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

[David Miller]

From: Hugh Dickins <hughd@xxxxxxxxxx>
Date: Thu, 26 Aug 2010 03:54:28 -0700

> On Wed, Aug 25, 2010 at 11:41 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>> From: Hugh Dickins <hughd@xxxxxxxxxx>
>> 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@xxxxxxxxxx>
>>
>> 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).

Ok, because I've been seeing mysterious hangs recently on one of my
sparc64 boxes and I was wondering if this might explain them :-)

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