Re: futex(2) man page update help request

[Michael Kerrisk (man-pages)]

Hi Darren,

On Wed, May 14, 2014 at 6:18 PM, Darren Hart <dvhart@xxxxxxxxxxxxxxx> wrote:
> On 5/14/14, 3:35, "Michael Kerrisk (man-pages)" <mtk.manpages@xxxxxxxxx>
> 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(). (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*".)

> something users even have access to anymore. I had to revert to calling
> the syscalls directly in the futextest test suite because of this:
>
> http://git.kernel.org/cgit/linux/kernel/git/dvhart/futextest.git/tree/inclu
> de/futextest.h#n67
>
>
> Which basically defines:
>
> #define futex(uaddr, op, val, timeout, uaddr2, val3, opflags) \
>         syscall(SYS_futex, uaddr, op | opflags, val, timeout, uaddr2, val3)
>
>
> Adding Carlos for his perspective.
>
> If I'm wrong, or we can restore the futex() call, great. If not... Should
> we keep the man-pages and document it as syscall(SYS_futex, ..., op, ...) ?

We should keep the man page ;-). I just need to add some words to
point out the use of syscall(). Mostly though, I'm interested in
getting documentation of the operations listed below :-).

Cheers,

Michael



>>    commit 4732efbeb997189d9f9b04708dc26bf8613ed721
>>    Author: Jakub Jelinek <jakub@xxxxxxxxxx>
>>    Date:   Tue Sep 6 15:16:25 2005 -0700
>>    See also https://bugzilla.kernel.org/show_bug.cgi?id=14303
>>
>>2.6.18 adds priority inheritance support:
>>FUTEX_LOCK_PI, FUTEX_UNLOCK_PI, and FUTEX_TRYLOCK_PI.
>>    commit c87e2837be82df479a6bae9f155c43516d2feebc
>>    Author: Ingo Molnar <mingo@xxxxxxx>
>>    Date:   Tue Jun 27 02:54:58 2006 -0700
>>
>>    commit e2970f2fb6950183a34e8545faa093eb49d186e1
>>    Author: Ingo Molnar <mingo@xxxxxxx>
>>    Date:   Tue Jun 27 02:54:47 2006 -0700
>>
>>    See Documentation/pi-futex.txt
>>
>>2.6.25 adds FUTEX_WAKE_BITSET, FUTEX_WAIT_BITSET
>>    commit cd689985cf49f6ff5c8eddc48d98b9d581d9475d
>>    Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>>    Date:   Fri Feb 1 17:45:14 2008 +0100
>>
>>2.6.31 adds FUTEX_WAIT_REQUEUE_PI, FUTEX_CMP_REQUEUE_PI
>>    commit 52400ba946759af28442dee6265c5c0180ac7122
>>    Author: Darren Hart <dvhltc@xxxxxxxxxx>
>>    Date:   Fri Apr 3 13:40:49 2009 -0700
>>
>>    commit ba9c22f2c01cf5c88beed5a6b9e07d42e10bd358
>>    Author: Darren Hart <dvhltc@xxxxxxxxxx>
>>    Date:   Mon Apr 20 22:22:22 2009 -0700
>>
>>    See Documentation/futex-requeue-pi.txt
>>
>>Thanks,
>>
>>Michael
>>
>>
>>--
>>Michael Kerrisk
>>Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>>Linux/UNIX System Programming Training: http://man7.org/training/
>>
>
>
> --
> Darren Hart                                     Open Source Technology Center
> darren.hart@xxxxxxxxx                                       Intel Corporation
>
>
>



-- 
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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