This routine acquires a read-write lock for read access. This
    routine waits for any existing lock holder for write access to
    relinquish its lock before granting the lock for read access.
    This routine returns when the lock is acquired. If the lock is
    already held simply for read access, the lock is granted.

    For each call to tis_read_lock() that successfully acquires the
    lock for read access, a corresponding call to tis_read_unlock()
    must be issued.