linux-kernel - Re: [patch V2 2/7] futex: Hash private futexes per process

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Date:	Mon, 30 May 2016 16:02:00 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:	Sebastian Andrzej Siewior <sebastian@...akpoint.cc>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Darren Hart <darren@...art.com>,
	Ingo Molnar <mingo@...nel.org>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Davidlohr Bueso <dave@...olabs.net>, Chris Mason <clm@...com>,
	Carlos O'Donell <carlos@...hat.com>,
	Torvald Riegel <triegel@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>
Subject: Re: [patch V2 2/7] futex: Hash private futexes per process

On Mon, May 30, 2016 at 03:59:51PM +0200, Sebastian Andrzej Siewior wrote:
> On 05/30/2016 03:49 PM, Peter Zijlstra wrote:
> >> The same thing can happen within a Task if you take my story from above
> >> and replace task with thread. Completely understood.
> > 
> > Right; so I don't see the point of PREALLOCATE_HASH to cater for RT
> > workloads if it still doesn't guarantee anything, esp. if the failure
> > case is silent and obscure.
> 
> So what do you suggest? Adding trace points in order to learn about
> possible collisions or using tickets (on top of this) to guarantee
> being collision free?

I have no idea about the ticket stuff, i've not seen it. But yes, you
need to somehow guarantee no collisions.