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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20100826235052.GZ6803@random.random>
Date:	Fri, 27 Aug 2010 01:50:52 +0200
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH] mm: fix hang on anon_vma->root->lock

Hi Hugh,

On Wed, Aug 25, 2010 at 11:12:54PM -0700, Hugh Dickins wrote:
> 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.

I don't get it, the anon_vma can be freed and reused only after we run
rcu_read_unlock(). And the anon_vma->root can't change unless the
anon_vma is freed and reused. Last but not the least by the time
page->mapping points to "anon_vma" the "anon_vma->root" is already
initialized and stable.

The page_mapped test is only relevant against the rcu_read_lock, not
the spin_lock, so how it can make a difference to run it twice inside
the same rcu_read_lock protected critical section? The first one still
is valid also after the anon_vma_lock() returns, it's not like that
anon_vma_lock drops the rcu_read_lock internally.

Furthermore no need of ACCESS_ONCE on the anon_vma->root because it
can't change from under us as the anon_vma can't be freed from under
us until rcu_read_unlock returns (after we verified the first time
that page_mapped is true under the rcu_read_lock, which we already do
before trying to take the anon_vma_lock).
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/