Subject: Re: [RFC][PATCH 9/7] mm: fix mm_take_all_locks() locking order
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Hugh Dickins <hugh@veritas.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       David Miller <davem@davemloft.net>, jeremy@goop.org, mingo@elte.hu,
       akpm@linux-foundation.org, linux-kernel@vger.kernel.org,
       davej@redhat.com
In-Reply-To: <Pine.LNX.4.64.0808071257200.4594@blonde.site>
References: <20080804130317.994042639@chello.nl>
	 <1218108349.8625.25.camel@twins>
	 <Pine.LNX.4.64.0808071257200.4594@blonde.site>
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Date: Thu, 07 Aug 2008 14:41:21 +0200
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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();
  }

?

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.


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