  In this study, a new linear theory of tearing instability is shown, where the
modified LSC (Loureiro, Schekochihin, and Cowley) theory [36] developed from
the original LSC theory [8] is extended from inviscid-resistive MHD to
viscous-resistive MHD. In contrast to FKR [2] and original LSC theories, the
upstream open boundary condition is implemented at a finite point $\xi_c$,
which is an additional control parameter to determine the solutions. This paper
firstly studies when the resistivity and viscosity are uniform in space. In
addition, some variations in the nonuniformity are studied. It is shown that
the non-uniformity can enhance the linear growth rate, rather than uniform
case. Unexpectedly, this suggests that the forward cascade process of plasmoid
instability (PI) does not stop, i.e., the finite differential MHD simulations
fail. To stop the forward cascade, uniform viscosity is required not only in
the inner region of the current sheet but also in the outer region. In the
uniform case, the critical condition is predicted to be $2P_m/(S
\xi_{c})=0.06$, beyond which the tearing instability, i.e., the forward
cascade, stops. Here, $S$ is the Lundquist number, $P_m$ is the magnetic
Prandtl number, and $\xi_{c}$ is the distance between the upstream open
boundary and neutral sheet, where the current sheet thickness is fixed at
$\xi_0=1.307$. According to the critical condition, the resistivity and
viscosity employed in most MHD simulations of PI are too small to stop the
forward cascade. This critical condition may be also applicable for the trigger
problem of the current sheet destabilization in substorms and solar flares.
