linux-kernel - Re: [RFC PATCH 1/2] Fix PF_NOFREEZE and freezeable race

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Date:	Mon, 23 Apr 2007 21:49:34 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	akpm@...ux-foundation.org, "Rafael J. Wysocki" <rjw@...k.pl>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, vatsa@...ibm.com,
	paulmck@...ibm.com, pavel@....cz
Subject: Re: [RFC PATCH 1/2] Fix PF_NOFREEZE and freezeable race

On 04/23, Gautham R Shenoy wrote:
>
> On Fri, Apr 20, 2007 at 10:02:08PM +0400, Oleg Nesterov wrote:
> > 
> > Gautham, isn't it possible to make a more simpler patch ? Just add PF_NOFREEZE
> > check to frozen_process,
> > 
> > 	static inline void frozen_process(struct task_struct *p)
> > 	{
> > 		if (!unlikely(current->flags & PF_NOFREEZE)) {
> > 			p->flags |= PF_FROZEN;
> > 			wmb();
> > 		}
> > 		clear_tsk_thread_flag(p, TIF_FREEZE);
> > 	}
> > 
> 
> Btw, since frozen_process is currently being called only from
> refrigerator, I am wondering if we still need the struct task_struct *p
> parameter there. It's very unlikely that some other task would mark a
> particular task as frozen. No?

I agree. Only "current" can set PF_FROZEN anyway. I also think it is better
to move this function into kernel/power/process.c, no need to export it in
freezer.h. It is special, should be called from refrigerator() with task_lock()
held.

> Anyways, Andrew, Could you please replace the earlier sent patch titled
> "fix_pf_nofreeze_and_freezeable_race.patch" with the following one?
>
> ...
>
> --- linux-2.6.21-rc6.orig/include/linux/freezer.h
> +++ linux-2.6.21-rc6/include/linux/freezer.h
> @@ -57,8 +57,10 @@ static inline int thaw_process(struct ta
>   */
>  static inline void frozen_process(struct task_struct *p)
>  {
> -	p->flags |= PF_FROZEN;
> -	wmb();
> +	if (!unlikely(p->flags & PF_NOFREEZE)) {
> +		p->flags |= PF_FROZEN;
> +		wmb();
> +	}
>  	clear_tsk_thread_flag(p, TIF_FREEZE);
>  }

On the second thought, this patch doesn't help if the thread never
calls try_to_freeze() exactly because it marks itself PF_NOFREEZE.

Perhaps it is better to cancel freezing in soon-to-be-introduced
freezer_exempt().

In any case I agree, this one is better than current
	fix_pf_nofreeze_and_freezeable_race.patch

Oleg.

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