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

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Message-ID: <20140816175002.GA24994@redhat.com>
Date:	Sat, 16 Aug 2014 19:50:02 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	umgwanakikbuti@...il.com, 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 08/16, 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.

Oleg.

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