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Date: Mon, 13 Mar 2017 15:25:52 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
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Subject: Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism
In-Reply-To: <20170313092542.GJ3343@twins.programming.kicks-ass.net>
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References: <20170304092717.762954142@infradead.org> <20170304093559.696873055@infradead.org> <alpine.DEB.2.20.1703071526110.3584@nanos> <20170313092542.GJ3343@twins.programming.kicks-ass.net>
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On Mon, 13 Mar 2017, Peter Zijlstra wrote:

> On Tue, Mar 07, 2017 at 03:31:50PM +0100, Thomas Gleixner wrote:
> > On Sat, 4 Mar 2017, Peter Zijlstra wrote:
> > 
> > > The problem with returning -EAGAIN when the waiter state mismatches is
> > > that it becomes very hard to proof a bounded execution time on the
> > > operation. And seeing that this is a RT operation, this is somewhat
> > > important.
> > > 
> > > While in practise it will be very unlikely to ever really take more
> > > than one or two rounds, proving so becomes rather hard.
> > 
> > Oh no. Assume the following:
> > 
> > T1 and T2 are both pinned to CPU0. prio(T2) > prio(T1)
> > 
> > CPU0
> > 
> > T1 
> >   lock_pi()
> >   queue_me()  <- Waiter is visible
> > 
> > preemption
> > 
> > T2
> >   unlock_pi()
> >     loops with -EAGAIN forever
> 
> So this is true before the last patch; but if we look at the locking
> changes brought by that (pasting its changelog here):

I was referring to the state before the last patch and your wording in the
changelog of this being very unlikely.

Thanks,

	tglx