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Date: Wed, 07 Apr 2010 10:48:26 -0400
From: Rik van Riel <riel@redhat.com>
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To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
       Borislav Petkov <bp@alien8.de>,
       Andrew Morton <akpm@linux-foundation.org>,
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       Hugh Dickins <hugh.dickins@tiscali.co.uk>, sgunderson@bigfoot.com,
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Subject: Re: [PATCH] rmap: make anon_vma_prepare link in all the anon_vmas
 of a mergeable VMA
References: <alpine.LFD.2.00.1004061609320.3487@i5.linux-foundation.org> <20100406195459.554265e7@annuminas.surriel.com> <20100407151357.FB7E.A69D9226@jp.fujitsu.com>
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On 04/07/2010 03:00 AM, KOSAKI Motohiro wrote:

> Hmm.. probably I'm moron.

Someone might be, but it's not you :)

> I'm also confusing this locking rule as same as linus said.
>
> after this patch, new locking order are

> So, Why mmap_sem read lock can protect vma->anon_vma_chain?
> An another threads seems to be able to change avc list concurrentlly and freely.

You are right, the code needs to take the pagetable_lock
around the call to anon_vma_clone, so other threads
get locked out.

This means the locking order has now been inverted,
with the pagetable_lock on the outside and the
anon_vma locks on the inside.

I have checked all the other call sites to the
anon_vma code.  The direct callers of anon_vma_clone
and anon_vma_fork already hold the mmap_sem for
write.  The callers of anon_vma_prepare hold the
mmap_sem for read - so excluding other callers of
anon_vma_prepare with the page_table_lock is enough.

mm_take_all_locks has the mmap_sem for write.

There seem to be no other traversals of the same_vma
list, so changing the locking order to have the
page_table_lock on the outside of the anon_vma locks
works.

> plus, Why don't we need "vma->anon_vma = merge_vma->anon_vma" assignment?
> if vma->anon_vma keep NULL, I think anon_vma_prepare() call anon_vma_clone()
> multiple times.

Added in the new version.  See the next email.