linux-kernel - Re: + taskstats-add-all-thread-stats-for-taskstats_cmd_attr

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Message-ID: <20070917162627.GB1398@tv-sign.ru>
Date:	Mon, 17 Sep 2007 20:26:27 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Guillaume Chazarain <guichaz@...oo.fr>
Cc:	akpm@...ux-foundation.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	balbir@...ibm.com, jlan@...r.sgi.com, jlim@....com
Subject: Re: + taskstats-add-all-thread-stats-for-taskstats_cmd_attr_tgid.patch added to -mm tree

On 09/15, Guillaume Chazarain wrote:
>
> Le Thu, 13 Sep 2007 16:41:41 +0400,
> Oleg Nesterov <oleg@...sign.ru> a ??crit :
> 
> > > +	do
> > > +		if (!tsk->exit_state)
> > > +			add_tsk(stats, tsk);
> > > +	while_each_thread(first, tsk);
> > 
> > This reminds me. The "!tsk->exit_state" is not good and racy, it
> > can't prevent the double accounting (of course, I don't blame this
> > patch).
> 
> I documented this in v4, but I was wondering if always setting
> tsk->exit_state under the protection of lock_task_sighand() would fix
> the race.

No, this is not enough. There is a window between do_exit()->taskstats_exit()
and do_exit()->exit_notify() which sets ->exit_state. Probably, we can
shift taskstats_exit() into exit_notify() but this is not good.

Oleg.

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