linux-kernel - Re: [PATCH 3/3] futex: fix miss ordered wakeups

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Message-ID: <alpine.LFD.1.10.0805241039220.3295@apollo.tec.linutronix.de>
Date:	Sat, 24 May 2008 10:55:15 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Walker <dwalker@...sta.com>
cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] futex: fix miss ordered wakeups

On Thu, 22 May 2008, Daniel Walker wrote:

> When the plist was added to futexes it added overhead to sort based
> on priority for the futex waiters. If there is a miss order the value of
> this, from my perspective, is lost. Since we don't re-order tasks
> when their priority is changed after they sleep then we get a miss ordered
> scenerio, and tasks aren't woken in priority order.

This is a solution looking for a problem. 

Normal futexes have no ordering guarantees at all. There is no
mechanism to prevent lock stealing from lower priority tasks. So why
should we care about the once a year case, where a sleepers priority
is modified ?

If you need ordering guarantees then use PI futexes.

> This patch corrects this issue, so the tasks are always woken in priority
> order.

The patch corrects a non issue and introduces lock order issues:

> +void futex_adjust_waiters(struct task_struct *p)
> +{
> +	spin_lock(&p->pi_lock);
> +		spin_lock(&hb->lock);
> ...
> +		spin_unlock(&hb->lock);
> +	}
> +	spin_unlock(&p->pi_lock);
> +}

vs.

> @@ -1155,6 +1191,8 @@ static int futex_wait(u32 __user *uaddr,
{
....
        hb = queue_lock(&q);

> +	spin_lock(&current->pi_lock);
> +	current->blocked_on = &blocked_on;
> +	spin_unlock(&current->pi_lock);

There are more issues vs. pi futexes as well. The simple case of
futex_wait() vs. futex_adjust_waiters will just upset lockdep, but
there are real dealocks vs. unqueue_me_pi waiting.

Thanks,
	tglx

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