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Date: Thu, 10 Oct 2013 15:04:44 -0400
From: Steven Rostedt <rostedt@goodmis.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
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On Thu, 10 Oct 2013 11:49:15 -0700
Linus Torvalds <torvalds@linux-foundation.org> wrote:
 
> Oh, and I'm sure there are several users that currently depend on
> being able to sleep over get_online_cpu's.  But I'm pretty sure it's
> "several", not "hundreds", and I think we could fix them up.

I'm wondering if we can have a for_each_cpu() that only disables
preemption in the loop. That is, each iteration will enable preemption,
but the loop itself will guarantee that the current cpu to process
still exists.

	rcu_read_lock();
	for_each_cpu(cpu, mask) {
		rcu_read_unlock();
		rcu_read_lock();
		if (!cpu_online(cpu))
			continue;
		[...]
	}
	rcu_read_unlock();

That way expensive loops wont stop the current CPU to process all
online CPUs.

Of course, it will miss a CPU that comes online. But it would still
need to handle the case of a CPU coming online after the final
put_online_cpus(), so I'm not sure that's a problem.

-- Steve