If I can drill a hole to mantle, does it form a volcano?
As Bon says, the mantle is solid, albeit somewhat viscous / plastic under the prevailing pressure and temperature conditions, so your hypothetical borehole isn't going to puncture any liquid magma chamber. Could it liquify if suddenly depressurized in a borehole? I doubt it, although it might do so if there was a local concentration of volatiles to depress the melting temperature. 

But the question is forever going to be hypothetical because there is no drill string capable of cutting into the lower crust, let alone the upper mantle. The heat would soften the drilling bit to the point where it was like putty. Also, the lateral rock pressure would be so high that the borehole walls would implode.