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

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Date:	Sat, 24 Jan 2015 17:25:05 +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>,
	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, 24 Jan 2015, Torvald Riegel wrote:
> 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.

It would be a constraint, but I don't think it would be a horrible
one. Though I have my doubts, that we can actually guarantee it under
all circumstances.

One thing comes to my mind right away: spurious wakeups. There is no
way that we can guarantee FIFO ordering in the context of those. And
we cannot prevent them either.

Thanks,

	tglx


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