linux-kernel - Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

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Message-Id: <200702181132.41264.rjw@sisk.pl>
Date:	Sun, 18 Feb 2007 11:32:39 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Oleg Nesterov <oleg@...sign.ru>
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: Re: [RFC PATCH(Experimental) 0/4] Freezer based Cpu-hotplug

On Sunday, 18 February 2007 00:42, Oleg Nesterov wrote:
> On 02/17, Rafael J. Wysocki wrote:
> >
> > On Saturday, 17 February 2007 22:34, Oleg Nesterov wrote:
> > > 
> > > 	static inline int is_user_space(struct task_struct *p)
> > > 	{
> > > 		return p->mm && !(p->flags & PF_BORROWED_MM);
> > > 	}
> > > 
> > > This doesn't look right. First, an exiting task has ->mm == NULL after
> > > do_exit()->exit_mm(). Probably not a problem. However, PF_BORROWED_MM
> > > check is racy without task_lock(), so we can have a false positive as
> > > well. Is it ok? We can freeze aio_wq prematurely.
> > 
> > Right now aio_wq is not freezeable (PF_NOFREEZE).
> 
> Right now yes, but we are going to change this?

Well, is there any more reliable (and not racy) method of differentiating
between kernel threads and user space processes?

> > > 				cancel_freezing(p);
> > > 				continue;
> > > 
> > > Is it right? Shouldn't we increment "todo" counter?
> > 
> > No.  It would be wrong to do that, because TASK_TRACED tasks with frozen
> > parents cannot be frozen any further.
> 
> TASK_TRACED task could be woken by SIGKILL. cancel_freezing() clears TIF_FREEZE.
> The task may start do_exit() when try_to_freeze_tasks() returns "success".
> Probably not a problem.

Yup.

Greetings,
Rafael
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