What is PTHREAD_MUTEX_ADAPTIVE_NP

Where can I find documentation for "adaptive" pthread mutexes? The symbol PTHREAD_MUTEX_ADAPTIVE_NP is defined on my system, but the only documentation I can find online says nothing about what an adaptive mutex is, or when it's appropriate to use.

So... what is it, and when should I use it?

For reference, my version of libc is:

GNU C Library (Ubuntu EGLIBC 2.15-0ubuntu10.5) stable release version 2.15, by Roland McGrath et al.
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.
There is NO warranty; not even for MERCHANTABILITY or FITNESS FOR A
PARTICULAR PURPOSE.
Compiled by GNU CC version 4.6.3.
Compiled on a Linux 3.2.50 system on 2013-09-30.
Available extensions:
    crypt add-on version 2.1 by Michael Glad and others
    GNU Libidn by Simon Josefsson
    Native POSIX Threads Library by Ulrich Drepper et al
    BIND-8.2.3-T5B
libc ABIs: UNIQUE IFUNC
For bug reporting instructions, please see:
<http://www.debian.org/Bugs/>.

and "uname -a" gives

Linux desktop 3.2.0-55-generic #85-Ubuntu SMP Wed Oct 2 12:29:27 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux

Solution

Here you go. As I read it, it's a brutally simple mutex that doesn't care about anything except making the no-contention case run fast.