linux-kernel - Re: [RFC][PATCH 9/7] mm: fix mm_take_all

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Date:	Thu, 7 Aug 2008 14:27:34 +0100 (BST)
From:	Hugh Dickins <hugh@...itas.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Miller <davem@...emloft.net>, jeremy@...p.org,
	mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, davej@...hat.com
Subject: Re: [RFC][PATCH 9/7] mm: fix mm_take_all_locks() locking order

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
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