共享内存访问CUDA中的垃圾值

I am trying to implement a Navier-Stokes solver in 2D using CUDA. I am using Jacobi's method to solve the system of difference equations. I am dividing the code in 4x4 blocks consisting of 16x16 threads. As every inner point in my matrix (of dimension 64x64) requires its top, bottom, left and right element to compute its new value, I create a new shared matrix of 18x18 dimension for every block. I read all the values into the matrix in this fashion - The thread with indices (0, 0) will write its value into the (1, 1) element in the matrix and will also attempt to read the element above it and the one to its left if this access is not exceeding the boundary. Once this read is done, I update the values of all the internal points and then write them back into memory.I am trying to implement a Navier-Stokes solver