linux-kernel - Re: [PATCH] mm: fix hang on anon

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Message-Id: <20100825.234149.189710316.davem@davemloft.net>
Date:	Wed, 25 Aug 2010 23:41:49 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	hughd@...gle.com
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	riel@...hat.com, aarcange@...hat.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: fix hang on anon_vma->root->lock

From: Hugh Dickins <hughd@...gle.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@...gle.com>

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