From the original change log:
It was possible for a reference to be added even with the lock held, and
for references added just after a lock release to be lost.
This bug was also independently found and reported in wesunsolve.net
issues 6985013 6995524.
In zrl_add(), always use an atomic operation to update the refcount.
The mutex in the ZRL only guarantees that wakeups occur for waiters on the
lock. It offers no protection against concurrent updates of the refcount.
The only refcount transition that is safe to perform without an atomic
operation is from ZRL_LOCKED back to 0, since this can only be performed
by the thread which has the ZRL locked.