Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

[Andrew Morton]

On Thu, 10 Oct 2013 13:13:05 -0400 Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> > 
> > > > Why prevent all CPUs from running when we want to remove
> > > > one?
> > > 
> > > So get_online_cpus() goes away.  Nothing is more scalable than nothing!
> > 
> > Very much agreed; now stop_machine() wouldn't actually work for hotplug
> > because it will instantly preempt everybody, including someone who might
> > be in the middle of using per-cpu state of the cpu we're about to
> > remove.
> 
> Well, stop machine doesn't instantly preempt everybody. Only those that
> don't have preemption disabled. Using per_cpu without preemption
> disabled can be dangerous.

Yes, I'd have thought that the cases where a CPU is fiddling with
another CPU's percpu data with preemption enabled would be rather rare.

I can't actually think of any off the top.  Are there examples we can
look at?

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