Re: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock_t

[Thomas Gleixner <tglx@linutronix.de>]

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.

Thanks,

	tglx