Hello Chen,

On Sun, Oct 04, 2015 at 12:55:29PM +0800, Chen Gang wrote:
> Theoretically, the lock and unlock need to be symmetric, if we have to
> lock f_mapping all firstly, then lock all anon_vma, probably, we also
> need to unlock anon_vma all, then unlock all f_mapping.

They don't need to be symmetric because the unlocking order doesn't
matter. To avoid lock inversion deadlocks it is enough to enforce the
lock order.