linux-kernel - Re: [PATCH 3/8] Use process freezer for cpu-hotplug

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Date:	Thu, 5 Apr 2007 17:34:49 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	akpm@...ux-foundation.org, paulmck@...ibm.com,
	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	vatsa@...ibm.com, "Rafael J. Wysocki" <rjw@...k.pl>, mingo@...e.hu,
	dipankar@...ibm.com, dino@...ibm.com,
	masami.hiramatsu.pt@...achi.com
Subject: Re: [PATCH 3/8] Use process freezer for cpu-hotplug

On 04/05, Gautham R Shenoy wrote:
>
> > > @@ -147,6 +147,7 @@ cpu_callback(struct notifier_block *nfb,
> > >  	case CPU_DEAD:
> > >  		p = per_cpu(watchdog_task, hotcpu);
> > >  		per_cpu(watchdog_task, hotcpu) = NULL;
> > > +		thaw_process(p);
> > >  		kthread_stop(p);
> > 
> > As it was already discussed, this is racy. As Srivatsa (imho rightly)
> > suggested, kthread_stop(p) should thaw process itself. This also allows
> > us to kill at least some of wait_for_die loops.
> > 
> 
> Well, in this case this is not racy. Remember, we're doing a
> thaw_process(p) in CPU_DEAD where p *is* frozen for cpu hotplug. So
> the where we might call a freeze_process(p) after we do a thaw_process
> doesn't seem to be feasible.

Oops, yes.

> > However, the change in kthread_stop(p) in not enough to close the race.
> > We can check kthread_should_stop() in refrigerator(), this looks like
> > a most simple approach for now.
> > 
> 
> Why the check kthread_should_stop() refrigerator() ?
> As vatsa mentioned, we would be doing 
> 
> task_lock(p);
> freezer_exempt(p, FE_ALL); /* Doesn't exist as of now, but we can work
> 				it out */
> thaw_process(p);
> task_unlock(p);

Please look at http://marc.info/?l=linux-kernel&m=117562018530190, we can't
change p->flags unless we know it is frozen.

> > Alternatively, Srivatsa suggests to introduce a new task_lock() protected
> > task_struct->freezer_state (so we can reliably set FE_ALL). Surely this is
> > more poweful, but needs more changes. I am not sure. Perhaps we can do
> > this later.
> 
> This needs an extra field! We're supposed to be miserly when it comes to
> adding new fields to task_struct, now aren't we :-)

That is why "Perhaps we can do this later" :)

> > In any case, imho "try3" should add thaw_process() to kthread_stop().
> > Gautham, Srivatsa, do you agree?
> > 
> 
> Completely. Working on it now.

Great!

Oleg.

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