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


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