Re: [PATCH 3/6] futex: add double_unlock_hb()

[Darren Hart]

Thomas Gleixner wrote:
> On Thu, 12 Mar 2009, Ingo Molnar wrote:
> > * Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > > On Thu, 2009-03-12 at 00:55 -0700, Darren Hart wrote:
> > > > The futex code uses double_lock_hb() which locks the hb->lock's in pointer
> > > > value order.  There is no parallel unlock routine, and the code unlocks them
> > > > in name order, ignoring pointer value.  This opens up a window for an ABBA
> > > > deadlock.  This patch adds double_unlock_hb() to remove the window as well
> > > > as refactor the duplicated code segments.
> > > While I don't mind the patch per-se, I'm hard pressed to see 
> > > any deadlock potential in the unordered unlock.
> > >
> > > All sites (at least those in the patch) always release both 
> > > locks without taking another in between, therefore one would 
> > > think there's no deadlock possible.
> > yeah.
>
> I can't see a deadlock either.

Right, sorry, it's the double_lock that requires the test.  Duh.  I need 
to find a way to do some of this work during more regular hours I guess 
;-)  Thanks for the catch everyone.

Ingo shall I resubmit?  Or did you already clean it up?

Thanks,

Darren

> > The patch is still nice (as you mention), it factors out the 
> > unlock sequence. I'll change the commit message accordingy.
>
> We do not need the comparison magic. Can we just put the code into
> double_unlock_hb() which gets replaced ?
>
> i.e:
>
>         spin_unlock(&hb1->lock);
>         if (hb1 != hb2)
>                 spin_unlock(&hb2->lock);
>
> This code is confusing enough.
>
> Thanks,
>
> 	tglx


--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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