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

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Date:	Thu, 10 Oct 2013 10:48:56 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

On Thu, 10 Oct 2013 13:13:05 -0400 Steven Rostedt <rostedt@...dmis.org> 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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