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    SubjectRe: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t
    On Fri, 9 Jul 2010, Darren Hart wrote:

    > The requeue_pi mechanism introduced proxy locking of the rtmutex. This creates
    > a scenario where a task can wake-up, not knowing it has been enqueued on an
    > rtmutex. In order to detect this, the task would have to be able to take either
    > task->pi_blocked_on->lock->wait_lock and/or the hb->lock. Unfortunately,
    > without already holding one of these, the pi_blocked_on variable can change
    > from NULL to valid or from valid to NULL. Therefor, the task cannot be allowed
    > to take a sleeping lock after wakeup or it could end up trying to block on two
    > locks, the second overwriting a valid pi_blocked_on value. This obviously
    > breaks the pi mechanism.
    > This patch increases latency, while running the ltp pthread_cond_many test
    > which Michal reported the bug with, I see double digit hrtimer latencies
    > (typically only on the first run after boo):
    > kernel: hrtimer: interrupt took 75911 ns

    Eewwww. There must be some more intelligent and less intrusive way to
    detect this.



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