From: Hugh Dickins <hughd@google.com>
Date: Wed, 25 Aug 2010 23:12:54 -0700 (PDT)

quoted text
> After several hours, kbuild tests hang with anon_vma_prepare() spinning on
> a newly allocated anon_vma's lock - on a box with CONFIG_TREE_PREEMPT_RCU=y
> (which makes this very much more likely, but it could happen without).
> 
> The ever-subtle page_lock_anon_vma() now needs a further twist: since
> anon_vma_prepare() and anon_vma_fork() are liable to change the ->root
> of a reused anon_vma structure at any moment, page_lock_anon_vma()
> needs to check page_mapped() again before succeeding, otherwise
> page_unlock_anon_vma() might address a different root->lock.
> 
> Signed-off-by: Hugh Dickins <hughd@google.com>

Interesting, is the condition which allows this to trigger specific
to this merge window or was it always possible?
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