It is useful to have RwLock prefer writers over readers since this avoids writer starvation if there is a continuous stream of overlapping readers. To make this work, new readers need to block if there is a pending writer, even if they could acquire a read lock directly (i.e. the lock is currently owned by existing readers). The downside is that if you try to acquire a read lock when you already own one in the current thread, you could deadlock if there is a writer currently waiting on the lock.
read_recursive (I don't mind changing the name) simply changes the behavior to always acquire a read lock if possible, even if there is a pending writer. The downside, as mentioned before, is that it may lead to writer starvation.