Re: Revised futex(2) man page for review

["Michael Kerrisk (man-pages)" <mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

Hi David,

On 07/28/2015 05:16 AM, Davidlohr Bueso wrote:
> On Mon, 2015-07-27 at 13:10 +0200, Michael Kerrisk (man-pages) wrote:
>> Hi David,
>>
>> On 03/31/2015 04:45 PM, Davidlohr Bueso wrote:
>>> On Sat, 2015-03-28 at 12:47 +0100, Peter Zijlstra wrote:
>>>
>>>>        The condition is represented by the futex word, which is an address  in
>>>>        memory  supplied to the futex() system call, and the value at this mem‐
>>>>        ory location.  (While the virtual addresses for the same memory in sep‐
>>>>        arate  processes  may  not be equal, the kernel maps them internally so
>>>>        that the same memory mapped in different locations will correspond  for
>>>>        futex() calls.)
>>>>
>>>>        When  executing  a futex operation that requests to block a thread, the
>>>>        kernel will only block if the futex word has the value that the calling
>>>
>>> Given the use of "word", you should probably state right away that
>>> futexes are only 32bit.
>>
>> So, I made the opening sentence here:
>>
>>        The  condition  is  represented  by  the  futex word, which is an
>>        address in memory supplied to the futex() system  call,  and  the
>>        32-bit  value  at  this  memory  location. 
>>
>> Okay?
> 
> I think we can still improve :)
> 
> I've re-read the whole first paragraphs, and have a few comments that
> touch upon this specific wording. Lets see. You have:
> 
>>        The  futex()  system call provides a method for waiting until a certain
>>        condition becomes true.  It is typically used as a  blocking  construct
>>        in the context of shared-memory synchronization: The program implements
>>        the majority of the synchronization in user  space,  and  uses  one  of
>>        operations  of  the  system call when it is likely that it has to block
>>        for a longer time until the condition becomes true.  The  program  uses
>>        another  operation of the system call to wake anyone waiting for a par‐
>>        ticular condition.
> 
> I've rephrased the next paragraph. How about adding this to follow?
> 
>        A futex is in essence a 32-bit user-space address. All futex operations and
>        conditions are governed by this variable, from now on referred to as 'futex
>        word'. As such, a futex is identified by the address in shared memory, which
>        may or may not be shared between different processes. For virtual memory, the
>        kernel will internally handle and resolve the later. This futex word, along
>        with the value at its the memory location, are supplied to the futex() system
>        call.
> 
> Feel free to reword however you think is better.


I agree with you that that second paragraph is a bit broken. But, like Heinrich,
I'm confused by this term "32-bit ... address".

I've rewritten the paragraph as:

       A futex is a 32-bit value—referred to below as a futex word—whose
       address is supplied to the futex()  system  call.   (Futexes  are
       32-bits in size on all platforms, including 64-bit systems.)  All
       futex operations are governed by this value.  In order to share a
       futex  between  processes,  the  futex  is  placed in a region of
       shared memory, created using (for example) mmap(2)  or  shmat(2).
       (Thus the futex word may have different virtual addresses in dif‐
       ferent processes, but these addresses all refer to the same loca‐
       tion in physical memory.)

Maybe you still have some further improvements for the paragraph?

Cheers,

Michael

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