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DateWed, 7 May 2008 16:19:05 -0700 (PDT)
FromLinus Torvalds <>
SubjectRe: [PATCH 08 of 11] anon-vma-rwsem
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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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