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

[Hugh Dickins <hughd@google.com>]

On Wed, Aug 25, 2010 at 11:41 PM, David Miller <davem@davemloft.net> wrote:
> From: Hugh Dickins <hughd@google.com>
> Date: Wed, 25 Aug 2010 23:12:54 -0700 (PDT)
>
>> 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?

Just specific to this merge window, which started using
anon_vma->root->lock in place of anon_vma->lock (anon_vma->root is
often anon_vma itself, but not always).  I _think_ that change was
itself a simplification of the locking in 2.6.35, rather than plugging
a particular hole (it's not been backported to -stable), but I may be
wrong on that - Rik?

Hugh

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