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Subject: Re: [patch] cpusets, cgroups: disallow attaching kthreadd
From: Mike Galbraith <efault@gmx.de>
To: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Tejun Heo <htejun@gmail.com>, LKML <linux-kernel@vger.kernel.org>,
        Li Zefan <lizf@cn.fujitsu.com>, Paul Menage <paul@paulmenage.org>,
        David Rientjes <rientjes@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>
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Date: Tue, 11 Oct 2011 18:57:16 +0200
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On Tue, 2011-10-11 at 16:08 +0200, Peter Zijlstra wrote:
> On Mon, 2011-10-10 at 09:43 -0700, Tejun Heo wrote:
> 
> > Yes, I think we need something like this.  wq workers were using
> > PF_THREAD_BOUND to prevent diddling from userland which made some
> > unhappy. 
> 
> But that can be properly fixed.
> 
> >  Maybe we need a flag to properly indicate "don't diddle with
> > this thread from userland"?  But, then, mainline kernel wouldn't need
> > the current PF_THREAD_BOUND at all.  Peter, Steven, what do you think?
> 
> Strict per-cpu affinity that is needed for correctness and disallows
> sched_setaffinity() is something entirely different from not being
> allowed to put something in a cgroup.

Agreed.  The proposed patchlet is purely a practical matter.

> As to not allowing to put in a cgroup thing, is there anything other
> than kthreadd for which we need to enforce that? So far I've mostly
> treated it like: root can do stupid things, this is one of them, don't
> do that then.

Yeah, that's the other side of the coin.  The only thing I can think of
justifying a response other than "well, gee, don't do that" is that Joe
User shouldn't need to know or care about kernel workqueue details.

I don't know of anything other than kthreadd where moving it here, or
over yonder matters one bit.

	-Mike