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

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Date:	Wed, 14 May 2014 15:21:20 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Darren Hart <dvhart@...ux.intel.com>
Cc:	Davidlohr Bueso <davidlohr@...com>, mtk.manpages@...il.com,
	Thomas Gleixner <tglx@...utronix.de>,
	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>,
	"Carlos O'Donell" <carlos@...hat.com>,
	mathieu.desnoyers@...icios.com
Subject: Re: futex(2) man page update help request

On Wed, May 14, 2014 at 02:03:13PM -0700, Darren Hart wrote:
> On 5/14/14, 13:56, "Davidlohr Bueso" <davidlohr@...com> wrote:
> 
> >On Wed, 2014-05-14 at 21:03 +0200, Michael Kerrisk (man-pages) wrote:
> >> Hi Darren,
> >> 
> >> On Wed, May 14, 2014 at 6:18 PM, Darren Hart <dvhart@...ux.intel.com>
> >>wrote:
> >> > On 5/14/14, 3:35, "Michael Kerrisk (man-pages)"
> >><mtk.manpages@...il.com>
> >> > wrote:
> >> >
> >> >>[So, some futex recent discussions remind me I should make this
> >>request]
> >> >>
> >> >>Hello all (especially those in the To:, namely Thomas, Darren, Ingo,
> >> >>Jakub),
> >> >>
> >> >>The futex man pages:
> >> >>http://man7.org/linux/man-pages/man2/futex.2.html
> >> >>http://man7.org/linux/man-pages/man7/futex.7.html
> >> >>are currently in a sorry state. I'm by no means convinced that all of
> >>the
> >> >>futex operations described there are explained fully and correctly.
> >>And
> >> >>probably not all error cases for each operation are properly
> >>documented.
> >> >>I'd be very happy if some folk could review those pages and send me
> >> >>corrections (Git:
> >> >>http://git.kernel.org/pub/scm/docs/man-pages/man-pages).
> >> >>
> >> >>But worse, a number of futex operations remain undocumented in
> >>futex(2)
> >> >>(see the list below).
> >> >>
> >> >>I am aware of Documentation/pi-futex.txt and
> >> >>Documentation/futex-requeue-pi.txt. However, both of those documents
> >> >>are rather thin on details / explain what certain FUTEX_* operations
> >>are
> >> >>used for rather than what they do / focus on the implementation,
> >>rather
> >> >>than the semantics.
> >> >>
> >> >>What I would like is that the futex(2) page documenta each one of
> >> >>these operations with a focus on the semantics in a way that might be
> >> >>useful to writers of library functions or those who simply wish to
> >> >>better understand (from a user-space perspective) what futexes are
> >> >>and how they are used. However, I don't have the knowledge to do
> >> >>this well in any reasonable time.
> >> >>
> >> >>Would the folk in the To: list (or anyone else who is knowledgeable)
> >> >>be willing to write patches
> >> >>(Git: http://git.kernel.org/pub/scm/docs/man-pages/man-pages )
> >> >>or just supply me with some raw text that documents these currently
> >> >>undocumented futex operations, in the manner suggested?
> >> >
> >> > Yes, I'll be glad to help.
> >> 
> >> Thanks!
> >> 
> >> > However, unless I'm sorely mistaken, the larger problem is that glibc
> >> > removed the futex() call entirely, so these man pages don't describe
> >> 
> >> I don't think futex() ever was in glibc--that's by design, and
> >> completely understandable: no user-space application would want to
> >> directly use futex().
> >
> >That's actually not quite true. There are plenty of software efforts out
> >there that use futex calls directly to implement userspace serialization
> >mechanisms as an alternative to the bulky sysv semaphores. I worked
> >closely with an in-memory DB project that makes heavy use of them. Not
> >everyone can simply rely on pthreads.
> 
> Userspace RCU from Mattieu Desnoyers / Efficios uses it as well - or did.

It still does.  ;-)

							Thanx, Paul

> >> (BTW, I mispoke in my earlier mail when I said I
> >> wanted documentation suitable for "writers of library functions" -- I
> >> meant suitable for "writers of *C library*".)
> >
> >While kind of beyond the scope of documentation like manpages, I would
> >very much welcome this! The internet is full of
> >misleading/false/stupid/incomplete references to futexes and how to use
> >them. Having a best practises helps the kernel directly avoiding users
> >from misusing them.
> 
> Agreed.
> 
> -- 
> Darren Hart					Open Source Technology Center
> darren.hart@...el.com				            Intel Corporation
> 
> 
> 

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