linux-kernel - Re: [PATCH 6/6] futex: fix miss ordered wakeups

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Message-ID: <alpine.LFD.1.10.0806252053400.3297@apollo.tec.linutronix.de>
Date:	Wed, 25 Jun 2008 21:06:33 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Walker <dwalker@...sta.com>
cc:	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] futex: fix miss ordered wakeups

On Wed, 25 Jun 2008, Daniel Walker wrote:
> On Wed, 2008-06-25 at 18:17 +0200, Peter Zijlstra wrote:
> > Also, the main point was about mixing user and kernel space state, you
> > still do so by including the futex waiter in the same union. That's a
> > fundamental fugly - no matter if you can make it work.
> 
> I don't think it's ugly at all, but I'm open to suggestion for alternate
> methods of implementing it .. I don't need to unify the blocked_on
> structures, but it does allow for some nice things like reducing the
> size of the task struct, and potentially later doing PI across different
> API's.

Just get it. Mixing concurrency controls and futex waiters is
fundamentally wrong. A task can be blocked on exactly one concurrency
control, but it can be on a futex waiter _AND_ then block on a
concurrency control. 

Unifying the mutex and the rtmutex blocked_on is semantically correct
and is a worthwhile thing to do, but adding the futex waiter to it is
simply a semantical brain fart which can not be excused by reducing
the size of task struct.

No thanks,

	tglx
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