watchdog: refuse to unload softdog module when its timer is running

the softdog has static variables which are accessed if its timer is
still running after the driver is unloaded. and lead to crash:

   $modprobe softdog
   $echo 1 >/dev/watchdog
   $modprobe -r softdog

   CPU 20 Unable to handle kernel paging request at virtual address
   Oops[#1]:
   CPU: 20 PID: 0 Comm: swapper/20 Not tainted 4.1.13-WR8.0.0.0_standard
   ...
   Modules linked in: [last unloaded: softdog]
    ....
   Call Trace:
   [<ffffffff801e142c>] cascade+0x34/0xb0
   [<ffffffff801e1964>] run_timer_softirq+0x30c/0x368
   [<ffffffff80181044>] __do_softirq+0x1ec/0x418
   [<ffffffff801815d0>] irq_exit+0x90/0x98
   [<ffffffff8010749c>] plat_irq_dispatch+0xa4/0x140
   [<ffffffff80152740>] ret_from_irq+0x0/0x4
   [<ffffffff801529e0>] __r4k_wait+0x20/0x40
   [<ffffffff801c2278>] cpu_startup_entry+0x2a0/0x368
   [<ffffffff8015fa64>] start_secondary+0x444/0x4d8

add the module ref when timer is running to avoid to unload the softdog
module

Signed-off-by: Li RongQing <roy.qing.li@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>

drivers/watchdog/softdog.c

@@ -86,6 +86,7 @@ static struct timer_list watchdog_ticktock =
 
 static void watchdog_fire(unsigned long data)
 {
+	module_put(THIS_MODULE);
 	if (soft_noboot)
 		pr_crit("Triggered - Reboot ignored\n");
 	else if (soft_panic) {
@@ -104,13 +105,16 @@ static void watchdog_fire(unsigned long data)
 
 static int softdog_ping(struct watchdog_device *w)
 {
-	mod_timer(&watchdog_ticktock, jiffies+(w->timeout*HZ));
+	if (!mod_timer(&watchdog_ticktock, jiffies+(w->timeout*HZ)))
+		__module_get(THIS_MODULE);
 	return 0;
 }
 
 static int softdog_stop(struct watchdog_device *w)
 {
-	del_timer(&watchdog_ticktock);
+	if (del_timer(&watchdog_ticktock))
+		module_put(THIS_MODULE);
+
 	return 0;
 }