This was a rare occurence:
Tue Jan 29 01:12:51 2019 pid=531 tid=951 op=In BackendRados::lockBackoff(): radosCtx.lock_exclusive() obj=Frontend-ctafrontend-531-20190128-18:02:44-0 duration=51.5958

(One out of 5 operations taking more than 1s in the whole day.