linux-kernel - Re: [PATCH 1/7] work_on_cpu: helper for doing task on a CPU.

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Date:	Fri, 24 Oct 2008 22:18:50 +1100
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Gautham R Shenoy <ego@...ibm.com>, linux-kernel@...r.kernel.org,
	travis@....com, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH 1/7] work_on_cpu: helper for doing task on a CPU.

On Friday 24 October 2008 21:29:57 Oleg Nesterov wrote:
> On 10/24, Gautham R Shenoy wrote:
> > On Fri, Oct 24, 2008 at 02:04:35PM +1100, Rusty Russell wrote:
> > > I think we should BUG_ON(per_cpu(cpu_state, cpuid) != CPU_DEAD) to
> > > ensure we never use work_on_cpu in the hotplug cpu path.  Then we use
> > > smp_call_function() for that hard intel_cacheinfo case.  Finally, we
> > > fix the cpu hotplug path to use schedule_work_on() itself rather than
> > > playing games with cpumask.
> > >
> > > If you agree, I'll spin the patches...
> >
> > How about the following?
> >
> > We go with this method, but instead of piggybacking on
> > the generic kevents workqueue, we create our own on_each_cpu_wq, for this
> > purpose.
>
> Gautham, Rusty, I am a bit lost on this discussion...
>
> Why should we care about this deadlock? Just do not use work_on_cpu() from
> the hotplug cpu path, that is all.

No, I agree with you (Oleg).  Gautham's proposal would work, but at the cost 
of yet another thread per CPU :(

Since we know how to handle the one problematic case Oleg spotted, *and* we 
know how to BUG_ON to make sure noone introduces new ones, I think this is 
clearest.

Thanks,
Rusty.
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