2012/11/28 Hakan Akkan <hakanak...@gmail.com>:
> +static int check_drop_timer_duty(int cpu)
> +{
> + int curr_handler, prev_handler, new_handler;
> + int nrepeat = -1;
> + bool drop_recheck;
> +
> +repeat:
> + WARN_ON_ONCE(++nrepeat > 1);
> + drop_recheck = false;
> + curr_handler = cpu;
> + new_handler = TICK_DO_TIMER_NONE;
> +
> +#ifdef CONFIG_CPUSETS_NO_HZ
> + if (atomic_read(&nr_cpus_user_nohz) > 0) {
Note atomic_read() is not SMP ordered. If another CPU does
atomic_add() or atomic_add_return(), you may not see the new value as
expected with atomic_read(). Doing atomic_add_return(0, &atomic_thing)
returns the fully ordered result.
You also need to do that to ensure full ordering against
tick_cpu_sched.user_nohz.
On the current layout we have:
(Write side) (Read side)
ts->user_nohz = 1;
atomic_inc(&nr_cpus_user_nohz)
if
(atomic_read(&nr_cpus_user_nohz))
if
(per_cpu(tick_cpu_sched, curr_handler).user_nohz)
....
If you want to make sure that you see the expected value on user_nohz
from the read side, you need to correctly order the write and read
against nr_cpus_user_nohz.
For this you can use atomic_add_return() which implies the full barrier:
(Write side) (Read side)
ts->user_nohz = 1;
atomic_inc_return(&nr_cpus_user_nohz)
if
(atomic_add_return(0, &nr_cpus_user_nohz))
if
(per_cpu(tick_cpu_sched, curr_handler).user_nohz)
....
I have much more comments on this patch, I will come back on this soon.
Thanks.
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