field calculation trigger concurrency issue

Question

Does this trigger have concurrency issues? Will I always get unique priorities? Is the trigger run sequentially per row?

create trigger priority_SuperRuleSamples before insert on SuperRuleSamples 
FOR EACH ROW 
SET NEW.Priority=coalesce(NEW.Priority, 
  coalesce(
  (select Priority from SuperRuleSamples order by Priority desc limit 1), 
  -1
  )+1
)