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

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Message-ID: <alpine.LFD.1.10.0806130129290.3193@apollo.tec.linutronix.de>
Date:	Fri, 13 Jun 2008 01:30:58 +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 Fri, 2008-06-13 at 00:43 +0200, Thomas Gleixner wrote:
> > On Thu, 12 Jun 2008, Daniel Walker wrote:
> > 
> > > 
> > > On Thu, 2008-06-12 at 21:55 +0200, Thomas Gleixner wrote:
> > > > Also your interpretation of the POSIX requirement is very
> > > > questionable:
> > > > 
> > > >  "If there are threads blocked on the mutex object referenced by mutex
> > > >  when pthread_mutex_unlock() is called, resulting in the mutex
> > > >  becoming available, the scheduling policy shall determine which
> > > >  thread shall acquire the mutex."
> > > 
> > > The key is "scheduling policy" .. When the mutex is un-blocked the next
> > > task to run is the same as if the scheduler was selecting tasks from the
> > > list of blocked tasks .. For Linux, that means the highest priority
> > > tasks should be selected.. So it's no more acceptable for the scheduler
> > > to priority invert some tasks than it is for the futex to do it.
> > 
> > Sigh, when do you actually get a gripe that the default futex
> > implementation does not and can not guarantee that at all and therefor
> > your "correctness" patch is as important as a bag of rice which
> > toopled over in China ?
> 
> Well, the last email I got from Arjan said this,
> 
> ".. Don't look at the release path... look at the acquire path.
> If a thread sees the futex is free, it'll take it, without even going
> to the kernel at all."
> 
> And yes, I understand that fully.

Great. Case closed, nothing to argue about.

Thanks,

	tglx
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