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

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Message-ID: <20081024102957.GC4583@redhat.com>
Date:	Fri, 24 Oct 2008 12:29:57 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Gautham R Shenoy <ego@...ibm.com>
Cc:	Rusty Russell <rusty@...tcorp.com.au>,
	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 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.

Once again, the "cpu_hotplug_begin()" lock is not special. You can't use
work_on_cpu() under (say) rtnl_lock() for the same reason, this lock is
used by work->func() too.

Perhaps I missed something, and work_on_cpu() is really important for
cpu-hotplug path?

Oleg.

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