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    SubjectRe: [PATCH 5/5] futex: fix miss ordered wakeups
    On Thu, 12 Jun 2008, Daniel Walker wrote:
    > On Thu, 2008-06-12 at 15:33 +0200, Thomas Gleixner wrote:
    > > On Thu, 12 Jun 2008, Daniel Walker wrote:
    > > > On Thu, 2008-06-12 at 10:56 +0200, Thomas Gleixner wrote:
    > > > > Please stop wasting everyone's time with that.
    > > >
    > > > It achieves correct ordering of the futex waiters inside the kernel,
    > > > that is in fact _something_ ..
    > >
    > > Yeah, just something _useless_
    >
    > Just because you don't use it, doesn't make it useless .. At least
    > there's enough people asking for this that it warrants me writing it..

    Which is not really a good technical reason to merge such a
    patch. Your handwaving about "enough people" is just irrelevant. Are
    you going to implement a root hole as well when enough people ask for
    it ?

    But there is also a Real Good technical reason why these patches are
    going nowhere else than into /dev/null:

    your approach of hijacking blocked_on is fundamentaly wrong as it
    mixes kernel internal state with user space state.

    It will break in preempt-rt at the point when this state is set and
    the code blocks on a spinlock, which uses the rtmutex based sleeping
    spinlock implementation and overwrites blocked_on.

    If it can acquire the spinlock in the fast path without modifying
    blocked_on it will cause trouble with the priority boosting chain
    when a higher priority task becomes blocked on the spinlock.


    If there would be a real good technical reason to fix this priority
    ordering it could be done with less than 20 lines of code without
    extra locking and wreckage waiting left and right, but I have not yet
    seen a single convincing technical argument or a relevant use case
    which might justify that.

    Thanks,

    tglx


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