Re: [RFC][PATCH 9/7] mm: fix mm_take_all_locks() locking order

Hugh Dickins
2008-08-07T13:28:28Z

On Thu, 7 Aug 2008, Peter Zijlstra wrote:
> On Thu, 2008-08-07 at 13:14 +0100, Hugh Dickins wrote:
> > On Thu, 7 Aug 2008, Peter Zijlstra wrote:
> > >
> > > Which the locking hierarchy in mm/rmap.c confirms as 'valid'.
> > >
> > > Although I don't think there are any users of these two locks that don't
> > > hold the mmap_sem, therefore the nesting is strictly ok, but since we
> > > already have an established order, we might as well respect it.
> >
> > Yes, I agree.
> >
> > > Fix this by first taking all the mapping->i_mmap_lock instances and then
> > > take all anon_vma->lock instances.
> >
> > Okay. I'd have preferred taking anon_vma lock after i_mmap_lock
> > each time around the loop, but imagine that's just as problematic
> > for lockdep as the original.
>
> I'm a little confused as to what you mean here, are you suggesting:
>
> for_each_vma() {
> if (file)
> vm_lock_mapping();
> if (anon)
> vm_lock_anon();
> }
>
> ?

Yes, I would have preferred that.

>
> That can still create the inverse lock order due to each vma being only
> of a single type, and therefore the lock order is set by the vma order,
> which can be anything.

Yes, I imagined it would be just as problematic for lockep.

Hugh