linux-kernel - Re: [PATCH 1/9] cpu: Introduce clear_tasks_mm

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Date:	Tue, 01 May 2012 12:45:33 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Anton Vorontsov <anton.vorontsov@...aro.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Russell King <linux@....linux.org.uk>,
	Mike Frysinger <vapier@...too.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Richard Weinberger <richard@....at>,
	Paul Mundt <lethal@...ux-sh.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	John Stultz <john.stultz@...aro.org>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	uclinux-dist-devel@...ckfin.uclinux.org,
	linuxppc-dev@...ts.ozlabs.org, linux-sh@...r.kernel.org,
	user-mode-linux-devel@...ts.sourceforge.net,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/9] cpu: Introduce clear_tasks_mm_cpumask() helper

On Thu, 2012-04-26 at 16:59 -0700, Andrew Morton wrote:
> > +void clear_tasks_mm_cpumask(int cpu)
> 
> The operation of this function was presumably obvious to you at the
> time you wrote it, but that isn't true of other people at later times.
> 
> Please document it?
> 
> 
> > +{
> > +     struct task_struct *p;
> > +
> > +     /*
> > +      * This function is called after the cpu is taken down and marked
> > +      * offline,
> 
> hm, well.  Who said that this function will only ever be called
> after that CPU was taken down?  There is nothing in the function name
> nor in the (absent) documentation which enforces this precondition.
> 
> If someone tries to use this function for a different purpose, or
> copies-and-modifies it for a different purpose, we just shot them in
> the foot.
> 
> They'd be pretty dumb to do that without reading the local comment,
> but still...

Methinks something simple like:

	WARN_ON(cpu_online(cpu));

Ought to cure that worry, no? :-)

> 
> >        so its not like new tasks will ever get this cpu set in
> > +      * their mm mask. -- Peter Zijlstra
> > +      * Thus, we may use rcu_read_lock() here, instead of grabbing
> > +      * full-fledged tasklist_lock.
> > +      */
> > +     rcu_read_lock();
> > +     for_each_process(p) {
> > +             struct task_struct *t;
> > +
> > +             t = find_lock_task_mm(p);
> > +             if (!t)
> > +                     continue;
> > +             cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
> > +             task_unlock(t);
> > +     }
> > +     rcu_read_unlock();
> > +} 
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