Re: futex(2) man page update help request

From: Thomas Gleixner <tglx@linutronix.de>
To: Darren Hart <dvhart@linux.intel.com>
Cc: "Michael Kerrisk (man-pages)" <mtk.manpages@gmail.com>,
	"Carlos O'Donell" <carlos@redhat.com>,
	Ingo Molnar <mingo@elte.hu>, Jakub Jelinek <jakub@redhat.com>,
	"linux-man@vger.kernel.org" <linux-man@vger.kernel.org>,
	lkml <linux-kernel@vger.kernel.org>,
	Davidlohr Bueso <davidlohr.bueso@hp.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Linux API <linux-api@vger.kernel.org>,
	Torvald Riegel <triegel@redhat.com>,
	Roland McGrath <roland@redhat.com>,
	Darren Hart <dvhart@infradead.org>,
	Anton Blanchard <anton@samba.org>, Petr Baudis <pasky@suse.cz>,
	Eric Dumazet <edumazet@google.com>,
	bill o gallmeister <bgallmeister@gmail.com>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	Daniel Wagner <wagi@monom.org>, Rich Felker <dalias@libc.org>
Subject: Re: futex(2) man page update help request
Date: Mon, 19 Jan 2015 11:45:48 +0100 (CET)	[thread overview]
Message-ID: <alpine.DEB.2.11.1501191144320.5526@nanos> (raw)
In-Reply-To: <D0DEECA2.B7EAD%dvhart@linux.intel.com>

On Fri, 16 Jan 2015, Darren Hart wrote:
> On 1/16/15, 12:54 PM, "Michael Kerrisk (man-pages)"
> <mtk.manpages@gmail.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