linux-kernel - Re: + work_on_cpu-rewrite-it-to-create-a-kernel-thread-on-demand.patch added to -mm tree

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Date:	Thu, 12 Feb 2009 14:20:21 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	ebiederm@...ssion.com (Eric W. Biederman)
Cc:	fweisbec@...il.com, mingo@...e.hu, linux-kernel@...r.kernel.org,
	oleg@...hat.com, travis@....com, a.p.zijlstra@...llo.nl,
	mm-commits@...r.kernel.org, rusty@...tcorp.com.au
Subject: Re: +
 work_on_cpu-rewrite-it-to-create-a-kernel-thread-on-demand.patch  added to
 -mm tree

On Thu, 12 Feb 2009 14:08:09 -0800
ebiederm@...ssion.com (Eric W. Biederman) wrote:

> Andrew Morton <akpm@...ux-foundation.org> writes:
> 
> > The problem with set_cpus_allowed() is that some other
> > suitably-privileged userspace process can come in from the side and
> > modify your cpus_allowed at any time.
> 
> According to the comments the only reason we care is so that
> we get the appropriate NUMA affinity by default.  I don't
> think it would be fatal if userspace messed around and we
> had a wrong value.

Right.  In this particular case, if you are fantastically unlucky and
hit the race window, all that will happen is that one particular device
will run a bit more slowly.

But at other codesites, the effects of a racing cpus_allowed rewrite
can be fatal.

> Does work_on_cpu prevent that?

Yup.

I think.

Nope.

I don't think there's actually anything which would prevent a
sufficiently stupid/malicious/unlucky administrator from moving the
work_on_cpu thread onto the wrong CPU at the wrong time.  hrm.

Another reason to use smp_call_function_single().
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