linux-kernel - Re: [RFC patch 4/7] futex: Add support for attached futexes

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Date:	Sun, 3 Apr 2016 17:56:51 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Andy Lutomirski <luto@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	Darren Hart <darren@...art.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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: [RFC patch 4/7] futex: Add support for attached futexes

On Sun, 3 Apr 2016, Andy Lutomirski wrote:

> On Sun, Apr 3, 2016 at 2:57 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Sat, 2 Apr 2016, Andy Lutomirski wrote:
> >
> >> On 04/02/2016 04:09 AM, Thomas Gleixner wrote:
> >> [omitted due to some Thunderbird bug, sigh]
> >>
> >> What happens if you mix attached an non-attached ops on the same futex?
> >
> > Not much. You might get an error code, sleep forever or the call will just
> > result in a NOP wasting cpu cycles. That's the same when you mix
> > shared/private operations on the same futex.
> 
> What's the workflow?
> 
> Can the creation of an attached futex fail due to memory allocation
> problems or any other reason?  If so, how does a library make sure it
> falls back to a normal futex safely?

Well, other operations on futexes can fail as well and the library or the
usage site has to take care of it. It's not that different.

> Why can't private futexes be attached by default?

We _can_ attach any futex - private or not - by default.

Thanks,

	tglx