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

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Message-ID: <CAKgNAkjQ5Dd_U9OojXdgeforpRevvPHNFAw99kBFPCwHgf7Ggg@mail.gmail.com>
Date:	Thu, 15 May 2014 06:53:38 +0200
From:	"Michael Kerrisk (man-pages)" <mtk.manpages@...il.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	"Carlos O'Donell" <carlos@...hat.com>,
	Darren Hart <dvhart@...ux.intel.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>
Subject: Re: futex(2) man page update help request

Hi Thomas,

On Thu, May 15, 2014 at 1:34 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 14 May 2014, Carlos O'Donell wrote:
>
>> On 05/14/2014 03:03 PM, Michael Kerrisk (man-pages) wrote:
>> >> 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(). (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*".)
>>
>> I fully agree with Michael here.
>>
>> The futex() syscall was never exposed to userspace specifically because
>> it was an interface we did not want to support forever with a stable ABI.
>> The futex() syscall is an implementation detail that is shared between
>> the kernel and the writers of core runtimes for Linux.
>
> Nonsense.
>
> If we change that interface (aside of adding functionality or some new
> error return) it would break the world and some more, simply because
> out of the blue glibc-2.xx would stop to work on linux-3.yy.
>
> Aside of that the futex syscall is used as a bare interface without
> any glibc interaction:
>
>  - It's handy to implement user space wait queues
>
>  - It's (ab)used in very interesting ways by data base apps
>
>  - It's (ab)used by some Java monstrosities.

Thanks for the education about user-space uses of futexes. I was unaware.

> Nothing you care about and you really don't want to see the gory
> details, but you have to accept that there is an universe which is
> happy to deal with the raw syscalls instead of going through some ill
> defined posix interfaces.

And that universe would love to have your documentation of
FUTEX_WAKE_BITSET and FUTEX_WAIT_BITSET ;-),

Cheers,

Michael

-- 
Michael Kerrisk
Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
Linux/UNIX System Programming Training: http://man7.org/training/
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