linux-kernel - Re: [PATCH 1/7] Freezer: Read PF_BORROWED

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Message-ID: <20070512165857.GA274@tv-sign.ru>
Date:	Sat, 12 May 2007 20:58:57 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

On 05/12, Rafael J. Wysocki wrote:
>
> Ah, I see.  We spawn a kernel thread from a code path that belongs to a
> user space task and we need to call deamonize() to make it become a
> 'real' kernel thread.
> 
> Still, this means that is_user_space() may return 'true' for this thread
> before it calls daemonize() and then the scenario described by me in the
> previous message may occur.  It seems.

Yes sure. Probably not so bad in practice. Most likely this fresh thread
is not "important" and could be freezed, I dunno.

> It's in freezer-fix-pf_nofreeze-vs-freezeable-race.patch (appended for
> convenience, white space may be broken).
>
> ---
> --- a/include/linux/freezer.h~freezer-fix-pf_nofreeze-vs-freezeable-race
> +++ a/include/linux/freezer.h
> @@ -63,8 +63,10 @@ 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);
>  }

This is OK if a kernel thread does try_to_freeze() eventually.

But what if it does not, because it marks itself as PF_NOFREEZE?
This means it may run with signal_pending() forever. That is why
I think we should clear TIF_FREEZE when we set PF_NOFREEZE.

Oleg.

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