Re: s/Mutex/ceph::mutex/ and Mutex::is_locked_by_me()

hi Patrick and list, i am trying replace Mutex with ceph::mutex in Ceph. the goal is to deprecate and then remove Mutex and Cond from our project. as we have already ceph::mutex and ceph::condition_variable. it's confusing and, in the long run, i think, it will hurt us as a technical debt. most of the refactory work is quite straightforward, and we can always replace Mutex::is_locked_by_me() with ceph_mutex_is_locked_by_me(). this macro will be expanded to `true` in release build. as we think it will be used only by `ceph_assert()` and `assert()`. but i realized that we are also using Mutex::is_locked_by_me() in functional code which won't be optimised out. for instance, in MDSRank::MDSRank() we check "mds_lock.is_locked_by_me()", if it's true, we just `handle_write_error(r)` without acquiring the lock, otherwise, we will call this function within `mds_lock`. the same applies to `MDSDaemon::handle_conf_change()`. so i am wondering if it's okay to change `mds_lock` to a recursive lock, so we don't need to query this information from the mutex. what do you think?