[Bug nptl/21119] New: Unify the pthread_mutex_t definitions

["hyc at symas dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=21119

            Bug ID: 21119
           Summary: Unify the pthread_mutex_t definitions
           Product: glibc
           Version: 2.23
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: nptl
          Assignee: unassigned at sourceware dot org
          Reporter: hyc at symas dot com
                CC: drepper.fsp at gmail dot com
  Target Milestone: ---

Currently the size of a pthread_mutex_t structure can be one of 3 different
sizes, depending on the CPU architecture and ABI. E.g., from
/usr/include/pthreadtypes.h:

#ifdef __x86_64__
# if __WORDSIZE == 64

#  define __SIZEOF_PTHREAD_MUTEX_T 40

# else

#  define __SIZEOF_PTHREAD_MUTEX_T 32

# endif
#else

# define __SIZEOF_PTHREAD_MUTEX_T 24

#endif

This limits the usefulness of process-shared mutexes, as they cannot safely be
used by both 32bit and 64bit processes at the same time.

It Would Be Nice if a cross-arch-compatible definition could be made. Since the
guts are all behind a futex syscall anyway this shouldn't be a huge problem,
should it?

Presumably this is also an issue for process-shared rwlocks.

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