linux-kernel - Re: [RFC][PATCH 1/2] Freezer: Introduce PF_FREEZER

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Date:	Wed, 7 May 2008 14:16:47 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@...e.cz>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Len Brown <lenb@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Matt Helsley <matthltc@...ibm.com>,
	Cedric Le Goater <clg@...ibm.com>,
	Paul Menage <menage@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [RFC][PATCH 1/2] Freezer: Introduce PF_FREEZER_NOSIG

On Wednesday, 7 of May 2008, Pavel Machek wrote:
> Hi!

Hi,

> > From: Rafael J. Wysocki <rjw@...k.pl>
> > 
> > The freezer currently attempts to distinguish kernel threads from
> > user space tasks by checking if their mm pointer is unset and it
> > does not send fake signals to kernel threads.  However, there are
> > kernel threads, mostly related to networking, that behave like
> > user space tasks and may want to be sent a fake signal to be frozen.
> > 
> > Introduce the new process flag PF_FREEZER_NOSIG that will be set
> > by default for all kernel threads and make the freezer only send
> > fake signals to the tasks having PF_FREEZER_NOSIG unset.  Provide
> > the set_freezable_with_signal() function to be called by the kernel
> > threads that want to be sent a fake signal for freezing.
> > 
> > This patch should not change the freezer's observable behavior.
> > 
> > Signed-off-by: Rafael J. Wysocki <rjw@...k.pl>
> 
> ACK.

Thanks!

> > -static int has_mm(struct task_struct *p)
> > +static inline bool should_send_signal(struct task_struct *p)
> >  {
> > -	return (p->mm && !(p->flags & PF_BORROWED_MM));
> > +	return !(current->flags & PF_FREEZER_NOSIG);
> >  }
> >
> 
> Note that we used to tell kernel threads by ->mm, and now you assume
> that anything created by ktrheadd is kernel thread, ->mm or not.
> 
> I'm not sure if those can differ (->mm = NULL somewhere? Or ->mm =
> something somewhere else?).

It's done in analogy with PF_NOFREEZE, ie. everything that has
set PF_NOFREEZE by default also has PF_FREEZER_NOSIG set by default, so I
really don't expect any problems here.

> I guess this should go to -mm for a long test...

Yes, it should.

Thanks,
Rafael
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