linux-kernel - Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

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Date:	Thu, 14 Aug 2014 16:24:04 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <peterz@...radead.org>,
	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Sanjay Rao <srao@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>
Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with
	seqlock

On 08/13, Rik van Riel wrote:
>
> @@ -862,11 +862,9 @@ void do_sys_times(struct tms *tms)
>  {
>  	cputime_t tgutime, tgstime, cutime, cstime;
>
> -	spin_lock_irq(&current->sighand->siglock);
>  	thread_group_cputime_adjusted(current, &tgutime, &tgstime);
>  	cutime = current->signal->cutime;
>  	cstime = current->signal->cstime;
> -	spin_unlock_irq(&current->sighand->siglock);

Ah, wait, there is another problem afaics...

thread_group_cputime_adjusted()->cputime_adjust() plays with
signal->prev_cputime and thus it needs siglock or stats_lock to ensure
it can't race with itself. Not sure it is safe to simply take the lock
in cputime_adjust(), this should be checked.

OTOH, do_task_stat() already calls task_cputime_adjusted() lockless and
this looks wrong or I missed something. So perhaps we need a lock in or
around cputime_adjust() anyway.

Oleg.

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