What is the likely concurrency issue with the static grid in the above code and how might it 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();
    }
}