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

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Date:	Mon, 18 Aug 2014 10:03:32 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Oleg Nesterov <oleg@...hat.com>
CC:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	fweisbec@...il.com, akpm@...ux-foundation.org, srao@...hat.com,
	lwoodman@...hat.com, atheurer@...hat.com
Subject: Re: [PATCH v2 2/3] time,signal: protect resource use statistics with
 seqlock

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.

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