On 04/24/2014 02:16 AM, Frederic Weisbecker wrote:
>> + again:
>> + active = ACCESS_ONCE(ts->idle_active);
>> + smp_rmb();
>> + count = ACCESS_ONCE(ts->idle_sleeptime);
>> + if (active == 1) {
>> + ktime_t delta, start;
>> +
>> + smp_rmb();
>> + start = ACCESS_ONCE(ts->idle_entrytime);
>> + if (start.tv64 == 0) {
>> + /*
>> + * Other CPU is updating the count.
>> + * We don't know whether fetched count is valid.
>> + */
>> + goto again;
>> + }
>> + delta = ktime_sub(now, start);
>> + count = ktime_add(count, delta);
>
> There is still a possibility that you race with an updater here.
> Lets take these initial values, all of which belong to ts(CPU 1):
>
> // idle_sleeptime == 0
> // idle_entrytime == 0
> // ktime_get() == 100
> // idle_active = 1
>
> CPU 0 CPU 1
>
> count = idle_sleeptime // = 0
> tick_nohz_stop_idle();
> tick_nohz_start_idle();
> /* now idle_sleeptime ==
> 100
> and idle_entrytime ==
> 100
> ktime_get() is still
> 100
> and idle_active is
> still 1
> as it has toggled two
> times */
> delta = now - idle_entrytime; // 100 - 100
> count += delta // == 0
>
> Then you get the spurious 0 result.
>
> So memory barriers probably aren't enough here. seqcount would solve the
> issue as it maintains
> update seq tokens.
I think you are right. I'll use your approach (seqcount) in the next version.
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