linux-kernel - Re: [PATCH 4/4] futex: convert hash_bucket locks to raw_spinlock

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Message-ID: <alpine.LFD.2.00.1007121503010.3321@localhost.localdomain>
Date:	Mon, 12 Jul 2010 15:05:30 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <dvhltc@...ibm.com>
cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>,
	Eric Dumazet <eric.dumazet@...il.com>,
	John Kacur <jkacur@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mike Galbraith <efault@....de>, linux-rt-users@...r.kernel.org
Subject: Re: [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.

Thanks,

	tglx
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