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

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Message-Id: <200902140751.15662.rusty@rustcorp.com.au>
Date:	Sat, 14 Feb 2009 07:51:15 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Frédéric Weisbecker <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
Subject: Re: + work_on_cpu-rewrite-it-to-create-a-kernel-thread-on-demand.patch  added to -mm tree

On Friday 13 February 2009 07:08:36 Eric W. Biederman wrote:
> Andrew Morton <akpm@...ux-foundation.org> writes:
> >
> > Series of four patches:
> >
> > - switch cstate.c frmo work_on_cpu to smp_call_function_single()
> >
> > - ditto acpi-cpufreq.c
> >
> > - ditto mce_amd_64.c
> >
> > The final work_on_cpu() caller is pci_call_probe().  I'd like to find a
> > way of removing that callsite as well, so we can finally remove this
> > turkey but for now, just fix the bugs in it:
> 
> As far as I can tell when we are doing the probes we are in a function
> that can sleep, so we should be able to just call set_cpus_allowed to
> remove the need for work_on_cpu in pci_call_probe.  Possibly with a
> save/restore of the allowed cpus.
> 
> Am I missing something?

Yes.  This is a questionable practice when it's a kernel thread (but not
really a problem), but a definite no-no on a real process.  Userspace is
allowed to change affinity on any process at any time; that's why we need
a real method to replace this meme.

(I'm also getting those cpumask's off the stack for core and x86 code, which
is why I'm hitting them all).

Thanks,
Rusty.
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