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

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Message-ID: <CALCETrU-SMcGuAGkJGYh-Vu9r3dcDU5hxNVO7GqWrVu1XE6oCQ@mail.gmail.com>
Date:	Sun, 3 Apr 2016 06:18:36 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Thomas Gleixner <tglx@...utronix.de>
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, 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?

Why can't private futexes be attached by default?

--Andy