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

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Date:	Mon, 19 Jan 2015 11:45:48 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Darren Hart <dvhart@...ux.intel.com>
cc:	"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>,
	Torvald Riegel <triegel@...hat.com>,
	Roland McGrath <roland@...hat.com>,
	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, 16 Jan 2015, Darren Hart wrote:
> On 1/16/15, 12:54 PM, "Michael Kerrisk (man-pages)"
> <mtk.manpages@...il.com> wrote:
> 
> >On 01/16/2015 04:20 PM, Thomas Gleixner wrote:
> >> On Fri, 16 Jan 2015, Michael Kerrisk (man-pages) wrote:
> >> 
> >>> Hello Thomas,
> >>>
> >>> On 01/15/2015 11:23 PM, Thomas Gleixner wrote:
> >>>> On Thu, 15 Jan 2015, Michael Kerrisk (man-pages) wrote:
> >>>>>> [EINVAL] uaddr equal uaddr2. Requeue to same futex.
> >>>>>
> >>>>> ??? I added this, but does this error not occur only for PI requeues?
> >>>>
> >>>> It's equally wrong for normal futexes. And its actually the same code
> >>>> checking for this for all variants.
> >>>
> >>> I don't understand "equally wrong" in your reply, I'm sorry. Do you
> >>> mean:
> >>>
> >>> a) This error text should be there for both normal and PI requeues
> >> 
> >> It is there for both. The requeue code has that check independent of
> >> the requeue type (normal/pi). It never makes sense to requeue
> >> something to itself whether normal or pi futex. We added this for PI,
> >> because there it is harmful, but we did not special case it. So normal
> >> futexes get the same treatment.
> >
> >Hello Thomas, 
> >
> >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?). Both
> tests ensuring uaddr1 != uaddr2 are under the requeue_pi conditional
> block. The second compares the keys in case they are not FUTEX_PRIVATE
> (uaddrs would be different, but still the same backing store).
> 
> Thomas, am I missing a test for this someplace else?

No, I had a short look at the code misread it. So, yes, it's a valid
operation for the non PI case. Sorry for the confusion.

Thanks,

	tglx
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