Re: [PATCH] watchdog: refuse to unload softdog module when its timer is running

[Guenter Roeck <linux@roeck-us.net>]

On 12/17/2015 05:30 AM, roy.qing.li@gmail.com wrote:
> From: Li RongQing <roy.qing.li@gmail.com>
>
> 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>
> Cc: Guenter Roeck <linux@roeck-us.net>
> ---
>   drivers/watchdog/softdog.c | 8 ++++++--
>   1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/watchdog/softdog.c b/drivers/watchdog/softdog.c
> index 0dc5e323d..86ce3f9 100644
> --- a/drivers/watchdog/softdog.c
> +++ b/drivers/watchdog/softdog.c
> @@ -87,6 +87,7 @@ static struct timer_list watchdog_ticktock =
>
>   static void watchdog_fire(unsigned long data)
>   {
> +	module_put(THIS_MODULE);

Don't bother with module_put() here.

Guenter

>   	if (soft_noboot)
>   		pr_crit("Triggered - Reboot ignored\n");
>   	else if (soft_panic) {
> @@ -105,13 +106,16 @@ 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;
>   }
>
>