Calling mod_timer from inside timer callback function - c

Question

I'm writing a kernel module for linux, and I want my timer to re-set itself. To this end, I thought to call mod_timer from inside the timer's callback function, as shown:
static void sched_send(unsigned long data)
{
send_now();
mod_timer(&test_timer, jiffies+(get_interval()*HZ));
}
static void timer_start(void)
{
set_log_msg("Meep meep!");
test_timer.function = sched_send;
test_timer.expires = jiffies + HZ*get_interval();
}
However, I've read mod_timer deletes the timer and re-adds it. Will it cause problems? If so, is there a better way to create a repeating timer for kernel modules?

Answer 1

It is safe to execute mod_timer from the timer callback.
From the kernel source (kernel/timer.c):
/* mod_timer(timer, expires) is equivalent to:
*
* del_timer(timer); timer->expires = expires; add_timer(timer);
* ...
*/
As for del_timer,
/*
* del_timer - deactive a timer.
* #timer: the timer to be deactivated
*
* del_timer() deactivates a timer - this works on both active and inactive
* timers.
* ...
*/
As noted by Peter, you need to invoke add_timer anytime you want to start/restart the timer.

Answer 2

Your function timer_start() will have to call add_timer() after it sets up the function and the expiration time. Once the timer function triggers, your timer is no longer active, so all you have to do is reset the .expires field to your new value and call add_timer() again. Be sure you provide a clean way to stop rescheduling the timer, for example on module unload.
send_now();
if(!terminate_timer) {
test_timer.expires = jiffies + HZ*get_interval();
add_timer(&test_timer);
}