linux-kernel - Re: [RFC][PATCH 1/3] Freezer: Fix vfork problem

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Date:	Tue, 27 Feb 2007 10:03:53 +0530
From:	"Aneesh Kumar" <aneesh.kumar@...il.com>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	"Oleg Nesterov" <oleg@...sign.ru>,
	"Srivatsa Vaddagiri" <vatsa@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Andrew Morton" <akpm@...l.org>, "Pavel Machek" <pavel@....cz>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Gautham R Shenoy" <ego@...ibm.com>
Subject: Re: [RFC][PATCH 1/3] Freezer: Fix vfork problem

>   * Only the _current_ task can read/write to tsk->flags, but other
> Index: linux-2.6.20-mm2/include/linux/freezer.h
> ===================================================================
> --- linux-2.6.20-mm2.orig/include/linux/freezer.h       2007-02-26 08:40:22.000000000 +0100
> +++ linux-2.6.20-mm2/include/linux/freezer.h    2007-02-27 00:51:44.000000000 +0100
> @@ -75,7 +75,32 @@ static inline int try_to_freeze(void)
>                 return 0;
>  }
>
> -extern void thaw_some_processes(int all);
> +/*
> + * Tell the freezer not to count current task as freezeable
> + */
> +static inline void freezer_do_not_count(void)
> +{
> +       current->flags |= PF_FREEZER_SKIP;
> +}
> +
> +/*
> + * Tell the freezer to count this task as freezeable again and if it's a user
> + * space one, try to freeze it
> + */
> +static inline void freezer_count(void)
> +{
> +       current->flags &= ~PF_FREEZER_SKIP;
> +       if (current->mm)
> +               try_to_freeze();
> +}
> +


This means that we are not going to wait for the kernel thread
(Parent) to freeze. I guess what vatsa suggested in previous mail is
better.

freeezer_do_not_count(void)
{
   if (current->mm) {

       current->flags |= PF_FREEZER_SKIP;
   }

}


freezer_count(void)
{
 if (current->mm) {
current->flags &= ~PF_FREEZER_SKIP;
try_to_freeze();
   }

}


Now if do_fork is called from kernel_thread with CLONE_VFORK we make
the freezer wait till that kernel thread is also frozen. I think this
is important.  with khelper thread it is okey because it is a single
threaded workqueue.

If do_fork is called via a user process we can skip the Parent if the
child got frozen before calling execve.

-aneesh
-
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