Procedures vary from airline to airline, but the final decision always rests with the pilot in command. If he has reason to believe that there is an actual fire on board, an immediate descent, landing and evacuation is called for. If there is reason to believe that the smoke is from an alternate source, dumping fuel and other standard procedures come into play. Every situation is different and it is very easy to play monday morning quarterback with the benefit of hindsight.
"The A340-300 may boast a long range, but the A340 is underpowered" -- Robert Milton, CEO - Air Canada