This is an optimization done by linux/glibc to make futexes faster when they’re not shared between processes. Glibc will use the _PRIVATE
versions of each of the futex calls unless the PTHREAD_PROCESS_SHARED
attribute is set on your mutex
It’s explained in more detail here: http://lwn.net/Articles/229668/
For the purposes of your debugging, you can just ignore the _PRIVATE
suffixes