Java Concurrency With Static Arrays

Given the code segment below, what is the primary concurrency issue related to the static grid variable, and how can this issue be resolved?

public class StaticMatrix {
    private static int[][] grid = new int[3][3];

    public static void updateGrid(int value) {
        for (int i = 0; i < grid.length; i++)
            for (int j = 0; j < grid[i].length; j++)
                grid[i][j] = value;
    }

    public static int readCell(int i, int j) {
        return grid[i][j];
    }

    public static void main(String[] args) {
        Thread t1 = new Thread(() -> StaticMatrix.updateGrid(5));
        Thread t2 = new Thread(() -> {
            System.out.println(StaticMatrix.readCell(1,1));
        });
        t1.start();
        t2.start();
    }
}