linux-kernel - Re: [RFC][PATCH 4/7] Freezer: Fix vfork problem

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Message-Id: <200702251605.42312.rjw@sisk.pl>
Date:	Sun, 25 Feb 2007 16:05:40 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	"Aneesh Kumar" <aneesh.kumar@...il.com>
Cc:	"Pavel Machek" <pavel@....cz>, LKML <linux-kernel@...r.kernel.org>,
	paulmck@...ux.vnet.ibm.com, ego@...ibm.com, akpm@...l.org,
	mingo@...e.hu, vatsa@...ibm.com, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, Oleg Nesterov <oleg@...sign.ru>
Subject: Re: [RFC][PATCH 4/7] Freezer: Fix vfork problem

On Sunday, 25 February 2007 15:33, Aneesh Kumar wrote:
> On 2/25/07, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Sunday, 25 February 2007 11:45, Rafael J. Wysocki wrote:
> > > Hi,
> > >
> > > =========
> > --- linux-2.6.20-mm2.orig/kernel/power/process.c        2007-02-22 23:44:04.000000000 +0100
> > +++ linux-2.6.20-mm2/kernel/power/process.c     2007-02-23 22:33:11.000000000 +0100
> > @@ -127,22 +127,12 @@ static unsigned int try_to_freeze_tasks(
> >                                 cancel_freezing(p);
> >                                 continue;
> >                         }
> > -                       if (is_user_space(p)) {
> > -                               if (!freeze_user_space)
> > -                                       continue;
> > -
> > -                               /* Freeze the task unless there is a vfork
> > -                                * completion pending
> > -                                */
> > -                               if (!p->vfork_done)
> > -                                       freeze_process(p);
> > -                       } else {
> > -                               if (freeze_user_space)
> > -                                       continue;
> > +                       if (is_user_space(p) == !freeze_user_space)
> > +                               continue;
> >
> 
> How about ?
> if ( ! (is_user_space(p) == freeze_user_space) )
>      continue;

I think it would be safer to do

if ( is_user_space(p) != !!freeze_user_space)
      continue;

which is equivalent to my previous version, but contains one '!' more. ;-)

Seriously, the one in the patch is consistent with the other occurrences of
it in the file and I'm going to change it anyway in a separate patch
(while freezing kernel threads we need to freeze userspace tasks too in case
one of the kernel threads called kernel_execve() in the meantime).

> BTW one of the concern that vatsa had was; is it ok to allow some of
> the tasks to be left running  ( the parent from vfork ) while
> freezing.  I guess we can solve this in a nice way.
> 
> in fork.c
> 
> if (clone_flags & CLONE_VFORK) {
>                         p->vfork_done = &vfork;
>                         p->flags |= PF_PARENT_WAKEUP_ON_FREEZE;
>                         init_completion(&vfork);
> }
> 
> 
> and in freeze_process(struct task_struct *p)
> 
> if ( p->flags & PF_PARENT_WAKEUP_ON_FREEZE ) {
>   wake_up_parent();
> }
> 
> now parent should be wating for these completion via
> 
> wait_for_completion_freezable(); // pavel's implementation.

Hm, I think this leaves us with an analogous problem: we need a method
to tell a vforking task that the child should set PF_PARENT_WAKEUP_ON_FREEZE.

In the approach with PF_FREEZER_SKIP we need a method to tell the
vforking task that it should skip try_to_freeze() in freezer_count(), and I
think there are some possible ways to do this.  The patch doesn't implement
any of them, because this is a different issue that can be deal with later.

Greetings,
Rafael
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