arcangeli]

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.
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