On 07/26/2015 10:27 PM, Nicholas Miell wrote:

Sounds good.

That was quick! Thanks for checking, and thanks for the report, Nicholas!

Cheers,

Michael

On Sun, Jul 26, 2015 at 1:24 PM, Michael Kerrisk (man-pages)
[off-list ref] wrote:

quoted

On 07/24/2015 05:51 PM, Nicholas Miell wrote:

quoted

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

quoted

On Fri, Jul 24, 2015 at 12:57 AM, Michael Kerrisk (man-pages) <
mtk.manpages-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org> wrote:

quoted

On 03/22/2015 10:38 PM, Nicholas Miell wrote:

quoted

On 03/22/2015 07:38 AM, Michael Kerrisk (man-pages) wrote:

quoted

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/

-- 
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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