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

Thomas Gleixner
2009-03-12T11:00:25Z

On Thu, 12 Mar 2009, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@infradead.org> 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.

> 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