linux-kernel - Re: [PATCH pm-freezer 2/4] freezer: set PF_NOFREEZE on a dying task right before TASK

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Date:	Mon, 29 Aug 2011 20:02:48 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Paul Menage <paul@...lmenage.org>,
	containers@...ts.linux-foundation.org,
	linux-pm@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH pm-freezer 2/4] freezer: set PF_NOFREEZE on a dying
	task right before TASK_DEAD

On 08/29, Tejun Heo wrote:
>
> There's no try_to_freeze() call in the exit path and the only
> necessary guarantee is that freezer doesn't hang waiting for zombies.
> Set PF_NOFREEZE right before setting tsk->state to TASK_DEAD instead.

Agreed.

But I'd like to repeat, this looks "asymmetrical". do_each_thread()
can't see the (auto)reaped tasks after they do exit_notify(). So we
can only see this PF_NOFREEZE if the thread becomes a zombie.

> @@ -1044,6 +1038,10 @@ NORET_TYPE void do_exit(long code)
>
>  	preempt_disable();
>  	exit_rcu();
> +
> +	/* this task is now dead and freezer should ignore it */
> +	current->flags |= PF_NOFREEZE;
> +
>  	/* causes final put_task_struct in finish_task_switch(). */
>  	tsk->state = TASK_DEAD;

May be freezing_slow_path() can check TASK_DEAD along with PF_NOFREEZE
instead? (or tsk->exit_state != 0 to avoid the asymmetry above). Just
to keep this logic in the freezer code. I dunno.

But this all is up to you and Rafael, I am not arguing. Just random
thoughts.

Oleg.

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