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Subject: Re: [PATCH 1/9] cpu: Introduce clear_tasks_mm_cpumask() helper
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Andrew Morton <akpm@linux-foundation.org>
Date: Tue, 01 May 2012 12:45:33 +0200
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Cc: linaro-kernel@lists.linaro.org, Mike Frysinger <vapier@gentoo.org>,
 user-mode-linux-devel@lists.sourceforge.net, linux-sh@vger.kernel.org,
 Richard Weinberger <richard@nod.at>, linuxppc-dev@lists.ozlabs.org,
 Oleg Nesterov <oleg@redhat.com>, linux-kernel@vger.kernel.org,
 linux-mm@kvack.org, Anton Vorontsov <anton.vorontsov@linaro.org>,
 Paul Mundt <lethal@linux-sh.org>, John Stultz <john.stultz@linaro.org>,
 patches@linaro.org, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>,
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On Thu, 2012-04-26 at 16:59 -0700, Andrew Morton wrote:
> > +void clear_tasks_mm_cpumask(int cpu)
>=20
> 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.
>=20
> Please document it?
>=20
>=20
> > +{
> > +     struct task_struct *p;
> > +
> > +     /*
> > +      * This function is called after the cpu is taken down and marked
> > +      * offline,
>=20
> 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.
>=20
> 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.
>=20
> 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? :-)

>=20
> >        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 =3D find_lock_task_mm(p);
> > +             if (!t)
> > +                     continue;
> > +             cpumask_clear_cpu(cpu, mm_cpumask(t->mm));
> > +             task_unlock(t);
> > +     }
> > +     rcu_read_unlock();
> > +}=20