linux-kernel - freezer problems

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Date:	Sun, 18 Feb 2007 17:52:46 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	ego@...ibm.com, akpm@...l.org, paulmck@...ibm.com, mingo@...e.hu,
	vatsa@...ibm.com, dipankar@...ibm.com,
	venkatesh.pallipadi@...el.com, linux-kernel@...r.kernel.org,
	Pavel Machek <pavel@....cz>
Subject: freezer problems

On 02/18, Rafael J. Wysocki wrote:
>
> On Sunday, 18 February 2007 12:31, Oleg Nesterov wrote:
> > > > 
> > > > However, this means that sys_vfork() makes impossible to freeze processes
> > > > until child exits/execs. Not good.
> > > 
> > I forgot to say that we have another problem: coredumping.
> > 
> > A thread which does do_coredump() send SIGKILL to ->mm users, and sleeps
> > on ->mm->core_startup_done. Now it can't be frozen if sub-thread goes to
> > refrigerator. I think this could be solved easily if we add a check to
> > refrigerator() as you suggested for ->vfork_donw.
> > 
> > > I think the real solution would be to use an interruptible completion in the
> > > vfork code.  It was discussed some time ago and, IIRC, Ingo had an experimental
> > > patch that implemented it.  Still, for the suspend this really is not an issue
> > > in practice, so it wasn't merged.
> > 
> > It is not (afaics) so trivial to do rightly, and with this change the parent
> > will be seen as TASK_INTERRUPTIBLE even without freezer in progress.
> > 
> > A very vague idea: what if parent will do
> > 
> > 	current->flags |= PF_PLEASE_CONSIDER_ME_AS_FROZEN_BUT_SET_TIF_FREEZE
> > 	wait_for_completion(&vfork);
> > 	try_to_freeze();
> > 
> > ?
> 
> This should work,

Good. So try_to_freeze_tasks() can forget about "if (!p->vfork_done)" check.
This needs more thinking, of course. For example, thaw_process() should be
carefull to clear TIF_FREEZE if we have the new flag set, but not PF_FROZEN.
frozen() should be changed to return true if PF_NEW_FLAG && TIF_FREEZE, but
it also called by refrigerator.

But IF we really can do this, it will be a general solution.

>                    but we'll need a separate process flag for it.  If that's
> acceptable,

I can't judge. Changed the subject to have more attention from experts.

>              I'd call it PF_VFORK_PARENT

I'd suggest not to put "VFORK" into the name. Probably we will find other
usage for this flag which in fact means: "I am sleeping TASK_UNINTERRUPTIBLE
at the safe place to freeze, I promise to do try_to_freeze() when I have
CPU".

And now another problem: exec. de_thread() sleeps in TASK_UNINTERRUPTIBLE
waiting for all sub-threads to die, and we have the same "deadlock" if
one of them is frozen. This is nasty. Probably we can change the ->state
to TASK_INTERRUPTIBLE and add try_to_freeze(), or play with the new PF_
flag, but I am not sure it is safe to freeze() the task which is deep
in the exec() path.

Oleg.

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