  Dislocations are shown to be smooth at zero temperature because of the
effective Coulomb-type interaction between kinks. Crossover to finite
temperature rougnehing is suggested to be a mechanism responsible for the
softening of \he4 shear modulus recently observed by Day and Beamish (Nature,
{\bf 450}, 853 (2007)). We discuss also that strong suppresion of superfuidity
along the dislocation core by thermal kinks can lead to locking in of the
mechanical and superfluid responses.
