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From: "Rafael J. Wysocki" <rjw@sisk.pl>
To: Oleg Nesterov <oleg@tv-sign.ru>
Subject: Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way
Date: Sat, 12 May 2007 19:16:22 +0200
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Cc: Linus Torvalds <torvalds@linux-foundation.org>,
       Andrew Morton <akpm@linux-foundation.org>,
       Gautham R Shenoy <ego@in.ibm.com>, LKML <linux-kernel@vger.kernel.org>,
       Pavel Machek <pavel@ucw.cz>,
       "Eric W. Biederman" <ebiederm@xmission.com>
References: <200705110035.32229.rjw@sisk.pl> <200705121835.21333.rjw@sisk.pl> <20070512165857.GA274@tv-sign.ru>
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On Saturday, 12 May 2007 18:58, Oleg Nesterov wrote:
> 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.

I don't know too, and that's why I'd like to prevent this from happening.

> > 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.

Yes, we should.

Greetings,
Rafael