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Subject: Re: patch] cpusets, cgroups: disallow attaching kthreadd
From: Mike Galbraith <efault@gmx.de>
To: David Rientjes <rientjes@google.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>,
        LKML <linux-kernel@vger.kernel.org>, Tejun Heo <htejun@gmail.com>,
        Li Zefan <lizf@cn.fujitsu.com>, Paul Menage <paul@paulmenage.org>
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Date: Wed, 19 Oct 2011 06:00:39 +0200
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On Tue, 2011-10-18 at 13:38 -0700, David Rientjes wrote:
> On Tue, 18 Oct 2011, Mike Galbraith wrote:
> 
> > From: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > 
> > cgroups: disallow attaching kthreadd
> > 
> > Allowing kthreadd to be moved to a non-root group makes no sense, it being
> > a global resource, and needlessly leads unsuspecting users toward trouble.
> > 
> > 1. An RT workqueue worker thread spawned in a task group with no rt_runtime
> > allocated is not schedulable.  Simple user error, but harmful to the box.
> > 
> > 2. A worker thread which acquires PF_THREAD_BOUND can never leave a cpuset,
> > rendering the cpuset immortal.
> > 
> 
> Eek, this seems like complete overkill, there may be cgroups now or in the 
> future that it would be perfectly acceptable to move kthreadd into without 
> any negative effects.  There's no reason to restrict it this broadly, 
> that's why I thought your change to cpusets was fine and acked it.  It's 
> the perfect candidate for what the ->can_attach() pointer for a cgroup can 
> identify is problematic for _that_ cgroup.

Oh my, I can't win.

Either of the two patchlets will do the trick, so maintainer's choice.

> > Save the user some unexpected trouble, just say no.
> > 
> > Signed-off-by: Mike Galbraith <efault@gmx.de>
> > 
> 
> A patch from Peter that only has your sign-off?

The only thing in it from me was a comment, figured I should post a from
line as a matter of propriety.  Guess not.  If this variant is the one
that flies, the from line can be dropped.  I doubt Peter counts his
credits, and even if he did, he wouldn't notice _this one_ missing :)

	-Mike