Re: [kvm-devel] [PATCH 08 of 11] anon-vma-rwsem

[Christoph L. <cla...@sg...>]

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.