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SubjectRe: [PATCH] timers: fix synchronization rules in comments of del_timer_sync
Hello maintainers,

In order to further prove the del_timer_sync() could stop the timer that
restart itself in its timer handler, I wrote the following kernel module
whoes part of code is shown below:

=================================================================

struct timer_list my_timer;
static void my_timer_callback(struct timer_list *timer);
static void start_timer(void);

static void start_timer(void){
del_timer(&my_timer);
my_timer.expires = jiffies+HZ;
my_timer.function = my_timer_callback;
add_timer(&my_timer);
}

static void my_timer_callback(struct timer_list *timer){
printk("In my_timer_function");
printk("the jiffies is %ld\n",jiffies);
start_timer();
}

static int __init del_timer_sync_init(void)
{
int result;
printk("my_timer will be create.\n");
printk("the jiffies is :%ld\n", jiffies);
timer_setup(&my_timer,my_timer_callback,0);
result = mod_timer(&my_timer,jiffies + SIXP_TXDELAY);
printk("the mod_timer is :%d\n\n",result);
return 0;
}

static void __exit del_timer_sync_exit(void)
{
int result=del_timer_sync(&my_timer);
printk("the del_timer_sync is :%d\n\n", result);
}

=================================================================

The timer handler is running from interrupts and del_timer_sync() could stop
the timer that rewind itself in its timer handler, the result is shown below:

# insmod del_timer_sync.ko
[ 103.505857] my_timer will be create.
[ 103.505922] the jiffies is :4294770832
[ 103.506845] the mod_timer is :0
[ 103.506845]
# [ 103.532389] In my_timer_function
[ 103.532452] the jiffies is 4294770859
[ 104.576768] In my_timer_function
[ 104.577096] the jiffies is 4294771904
[ 105.600941] In my_timer_function
[ 105.601072] the jiffies is 4294772928
[ 106.625397] In my_timer_function
[ 106.625573] the jiffies is 4294773952
[ 107.648995] In my_timer_function
[ 107.649212] the jiffies is 4294774976
[ 108.673037] In my_timer_function
[ 108.673787] the jiffies is 4294776001
rmmod del_timer_sync.ko
[ 109.649482] the del_timer_sync is :1
[ 109.649482]
#

The root cause is shown below:

do {
ret = try_to_del_timer_sync(timer);

if (unlikely(ret < 0)) {
del_timer_wait_running(timer);
cpu_relax();
}
} while (ret < 0);

https://elixir.bootlin.com/linux/latest/source/kernel/time/timer.c#L1381

If we call another thread such as a work_queue or the code in other places
to restart the timer instead of in its timer handler, the del_timer_sync()
could not stop it. So, I think the comments should be changed to "Callers
must prevent restarting of the timer in other places except for the timer's
handler".

Best regards,
Duoming Zhou
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