On Tue, Dec 29, 2009 at 9:15 PM, Robert P. J. Day <rpjday@xxxxxxxxxxxxxx> wrote:
>
> from kernel/timer.c:
>
> /*
> * The 64-bit jiffies value is not atomic - you MUST NOT read it
> * without sampling the sequence number in xtime_lock.
> * jiffies is defined in the linker script...
> */
>
> void do_timer(unsigned long ticks)
> {
> jiffies_64 += ticks;
> update_wall_time();
> calc_global_load();
> }
>
> i'm not sure how to interpret that comment since it clearly suggests
> that you can't simply access jiffies_64, but that's exactly what
> do_timer() is doing in that first line by incrementing it by a certain
> value. can anyone clarify whether the above makes any sense?
with the help of cscope, you could see that its callers: tick_periodic
and tick_do_update_jiffies64 defined in tick-common.c and tick-sched.c
respectively, already grab the seqlock before calling the do_timer and
release it after do_timer is done.
/*
* Periodic tick
*/
static void tick_periodic(int cpu)
{
if (tick_do_timer_cpu == cpu) {
write_seqlock(&xtime_lock); // here....
/* Keep track of the next tick event */
tick_next_period = ktime_add(tick_next_period, tick_period);
do_timer(1);
write_sequnlock(&xtime_lock); // and here....
}
update_process_times(user_mode(get_irq_regs()));
profile_tick(CPU_PROFILING);
}
--
regards,
Mulyadi Santosa
Freelance Linux trainer and consultant
blog: the-hydra.blogspot.com
training: mulyaditraining.blogspot.com
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