linux-kernel - Re: [PATCH -v5 14/14] futex: futex_unlock

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Date:   Mon, 13 Mar 2017 15:25:52 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     mingo@...nel.org, juri.lelli@....com, rostedt@...dmis.org,
        xlpang@...hat.com, bigeasy@...utronix.de,
        linux-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
        jdesfossez@...icios.com, bristot@...hat.com, dvhart@...radead.org
Subject: Re: [PATCH -v5 14/14] futex: futex_unlock_pi() determinism

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