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

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Date:	Wed, 7 May 2008 16:19:05 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
cc:	Andrea Arcangeli <andrea@...ranet.com>, clameter@....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, Andrew Morton wrote:
> 
> Now, if we need to take both anon_vma->lock AND i_mmap_lock in the newly
> added mm_lock() thing and we also take both those locks at the same time in
> regular code, we're probably screwed.

No, just use the normal static ordering for that case: one type of lock 
goes before the other kind. If those locks nest in regular code, you have 
to do that *anyway*.

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);

and now everybody follows the rule that "anon_vma->lock" precedes 
"i_mmap_lock". So there can be no ABBA deadlock between the normal users 
and the many-locks version, and there can be no ABBA deadlock between 
many-locks-takers because they use the global_lock to serialize.

This really isn't rocket science, guys.

(I really hope and believe that they don't nest anyway, and that you can 
just use a single for-loop for the many-lock case)

		Linus
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