A timer for processing data blocks is proposed for a receiver of a mobile communications system. If the timer is not running, the timer is started based a data block. The data block has a sequence number higher than a sequence number of another data block that was first expected to be received. If the timer is stopped or expires, the timer is based on a highest sequence number of a data block among data blocks that cannot be delivered to a higher entity. The timer can be used to prevent a stall condition in mobile communications.