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

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Message-ID: <1422104287.29655.13.camel@triegel.csb>
Date:	Sat, 24 Jan 2015 13:58:07 +0100
From:	Torvald Riegel <triegel@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
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>,
	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>,
	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 Sat, 2015-01-24 at 11:05 +0100, Thomas Gleixner wrote:
> 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.

OK.

But, just to be clear, do I correctly understand that you do not want to
guarantee FIFO behavior in the specified futex semantics?  I think there
are cases where being able to *rely* on FIFO (now and on all future
kernels) would be helpful for users (e.g., on POSIX/C++11 condvars and I
assume in other ordered-wakeup cases too) -- but at the same time, this
would constrain future futex implementations.

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