Re: [PATCH 2/3] nohz: Synchronize sleep time stats with memory barriers

[Denys Vlasenko <dvlasenk@redhat.com>]

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.