linux-kernel - Re: [PATCH 06/11] huge-memory: Use lockdep_assert

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Date:	Fri, 16 Mar 2012 14:18:57 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
cc:	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 06/11] huge-memory: Use lockdep_assert_held

On Fri, 16 Mar 2012, Andrea Arcangeli wrote:
> On Fri, Mar 16, 2012 at 12:00:59PM -0700, Andi Kleen wrote:
> > From: Andi Kleen <ak@...ux.intel.com>
> > 
> > Use lockdep_assert_held to check for locks instead of an opencoded
> > variant.
> > 
> > Cc: aarcange@...hat.com
> > Signed-off-by: Andi Kleen <ak@...ux.intel.com>
> > ---
> >  mm/huge_memory.c |    4 ++--
> >  mm/swap.c        |    2 +-
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 91d3efb..28669c6 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -2083,7 +2083,7 @@ static void collect_mm_slot(struct mm_slot *mm_slot)
> >  {
> >  	struct mm_struct *mm = mm_slot->mm;
> >  
> > -	VM_BUG_ON(NR_CPUS != 1 && !spin_is_locked(&khugepaged_mm_lock));
> > +	lockdep_assert_held(&khugepaged_mm_lock);
> 
> This check was intended to be a VM debug check, so getting enabled by
> DEBUG_VM, not through lockdep. I mean I always have DEBUG_VM enabled
> in all my kernels, but lockdep only enabled on my test system. So it's
> not an opencoded variant strictly speaking.
> 
> My estimate is that what gets more tested is BUG_ON, second VM_BUG_ON,
> third LOCKDEP.

My estimate too.

> But hey, this code has been tested for a while so I'm
> neutral and if you prefer the lockdep version it's up to you.

Yes, it's not as if these are public interfaces used by drivers and
filesystems: they have very few callsites, and by now those lines
are mostly useful as documentation.

Andrew did mention assert_spin_locked() when I added those NR_CPUS
conditions, but we preferred VM_BUG_ON to BUG_ON, and I'm content
with Andi's further restriction to lockdep.

Acked-by: Hugh Dickins <hughd@...gle.com>

Hugh
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