linux-kernel - Re: [PATCH 5/5] futex: fix miss ordered wakeups

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Date:	Thu, 12 Jun 2008 17:24:19 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Daniel Walker <dwalker@...sta.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Ulrich Drepper <drepper@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH 5/5] futex: fix miss ordered wakeups

On Thu, 12 Jun 2008, Daniel Walker wrote:
> On Thu, 2008-06-12 at 15:33 +0200, Thomas Gleixner wrote:
> > On Thu, 12 Jun 2008, Daniel Walker wrote:
> > > On Thu, 2008-06-12 at 10:56 +0200, Thomas Gleixner wrote:
> > > > Please stop wasting everyone's time with that.
> > > 
> > > It achieves correct ordering of the futex waiters inside the kernel,
> > > that is in fact _something_ ..
> > 
> > Yeah, just something _useless_
> 
> Just because you don't use it, doesn't make it useless .. At least
> there's enough people asking for this that it warrants me writing it..

Which is not really a good technical reason to merge such a
patch. Your handwaving about "enough people" is just irrelevant. Are
you going to implement a root hole as well when enough people ask for
it ?

But there is also a Real Good technical reason why these patches are
going nowhere else than into /dev/null:

 your approach of hijacking blocked_on is fundamentaly wrong as it
 mixes kernel internal state with user space state.

 It will break in preempt-rt at the point when this state is set and
 the code blocks on a spinlock, which uses the rtmutex based sleeping
 spinlock implementation and overwrites blocked_on.

 If it can acquire the spinlock in the fast path without modifying
 blocked_on it will cause trouble with the priority boosting chain
 when a higher priority task becomes blocked on the spinlock.

If there would be a real good technical reason to fix this priority
ordering it could be done with less than 20 lines of code without
extra locking and wreckage waiting left and right, but I have not yet
seen a single convincing technical argument or a relevant use case
which might justify that.

Thanks,

	tglx
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