The read threads block because the ReadWriteLock algorithm tries to prevent thread starvation (i.e. when the exclusive lock never gets acquired). Most ReadWriteLock implementation alternate between giving the read locks then the exclusive locks access to the lock.
of course - SRW lock allow shared access only if no waiters ( request to exclusive access) on lock. so even if lock in shared mode, new shared request can block, if was waiter(exclusive) already. in case OP no exactly this, but anyway - i be say that shared access is only hint to system, that it can optimize access - and allow multiple shared threads inside lock. but this was not always
