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

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Date:	Sat, 24 Jan 2015 11:05:29 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Torvald Riegel <triegel@...hat.com>
cc:	Darren Hart <dvhart@...ux.intel.com>,
	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>,
	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, 23 Jan 2015, Torvald Riegel wrote:

> 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.)

The (current) behaviour is:

    real-time threads:   FIFO per priority level
    sched-other threads: FIFO independent of nice level

The wakeup is priority ordered. Highest priority level first.

Thanks,

	tglx



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