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

Ingo Molnar
2009-03-12T10:11:14Z

* 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.

The patch is still nice (as you mention), it factors out the
unlock sequence. I'll change the commit message accordingy.

Ingo