Read this,it's from the link you gave me, under the The Serbs during the fighting between Byzantium and the Second Bulgarian Empire section. Then tell me if Petar was of Croatian or Serbian origin. 

During their rule in Rasa the Bulgarians completely exterminated the old Serbian royal family. From 1036 to 1042 Dobroslav, called Stjepan Vojislav, member of an old Croatian family from Duklja, revolted and liberated Duklja and Rasa.