Make DataGridView cell ReadOnly on RowsAdded event using VB.Net in Windows Forms Application

i have written the code for the update but its not working. when i run the break point debugger it goes to infinite loop.

my update button click code

    Public Sub Updates()
        If con.State = ConnectionState.Open Then
            con.Close()
        End If      
        Dim i As Integer     
        Try        
            For i = 0 To dgv.Rows.Count - 1
                Dim cmd As New SqlClient.SqlCommand("Update tblProductsve set ItemCode=('" & dgv.Rows(i).Cells(1).Value & "'),srno=('" & dgv.Rows(i).Cells(0).Value & "'), ItemDes=('" & dgv.Rows(i).Cells(2).Value & "'), Price=('" & dgv.Rows(i).Cells(3).Value & "'),Qty=('" & dgv.Rows(i).Cells(4).Value & "'),amt=('" & dgv.Rows(i).Cells(5).Value & "'),subtot=('" & txtBSearch.Text & "') where billid=('" & txtBSearch.Text & "'))", con)
                con.Open()
                cmd.ExecuteNonQuery()
                con.Close()              
            Next
            MessageBox.Show("Updated Successully ")           
        Catch ex As Exception
            MsgBox(ex.Message)
        End Try
    End Sub

below code is going to infinite loop

    Private Sub dgv_RowPostPaint(sender As Object, e As DataGridViewRowPostPaintEventArgs) Handles dgv.RowPostPaint
        If con.State = ConnectionState.Open Then
            con.Close()
        End If
        Me.dgv.Rows(e.RowIndex).Cells(0).Value = e.RowIndex + 1.ToString
        dgv.Columns(0).ReadOnly = True
    End Sub

please help me where should i make correction