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

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Date:	Sun, 25 Feb 2007 20:58:29 +0530
From:	"Aneesh Kumar" <aneesh.kumar@...il.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
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 2/25/07, Rafael J. Wysocki <rjw@...k.pl> wrote:
> 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.


But approach i outlined above make sure both parent and child get
frozen during the freeze_process. where as with PF_FREEZER_SKIP the
child waits in the completion wait_queue in an uninterruptible state.
I am not sure whether it really make any difference from any of the
freezer users point of view. (suspend, hotplug, kprobes etc ).

 -aneesh
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