Re: [PATCH -v2] rmap: make anon_vma_prepare link in all theanon_vmas of a mergeable VMA

[Peter Zijlstra]

On Mon, 2010-04-12 at 20:40 +0200, Peter Zijlstra wrote:

Hmm, if interleaved like so

> struct anon_vma *page_lock_anon_vma(struct page *page)
> {
>         struct anon_vma *anon_vma;
>         unsigned long anon_mapping;

page_remove_rmap()
anon_vma_unlink()
  anon_vma_free()

So that the below will all observe the old page->mapping:

>         rcu_read_lock();
> again:
>         anon_mapping = (unsigned long)rcu_dereference(page->mapping);
>         if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
>                 goto out;
>         anon_vma = (struct anon_vma *)(anon_mapping - PAGE_MAPPING_ANON);
> 
>         /*
>          * The RCU read lock ensures we can safely dereference anon_vma
>          * since it ensures the backing slab won't go away. It will however
>          * not guarantee it's the right object.
>          *
>          * First take the anon_vma->lock, this will, per anon_vma_unlink()
>          * avoid this anon_vma from being freed if it is a valid object.
>          */
>         spin_lock(&anon_vma->lock);
> 
>         /*
>          * Secondly, we have to re-read page->mapping, so ensure it
>          * has not changed, rely on spin_lock() being at least a
>          * compiler barrier to force the re-read.
>          */
>         if (unlikely(page_rmapping(page) != anon_vma)) {
>                 spin_unlock(&anon_vma->lock);
>                 goto again;
>         }

page_add_anon_rmap(), so that the page_mapped() test below would be
positive,

>         /*
>          * Ensure we read page->mapping before page->_mapcount,
>          * orders against atomic_add_negative() in page_remove_rmap().
>          */
>         smp_rmb();
> 
>         /*
>          * Finally check that the page is still mapped,
>          * if not, this can't possibly be the right anon_vma.
>          */
>         if (!page_mapped(page))
>                 goto unlock;

We could here return a non-valid and already freed anon_vma.

>         return anon_vma;
> 
> unlock:
>         spin_unlock(&anon_vma->lock);
> out:
>         rcu_read_unlock();
>         return NULL;
> }
> 
> 

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