Re: For review: nptl(7) man page

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

On 07/24/2015 05:51 PM, Nicholas Miell wrote:
> PTHREAD_PROCESS_SHARED says any thread with access to the memory containing
> the mutex can operate on the mutex and POSIX basically ignores the idea
> that different processes could be running completely incompatible
> executables or whatever.
> 
> pthread_mutex_t has a bunch of #ifdefs in the middle of it that change the
> structure size and layout between i386 and x86_64.
> 
> Most importantly, the positions of the __nusers and __kind fields are
> swapped (this looks to be an oversight dating back to 2003 when __nusers
> was first introduced and carefully preserved when the separate i386 and
> x86_64 versions of pthreadtypes.h were merged into the single x86 version),
> which means that when the lock and unlock functions attempt to figure out
> what kind of mutex it is (recursive/adaptive/whatever), they'll look at the
> wrong field if the mutex is from the wrong architecture and then things
> will break.
> 
> And then there's the fact that the rest of the struct is a union in the
> 32-bit version and flat in the 64-bit version, but that could have been
> worked around if you put a flag in the __kind field that tells the 64-bit
> pthread library that it is looking at a 32-bit mutex.

Thanks for the additional detail, Nicholas. So, how about a paragraph such 
as the following for the manual page:

       POSIX says that any thread in any process with access to the mem‐
       ory containing a  process-shared  (PTHREAD_PROCESS_SHARED)  mutex
       can  operate  on that mutex.  However, on 64-bit x86 systems, the
       mutex definition for x86-64 is incompatible with the mutex  defi‐
       nition  for  i386,  meaning that 32-bit and 64-bit binaries can't
       share mutexes on x86-64 systems.
?

Cheers,

Michael


> On Fri, Jul 24, 2015 at 12:57 AM, Michael Kerrisk (man-pages) <
> mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:
> 
>> On 03/22/2015 10:38 PM, Nicholas Miell wrote:
>>> On 03/22/2015 07:38 AM, Michael Kerrisk (man-pages) wrote:
>>>> Hello all,
>>>>
>>>> I wrote a short man page documented some of the details of
>>>> NPTL that might affect application developers. Review comments
>>>> would be most welcome.
>>>>
>>>> Thanks,
>>>>
>>>> Michael
>>>
>>> POSIX specifies that process-shared pthread_mutex_t's can be manipulated
>>> by independent processes, however the AMD64 and i386 versions of the
>>> mutex structures are incompatible and cannot be shared.
>>>
>>> This might be true of the other supposedly sharable primitives, I never
>>> bothered to check the rest when I first noticed the problem with mutexes.
>>
>> Nicholas,
>>
>> Can you provide pointers to further information on this topic?
>>
>> Cheers,
>>
>> Michael
>>
>>
>> --
>> Michael Kerrisk
>> Linux man-pages maintainer; http://www.kernel.org/doc/man-pages/
>> Linux/UNIX System Programming Training: http://man7.org/training/
>>
> 


-- 
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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