linux-kernel - Re: futex(2) man page update help request

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Date:	Fri, 23 Jan 2015 19:19:05 +0100
From:	Torvald Riegel <triegel@...hat.com>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Carlos O'Donell" <carlos@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Jakub Jelinek <jakub@...hat.com>,
	"linux-man@...r.kernel.org" <linux-man@...r.kernel.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Arnd Bergmann <arnd@...db.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux API <linux-api@...r.kernel.org>,
	Darren Hart <dvhart@...radead.org>,
	Anton Blanchard <anton@...ba.org>, Petr Baudis <pasky@...e.cz>,
	Eric Dumazet <edumazet@...gle.com>,
	bill o gallmeister <bgallmeister@...il.com>,
	Jan Kiszka <jan.kiszka@...mens.com>,
	Daniel Wagner <wagi@...om.org>, Rich Felker <dalias@...c.org>
Subject: Re: futex(2) man page update help request

On Fri, 2015-01-16 at 16:46 -0800, Darren Hart wrote:
> On 1/16/15, 12:54 PM, "Michael Kerrisk (man-pages)"
> <mtk.manpages@...il.com> wrote:
> 
> >Color me stupid, but I can't see this in futex_requeue(). Where is that
> >check that is "independent of the requeue type (normal/pi)"?
> >
> >When I look through futex_requeue(), all the likely looking sources
> >of EINVAL are governed by a check on the 'requeue_pi' argument.
> 
> 
> Right, in the non-PI case, I believe there are valid use cases: move to
> the back of the FIFO, for example (OK, maybe the only example?).

But we never guarantee a futex is a FIFO, or do we?  If we don't, then
such a requeue could be implemented as a no-op by the kernel, which
would sort of invalidate the use case.

(And I guess we don't want to guarantee FIFO behavior for futexes.)


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