linux-kernel - Re: [PATCH 08 of 11] anon-vma-rwsem

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Date:	Wed, 7 May 2008 16:39:39 -0700 (PDT)
From:	Christoph Lameter <clameter@....com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <andrea@...ranet.com>, steiner@....com,
	holt@....com, npiggin@...e.de, a.p.zijlstra@...llo.nl,
	kvm-devel@...ts.sourceforge.net, kanojsarcar@...oo.com,
	rdreier@...co.com, swise@...ngridcomputing.com,
	linux-kernel@...r.kernel.org, avi@...ranet.com, linux-mm@...ck.org,
	general@...ts.openfabrics.org, hugh@...itas.com,
	rusty@...tcorp.com.au, aliguori@...ibm.com, chrisw@...hat.com,
	marcelo@...ck.org, dada1@...mosbay.com, paulmck@...ibm.com
Subject: Re: [PATCH 08 of 11] anon-vma-rwsem

On Wed, 7 May 2008, Linus Torvalds wrote:

> The code that can take many locks, will have to get the global lock *and* 
> order the types, but that's still trivial. It's something like
> 
> 	spin_lock(&global_lock);
> 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
> 		if (vma->anon_vma)
> 			spin_lock(&vma->anon_vma->lock);
> 	}
> 	for (vma = mm->mmap; vma; vma = vma->vm_next) {
> 		if (!vma->anon_vma && vma->vm_file && vma->vm_file->f_mapping)
> 			spin_lock(&vma->vm_file->f_mapping->i_mmap_lock);
> 	}
> 	spin_unlock(&global_lock);

Multiple vmas may share the same mapping or refer to the same anonymous 
vma. The above code will deadlock since we may take some locks multiple 
times.
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