/usr/share/man/man3/pthread_mutex_lock.3.gz is in glibc-doc 2.19-0ubuntu6.14.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/man/man3/pthread_mutex_init.3.gz
/usr/share/man/man3/pthread_mutex_init.3.gz