Valdis Klētnieks, Thank you for your kind explanation.
Okay, so 'timer is running' means it is not expired yet.
and when timer is expired (as specified in 'expires' field),
the timer_callback is called.
and there can be race condition in running timer_callback.
so locking is needed before re-registering timer.
2021년 5월 5일 (수) 오전 1:17, Valdis Klētnieks <valdis.kletnieks@xxxxxx>님이 작성:
>
> On Tue, 04 May 2021 23:59:12 +0900, Hyeonggon Yoo said:
> > Does del_timer work well for timers that re-registers itself?
> > what if the timer is currently running, and del_timer is called,
> > and the running timer re-registers itself?
>
> Minor nit: while the timer is running, there's no problem.
> When the timer has *expired* and timer_callback() is running
> is when you have a race condition.
>
> So who's going to call del_timer()? The only thing that does that is
> timer_exit. Soo... Apply some silly locking:
>
> static int exiting = 0;
>
> void timer_callback(struct timer_list *timer) {
> struct timer_data *data = from_timer(data, timer, timer);
> data->value++;
> printk(KERN_INFO "[%s] value is = %d\n", __func__, data->value);
> if (!exiting)
> mod_timer(timer, jiffies + DELAY);
> }
>
> void __exit timer_exit(void) {
> exiting = 1;
> int ret = del_timer(&data.timer);
> printk("[%s] deleting timer..., ret = %d\n", __func__, ret);
> }
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