tty/serial: at91: correct the usage of tasklet

The tasklet may be scheduled and executed after serial port
was shutdown, for example, DMA rx callback will schedule the
tasklet while serial port is shutting down, especially serial
port is sending and receiving data in a higher baud rate and
it's killed by external program. In this case, tasklet_kill
can only clear the current scheduling out, so tasklet should
be disabled to prevent being executed in later scheduling.
Otherwise, the tasklet executed after serial port was shutdown
can lead to kernel crash.
Signed-off-by: Leilei Zhao <leilei.zhao@atmel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/atmel_serial.c

@@ -1755,6 +1755,8 @@ static int atmel_startup(struct uart_port *port)
 	if (retval)
 		goto free_irq;
 
+	tasklet_enable(&atmel_port->tasklet);
+
 	/*
 	 * Initialize DMA (if necessary)
 	 */
@@ -1858,6 +1860,7 @@ static void atmel_shutdown(struct uart_port *port)
 	 * Clear out any scheduled tasklets before
 	 * we destroy the buffers
 	 */
+	tasklet_disable(&atmel_port->tasklet);
 	tasklet_kill(&atmel_port->tasklet);
 
 	/*
@@ -2251,6 +2254,7 @@ static int atmel_init_port(struct atmel_uart_port *atmel_port,
 
 	tasklet_init(&atmel_port->tasklet, atmel_tasklet_func,
 			(unsigned long)port);
+	tasklet_disable(&atmel_port->tasklet);
 
 	memset(&atmel_port->rx_ring, 0, sizeof(atmel_port->rx_ring));