  It is an immediate conclusion from Bavula's papers \cite{Bavula:GWA-def},
\cite{Bavula:GWA-tensor-product} that if a generalized Weyl algebra
$A=\kk[z;\lambda,\eta,\varphi(z)]$ is homologically smooth, then the polynomial
$\varphi(z)$ has no multiple roots. We prove in this paper that the converse is
also true. Moreover, formal deformations of $A$ are studied when $\kk$ is of
characteristic zero.
