linux-kernel - Re: futex: priority wakeup miss ordering

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Date:	Thu, 8 May 2008 12:06:18 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Walker <dwalker@...sta.com>
cc:	Pierre Peiffer <peifferp@...il.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: futex: priority wakeup miss ordering

On Wed, 7 May 2008, Daniel Walker wrote:
> 
> from commit ec92d08292d3e9b0823eba138a4564d2d39f25c7
> 
> The wakeup ordering should be in priority order. However if the priority
> of a task is modified while it's waiting on the futex you will get an
> out of order wakeup.
> 
> If you use a priority inheriting mutex this is correct. It's because the
> rtmutex hooks into sched_setscheduler to catch any priority changes,
> then re-queues with the new priority.. But the futex doesn't have this
> hook.
> 
> Was this intentionally left out of this priority wake up patch? I
> thought I would let you know in case you had a specific reason for not
> handling this.. 

IIRC this has been discussed before. The priority ordering of the
plain mutex waiters is an optimization without guarantees of
correctness across a priority change. It's simply not worth the
additional complexity and overhead for that corner case.

Thanks,

	tglx
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