Re: [PATCH v2 2/3] time,signal: protect resource use statistics with seqlock

[Rik van Riel <riel@redhat.com>]

On 08/18/2014 12:44 AM, Mike Galbraith wrote:
> On Sat, 2014-08-16 at 19:50 +0200, Oleg Nesterov wrote:
>> On 08/16, Rik van Riel wrote:
>>>
>>> +	do {
>>> +		seq = nextseq;
>>> +		read_seqbegin_or_lock(&sig->stats_lock, &seq);
>>> +		times->utime = sig->utime;
>>> +		times->stime = sig->stime;
>>> +		times->sum_exec_runtime = sig->sum_sched_runtime;
>>> +
>>> +		for_each_thread(tsk, t) {
>>> +			task_cputime(t, &utime, &stime);
>>> +			times->utime += utime;
>>> +			times->stime += stime;
>>> +			times->sum_exec_runtime += task_sched_runtime(t);
>>> +		}
>>> +		/* If lockless access failed, take the lock. */
>>> +		nextseq = 1;
>>
>> Yes, thanks, this answers my concerns.
>>
>> Cough... can't resist, and I still think that we should take rcu_read_lock()
>> only around for_each_thread() and the patch expands the critical section for
>> no reason. But this is minor, I won't insist.
>
> Hm.  Should traversal not also disable preemption to preserve the error
> bound Peter mentioned?

The second traversal takes the spinlock, which automatically
disables preemption.